Handle the remainder in MemoryExtensions.Count vectorized - #82687

Merged
stephentoub merged 6 commits into
dotnet:mainfrom
gfoidl:memoryextensions_count
May 15, 2023
Merged

Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom
gfoidl:memoryextensions_count

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@gfoidl

@gfoidlgfoidl commented Feb 26, 2023

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Description

In the current implementation the remainder of the vectorized code-path is processed scalar.
This PR processes the remainder vectorized too.

This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.
As some elements may overlap now, we need to shift them off from the mask to get the correct count.

Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
Thus the remainder is done vectorized only if remaining length is more than half of a vector size.

Benchmarks

Benchmark-code, run on win-x64 with .NET 8 Preview 1.
Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.

byte

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...

@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost added the community-contribution Indicates that the PR has been added by a community member label Feb 26, 2023
@ghost

ghost commented Feb 26, 2023

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See info in area-owners.md if you want to be subscribed.

Issue Details

Description

In the current implementation the remainder of the vectorized code-path is processed scalar.
This PR processes the remainder vectorized too.

This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.
As some elements may overlap now, we need to shift them off from the mask to get the correct count.

Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
Thus the remainder is done vectorized only if remaining length is more than half of a vector size.

Benchmarks

Benchmark-code, run on win-x64 with .NET 8 Preview 1.
Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.

byte

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long

| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...

Author:gfoidl
Assignees:-
Labels:

area-System.Memory, community-contribution

Milestone:-

@build-analysisbuild-analysisBot mentioned this pull request Feb 26, 2023
Closed
@stephentoub

stephentoub commented Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

How much higher? The / 2 feels a bit arbitrary.

@gfoidl

gfoidl commented Feb 28, 2023

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The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.

Benchmarks for if (remaining > 0):

byte

| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short

| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int

| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long

| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.

stephentoub
stephentoub reviewed May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
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Thanks.

@gfoidl

gfoidl commented May 15, 2023

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I have some other ideas on how to improve perf for Count

Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as

@stephentoub
stephentoub merged commit 50d4ecb into dotnet:mainMay 15, 2023
@gfoidl
gfoidl deleted the memoryextensions_count branch May 16, 2023 09:32
@EgorBoEgorBo mentioned this pull request May 18, 2023
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}
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})();
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try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
Conversation
@gfoidl
@gfoidlgfoidl
commented
Feb 26, 2023

Copy link


Copy Markdown

Member
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
added
the 
community-contribution
Indicates that the PR has been added by a community member
label
Feb 26, 2023
@ghost
ghost
commented
Feb 26, 2023

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Tagging subscribers to this area: @dotnet/area-system-memory

See info in area-owners.md if you want to be subscribed.
Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
mentioned this pull request
Feb 26, 2023

Closed

@stephentoub
stephentoub
commented
Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

Copy link


Copy Markdown

MemberAuthor
The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
stephentoub
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approved these changes
May 15, 2023
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
gfoidl
deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

Closed

@kunalspathakkunalspathak
mentioned this pull request
May 18, 2023

Closed

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locked as resolved and limited conversation to collaborators
Jun 15, 2023
Sign up for freeto subscribe to this conversation on GitHub.
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
Conversation
@gfoidl
@gfoidlgfoidl
commented
Feb 26, 2023

Copy link


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Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
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Feb 26, 2023
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Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
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Feb 26, 2023

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@stephentoub
stephentoub
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Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

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The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
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deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

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@gfoidl@stephentoub@vcsjones

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
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@gfoidl
@gfoidlgfoidl
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Feb 26, 2023

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Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
added
the 
community-contribution
Indicates that the PR has been added by a community member
label
Feb 26, 2023
@ghost
ghost
commented
Feb 26, 2023

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Tagging subscribers to this area: @dotnet/area-system-memory

See info in area-owners.md if you want to be subscribed.
Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
mentioned this pull request
Feb 26, 2023

Closed

@stephentoub
stephentoub
commented
Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

Copy link


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MemberAuthor
The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
stephentoub
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
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deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

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May 18, 2023

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@gfoidl@stephentoub@vcsjones

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
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@gfoidl
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Feb 26, 2023

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Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
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Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
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Feb 26, 2023

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@stephentoub
stephentoub
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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

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MemberAuthor
The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
gfoidl
deleted the

memoryextensions_count

branch
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@EgorBoEgorBo
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@gfoidl@stephentoub@vcsjones

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
Conversation
@gfoidl
@gfoidlgfoidl
commented
Feb 26, 2023

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Member
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
added
the 
community-contribution
Indicates that the PR has been added by a community member
label
Feb 26, 2023
@ghost
ghost
commented
Feb 26, 2023

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Tagging subscribers to this area: @dotnet/area-system-memory

See info in area-owners.md if you want to be subscribed.
Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
mentioned this pull request
Feb 26, 2023

Closed

@stephentoub
stephentoub
commented
Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

Copy link


Copy Markdown

MemberAuthor
The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
stephentoub
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May 15, 2023
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
gfoidl
deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

Closed

@kunalspathakkunalspathak
mentioned this pull request
May 18, 2023

Closed

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Jun 15, 2023
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@gfoidl@stephentoub@vcsjones

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
Handle the remainder in MemoryExtensions.Count vectorized - #82687
Merged
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
Conversation
@gfoidl
@gfoidlgfoidl
commented
Feb 26, 2023

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Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
added
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community-contribution
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Feb 26, 2023
@ghost
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Feb 26, 2023

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Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
mentioned this pull request
Feb 26, 2023

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@stephentoub
stephentoub
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Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

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The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
gfoidl
deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

Closed

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May 18, 2023

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@gfoidl@stephentoub@vcsjones

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Handle the remainder in MemoryExtensions.Count vectorized - #82687
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stephentoub merged 6 commits into
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gfoidl:memoryextensions_count
May 15, 2023
Merged
Handle the remainder in MemoryExtensions.Count vectorized#82687
stephentoub merged 6 commits into
dotnet:mainfrom 
gfoidl:memoryextensions_count
Conversation
@gfoidl
@gfoidlgfoidl
commented
Feb 26, 2023

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Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
@gfoidl
Re-use targetVector from Vector256 in Vector128 code-path
375fec8
Saves an additional vpbroadcastb.
@gfoidl
Process remainder vectorized
325a194
@gfoidl
Use unsigned division for remaining elements
c3721ed
Avoids the signed integer division.
@gfoidl
Process remainder vectorized only if more than half of vector remains
36b74c7
Benchmarking showed that the cost is quite high, so for just a few elements the scalar loop seems better.
@ghostghost
added
the 
community-contribution
Indicates that the PR has been added by a community member
label
Feb 26, 2023
@ghost
ghost
commented
Feb 26, 2023

Copy link


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Tagging subscribers to this area: @dotnet/area-system-memory

See info in area-owners.md if you want to be subscribed.
Issue Details
Description
In the current implementation the remainder of the vectorized code-path is processed scalar.

This PR processes the remainder vectorized too.
This is done by reading a vector from the end, comparing with the target vector, and extracting the most significant bits as usual.

As some elements may overlap now, we need to shift them off from the mask to get the correct count.
Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.

Thus the remainder is done vectorized only if remaining length is more than half of a vector size.
Benchmarks
Benchmark-code, run on win-x64 with .NET 8 Preview 1.

Bencharks are done for the lengths Vector256<T>.Count + 1 and Vector256<T>.Count * 2 - 1, so the both extreme cases for the remainder.
byte
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |----------:|----------:|----------:|------:|--------:|
| Default | 33 | 2.550 ns | 0.0897 ns | 0.0795 ns | 1.00 | 0.00 |
| PR | 33 | 2.460 ns | 0.0574 ns | 0.0509 ns | 0.97 | 0.04 |
| | | | | | | |
| Default | 63 | 18.252 ns | 0.2371 ns | 0.2102 ns | 1.00 | 0.00 |
| PR | 63 | 2.816 ns | 0.0921 ns | 0.0862 ns | 0.15 | 0.00 |

short
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 17 | 2.729 ns | 0.0630 ns | 0.0589 ns | 1.00 | 0.00 |
| PR | 17 | 3.255 ns | 0.1044 ns | 0.2358 ns | 1.20 | 0.11 |
| | | | | | | |
| Default | 31 | 9.784 ns | 0.2347 ns | 0.4172 ns | 1.00 | 0.00 |
| PR | 31 | 3.707 ns | 0.1104 ns | 0.1473 ns | 0.38 | 0.03 |

int
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 9 | 1.960 ns | 0.0805 ns | 0.1452 ns | 1.00 | 0.00 |
| PR | 9 | 2.177 ns | 0.0427 ns | 0.0356 ns | 1.04 | 0.08 |
| | | | | | | |
| Default | 15 | 4.317 ns | 0.1263 ns | 0.2278 ns | 1.00 | 0.00 |
| PR | 15 | 2.205 ns | 0.0291 ns | 0.0272 ns | 0.51 | 0.02 |

long
| Method | Length | Mean | Error | StdDev | Ratio | RatioSD |
|-------- |------- |---------:|----------:|----------:|------:|--------:|
| Default | 5 | 2.274 ns | 0.0866 ns | 0.2414 ns | 1.00 | 0.00 |
| PR | 5 | 2.460 ns | 0.0922 ns | 0.2703 ns | 1.09 | 0.16 |
| | | | | | | |
| Default | 7 | 3.071 ns | 0.1044 ns | 0.2944 ns | 1.00 | 0.00 |
| PR | 7 | 2.469 ns | 0.0941 ns | 0.2199 ns | 0.79 | 0.09 |

I have some other ideas on how to improve perf for Count, but a) I'd like to keep the changes separate to make it easier to track the improvements, and b) at the moment it's a bit difficult with time for me...
Author:gfoidl
Assignees:-
Labels:
area-System.Memory, community-contribution
Milestone:-
@build-analysisbuild-analysisBot
mentioned this pull request
Feb 26, 2023

Closed

@stephentoub
stephentoub
commented
Feb 27, 2023

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Benchmarking showed that the cost of doing the remainder vectorized is higher than scalar processing if there are just a few elements.
How much higher? The / 2 feels a bit arbitrary.
@gfoidl
gfoidl
commented
Feb 28, 2023

Copy link


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MemberAuthor
The / 2 was chosen by some (rough) tests, and to make the code not too complicated, i.e. by avoiding type-based and remainder-count different threshoulds.
Benchmarks for if (remaining > 0):
byte
| Method | Length | Mean | Ratio |
|-------- |------- |----------:|------:|
| Default | 33 | 2.318 ns | 1.00 |
| PR | 33 | 2.465 ns | 1.06 |
| | | | |
| Default | 63 | 16.462 ns | 1.00 |
| PR | 63 | 2.404 ns | 0.15 |

short
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 17 | 2.486 ns | 1.00 |
| PR | 17 | 3.075 ns | 1.24 |
| | | | |
| Default | 31 | 8.185 ns | 1.00 |
| PR | 31 | 3.229 ns | 0.39 |

int
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 9 | 1.653 ns | 1.00 |
| PR | 9 | 2.384 ns | 1.44 |
| | | | |
| Default | 15 | 3.514 ns | 1.00 |
| PR | 15 | 2.387 ns | 0.68 |

long
| Method | Length | Mean | Ratio |
|-------- |------- |---------:|------:|
| Default | 5 | 1.936 ns | 1.00 |
| PR | 5 | 2.585 ns | 1.33 |
| | | | |
| Default | 7 | 2.835 ns | 1.00 |
| PR | 7 | 2.799 ns | 0.99 |

So if the remainder is small -- just a few elements -- the scalar loop seems faster than doing the bitmask + popcount on the full last vector. Thus for simplicity / 2 was chosen.
stephentoub
stephentoub
reviewed
May 15, 2023
View reviewed changes
Comment threadsrc/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
stephentoub
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May 15, 2023
View reviewed changes
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Thanks.
@gfoidl
gfoidl
commented
May 15, 2023

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MemberAuthor
I have some other ideas on how to improve perf for Count
Played around with these ideas (back then when creating this PR, may not be fully fleshed out), but I don't think that's something that should be merged here, as
@stephentoub
stephentoub
merged commit 50d4ecb
into
dotnet:mainMay 15, 2023
@gfoidl
gfoidl
deleted the

memoryextensions_count

branch
May 16, 2023 09:32
@EgorBoEgorBo
mentioned this pull request
May 18, 2023

Closed

@kunalspathakkunalspathak
mentioned this pull request
May 18, 2023

Closed

@ghostghost
locked as resolved and limited conversation to collaborators
Jun 15, 2023
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3 participants

@gfoidl@stephentoub@vcsjones