Vector<T>.SquareRoot no longer vectorizing in .NET 10.0

[alexcovington]

Description

The codegen for Vector<T>.SquareRoot in .NET 10.0 is no longer producing vectorized code like it did in .NET 9.0. This can be reproduced with this microbenchmark.

Regression?

Yes, this is a regression going from .NET 9.0 to .NET 10.0.

Data

.NET 9.0

BenchmarkDotNet v0.14.1-nightly.20250107.205, Windows 11 (10.0.26100.4946)
.NET SDK 10.0.100-preview.7.25380.108
 [Host] : .NET 9.0.8 (9.0.825.36511), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
 Job-DCSEPH : .NET 9.0.8 (9.0.825.36511), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

PowerPlanMode=00000000-0000-0000-0000-000000000000 IterationTime=250ms MaxIterationCount=20
MinIterationCount=15 WarmupCount=1

| Type | Method | Mean | Error | StdDev | Median | Min | Max | Allocated |
|---------------------- |-------------------- |----------:|----------:|----------:|----------:|----------:|----------:|----------:|
| Perf_VectorOf<Double> | SquareRootBenchmark | 0.6996 ns | 0.0113 ns | 0.0106 ns | 0.6984 ns | 0.6827 ns | 0.7205 ns | - |
| Perf_VectorOf<Single> | SquareRootBenchmark | 0.0445 ns | 0.0119 ns | 0.0111 ns | 0.0449 ns | 0.0257 ns | 0.0605 ns | - |

.NET 10.0

BenchmarkDotNet v0.14.1-nightly.20250107.205, Windows 11 (10.0.26100.4946)
.NET SDK 10.0.100-preview.7.25380.108
 [Host] : .NET 10.0.0 (10.0.25.38108), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
 Job-CCVXYZ : .NET 10.0.0 (10.0.25.38108), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

PowerPlanMode=00000000-0000-0000-0000-000000000000 IterationTime=250ms MaxIterationCount=20
MinIterationCount=15 WarmupCount=1

| Type | Method | Mean | Error | StdDev | Median | Min | Max | Allocated |
|---------------------- |-------------------- |---------:|----------:|----------:|---------:|---------:|---------:|----------:|
| Perf_VectorOf<Double> | SquareRootBenchmark | 5.737 ns | 0.0415 ns | 0.0389 ns | 5.735 ns | 5.678 ns | 5.798 ns | - |
| Perf_VectorOf<Single> | SquareRootBenchmark | 6.508 ns | 0.0496 ns | 0.0440 ns | 6.502 ns | 6.448 ns | 6.586 ns | - |

Analysis

The issue is related to codegen.

.NET 9.0 is able to produce vectorized codegen for Vector<T>.SquareRoot, but .NET 10.0 does not vectorize the same code.

Here is the codegen for Vector<Single>.SquareRoot as an example:

.NET 9.0

 vsqrtps ymm0,[7FF816FAB740]
 vmovups [rdx],ymm0
 mov rax,rdx
 vzeroupper
 ret

.NET 10.0

 sub rsp,58
 vbroadcastss ymm0,dword ptr [7FF7D9EDB4A8]
 vmovups [rsp],ymm0
 vsqrtss xmm0,xmm0,dword ptr [rsp]
 vmovss dword ptr [rsp+20],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+4]
 vmovss dword ptr [rsp+24],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+8]
 vmovss dword ptr [rsp+28],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+0C]
 vmovss dword ptr [rsp+2C],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+10]
 vmovss dword ptr [rsp+30],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+14]
 vmovss dword ptr [rsp+34],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+18]
 vmovss dword ptr [rsp+38],xmm0
 vsqrtss xmm0,xmm0,dword ptr [rsp+1C]
 vmovss dword ptr [rsp+3C],xmm0
 vmovups ymm0,[rsp+20]
 vmovups [rdx],ymm0
 mov rax,rdx
 vzeroupper
 add rsp,58
 ret

I think this can be resolved by checking the method name during import similar to these intrinsics to make sure that Vector<T>.SquareRoot gets generated as the appropriate NI_Vector128_Sqrt/NI_Vector256_Sqrt/NI_Vector512_Sqrt intrinsic.

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[tannergooding]

This is indeed due to the naming mismatch. We should fix this and backport it to .NET 10 RC1