src: fix build failure for clang 3.2; consolidate byte swapping code; fix buffer writes for unaligned ucs2 strings by zbjornson · Pull Request #7645 · nodejs/node

zbjornson · 2016-07-10T20:01:22Z

Fixes#7618
Alternative to #7644 that preserve performance.

Also consolidates all byte-swapping code (following what bnoordhuis did in #7644).

gibfahn · 2016-07-11T11:12:56Z

Couldn't get this to build on OSX 10.8 and Clang 3.2 (commit cc11a96)

 c++ '-D_DARWIN_USE_64_BIT_INODE=1' '-DNODE_ARCH="x64"' '-DNODE_WANT_INTERNALS=1' '-DV8_DEPRECATION_WARNINGS=1' '-DNODE_USE_V8_PLATFORM=1' '-DNODE_HAVE_I18N_SUPPORT=1' '-DNODE_HAVE_SMALL_ICU=1' '-DHAVE_INSPECTOR=1' '-DV8_INSPECTOR_USE_STL=1' '-DHAVE_OPENSSL=1' '-DHAVE_DTRACE=1' '-D__POSIX__' '-DNODE_PLATFORM="darwin"' '-DUCONFIG_NO_TRANSLITERATION=1' '-DUCONFIG_NO_SERVICE=1' '-DUCONFIG_NO_REGULAR_EXPRESSIONS=1' '-DU_ENABLE_DYLOAD=0' '-DU_STATIC_IMPLEMENTATION=1' '-DU_HAVE_STD_STRING=0' '-DUCONFIG_NO_BREAK_ITERATION=0' '-DUCONFIG_NO_LEGACY_CONVERSION=1' '-DUCONFIG_NO_CONVERSION=1' '-DHTTP_PARSER_STRICT=0' '-D_LARGEFILE_SOURCE' '-D_FILE_OFFSET_BITS=64' -I../src -I../tools/msvs/genfiles -I../deps/uv/src/ares -I/home/gib/node/out/Release/obj/gen -I../deps/v8_inspector -I../deps/v8_inspector/deps/wtf -I/home/gib/node/out/Release/obj/gen/blink -I../deps/v8/include -I../deps/icu-small/source/i18n -I../deps/icu-small/source/common -I../deps/openssl/openssl/include -I../deps/zlib -I../deps/http_parser -I../deps/cares/include -I../deps/uv/include -Os -gdwarf-2 -mmacosx-version-min=10.7 -arch x86_64 -Wall -Wendif-labels -W -Wno-unused-parameter -std=gnu++0x -fno-rtti -fno-exceptions -fno-threadsafe-statics -fno-strict-aliasing -MMD -MF /home/gib/node/out/Release/.deps//home/gib/node/out/Release/obj.target/node/src/async-wrap.o.d.raw -c -o /home/gib/node/out/Release/obj.target/node/src/async-wrap.o ../src/async-wrap.cc
/include -I../deps/uv/include -Os -gdwarf-2 -mmacosx-version-min=10.7 -arch x86_64 -Wall -Wendif-labels -W -Wno-unused-parameter -std=gnu++0x -fno-rtti -fno-exceptions -fno-threadsafe-statics -fno-strict-aliasing -MMD -MF /home/gib/node/out/Release/.deps//home/gib/node/out/Release/obj.target/node/src/handle_wrap.o.d.raw -c -o /home/gib/node/out/Release/obj.target/node/src/handle_wrap.o ../src/handle_wrap.cc
In file included from ../test/cctest/util.cc:2:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/inspector_socket.cc:3:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/env.cc:1:
In file included from ../src/env.h:7:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/cares_wrap.cc:4:
In file included from ../src/async-wrap-inl.h:8:
In file included from ../src/base-object-inl.h:7:
In file included from ../src/env.h:7:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/debug-agent.cc:22:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/fs_event_wrap.cc:2:
In file included from ../src/async-wrap-inl.h:8:
In file included from ../src/base-object-inl.h:7:
In file included from ../src/env.h:7:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/async-wrap.cc:2:
In file included from ../src/async-wrap-inl.h:8:
In file included from ../src/base-object-inl.h:7:
In file included from ../src/env.h:7:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^
In file included from ../src/handle_wrap.cc:3:
In file included from ../src/async-wrap-inl.h:8:
In file included from ../src/base-object-inl.h:7:
In file included from ../src/env.h:7:
In file included from ../src/debug-agent.h:29:
../src/util-inl.h:250:19: error: use of undeclared identifier '__builtin_bswap16'
data16[i] = BSWAP_INTRINSIC_2(data16[i]);
^
../src/util-inl.h:15:30: note: expanded from macro 'BSWAP_INTRINSIC_2'
#define BSWAP_INTRINSIC_2(x) __builtin_bswap16(x)
^

EDIT: I ran make clean && ./configure && make -j8 CXX.host=c++. I'm happy to help with debugging @zbjornson (assuming I didn't somehow configure the build wrong). The same process worked with #7644 FWIW.

bnoordhuis · 2016-07-11T12:59:23Z

In-place modification of a const buffer is definitely not allowed.

@bnoordhuis Does that mean this isn't possible?

No, I just botched and misread the pointer reassignment in the original code. Will fix. Thanks for catching @bnoordhuis.

zbjornson · 2016-07-11T17:31:15Z

Sorry, turns out clang also defines __GNUC__ -- fixed. Also fixed the const buffer problem. Can you try again please? Thanks for your help @gibfahn.

bnoordhuis · 2016-07-11T17:49:44Z

@zbjornson First off, thanks for taking the time to work on this. I don't think this PR is the way to go but that doesn't mean I don't appreciate you working on it.

Knowing what I know of clang and gcc (and some quick tests confirm that), I don't think it's the use of intrinsics/builtins that is responsible for the performance improvement, it's that the compiler can:

Assume proper alignment, and
Not worry about src and dst aliasing or overlapping.

If I rewrite #7644 to hint the same conditions to the compiler, I get comparable numbers on benchmark/buffers/buffer-swap.js in the aligned case. The compiler even emits the same machine code as when using the builtins.

(I still need to look into the unaligned case. Perhaps there is something we can tweak there as well.)

zbjornson · 2016-07-11T18:04:31Z

Interesting, okay. Keen to try out your reworked version. When I played with similar code, the compilers would only emit PSHUFB and similar with -O3, whereas the intrinsics always mapped correctly.

addaleax · 2016-07-11T18:10:05Z

When I played with similar code, the compilers would only emit PSHUFB and similar with -O3

I’d expect that to noticeably outperform the __bswap* intrinsics, but that instruction wouldn’t be available on all x64 CPUs, so at least the Linux binaries wouldn’t be able to leverage support for it.

zbjornson · 2016-07-11T18:18:18Z

@addaleax err, I meant to say that this type of thing:

char a = data[0];
data[0] = data[1];
data[1] = a;

mapped to PSHUFB only with -O3, whereas the builtins reliably mapped to PSHUFB without -O3 (and afaik node is compiled with -Os and /Od).

addaleax · 2016-07-11T18:28:40Z

@zbjornson Ah – Either way, be aware that, without CPU detection or extra compiler flags, pshufb is something that won’t end up in Linux release builds, so I’d be careful with performance measurements on Mac (I’m assuming from the above that you are using a Mac).

and afaik node is compiled with -Os and /Od

Pretty sure it’s -O3 by default:

node/common.gypi

Line 103 in fcae5e2

'cflags': [ '-O3' ],

zbjornson · 2016-07-11T18:43:57Z

@addaleax ahh I'd been trying to find that info about whether or not AVX/SSE extensions will be used in release for quite some time, thanks! If that's the case, then it makes sense to use the smaller code from #7644. (I'd been benchmarking on Windows.)

You're right on -O3. (The first line in #7645 (comment) has -Os for some reason, but a normal release build shows -O3.)

gibfahn · 2016-07-12T11:42:35Z

@zbjornson This PR is now building cleanly on my 10.8 machine (d6147aef8e2e9511149cba5809e330fd318e6e38)

jasnell · 2016-08-08T19:16:51Z

@addaleax ... any further thoughts on this one?

zbjornson · 2016-08-08T20:02:54Z

Note that I want to replace the std::swap calls with the char a = data[0]; data[0] = data[1]; data[1] = a; dance because it's faster. I delayed making that change because I wasn't sure if this would get merged.

I think that because this is substantially faster on at least Windows, and potentially on custom builds on other platforms (to be tested), is a good reason to pursue this PR. I don't see any downsides at least.

I can make the above change and do more benchmarking this week.

gibfahn · 2016-09-22T09:23:31Z

@bnoordhuis So does this updated PR seem better to you?

ref: #7618 (comment)

bnoordhuis · 2016-09-22T10:23:26Z

Nit: no space before (.

bnoordhuis · 2016-09-22T10:24:45Z

Consider writing it as sizeof(dst[0]).

bnoordhuis · 2016-09-22T10:27:26Z

Is this actually faster than std::swap or an inline function?

bnoordhuis · 2016-09-22T10:29:14Z

This is a strict-aliasing violation, strictly speaking. My PR operates on just char for that reason.

zbjornson · 2016-09-22T20:58:19Z

Thanks for your review @bnoordhuis and again sorry that this small thing has dragged on. Revisions submitted.

As far as the strict aliasing violation you pointed out:

I admit that this confused me. When I've asked about reading from char buffers in particular, I get answers along the lines of "it's not UB or a SA violation," "it looks like one but it isn't because you assume that the original pointer is a uint16_t" or "it is but everyone does it" (which seems to be the most correct of the three) (see ref1, ref2 (and later replies)).

Note that the violation already existed in string_bytes.cc and this PR just relocates it:
https://github.com/nodejs/node/pull/7645/files#diff-aab3e751fbd702712b90a419b21b58aeL324

As far as avoiding the violation:

Your method (memcpy) compiles the same as a reinterpret_cast by GCC for -Os, -O1 and -O2, but it prevents loop unrolling at -O3 (node's default) in the case of swap16. That's why there's the 2M/400k difference in the benchmarks. For swap32 and swap64 they are equivalent.
MSVC doesn't "have" global strict aliasing (i.e. it behaves like -fno-strict-aliasing), so while it's not spec-compliant there is no UB to worry about as far as I know; we've already checked alignment and are don't care about endianness. Using the memcpy method kills perf.

Thus, in the latest revision, I use reinterpret_cast on Windows and eliminated for the rest. What do you think about that?

That yields these benchmarks, which are about as good as they get (with node's default build config) aside from aligned 16 on linux (per above):

bnoordhuis · 2016-09-23T06:42:49Z

"it is but everyone does it" (which seems to be the most correct of the three)

It is and that is why node.js is currently built with -fno-strict-aliasing. Still, I'd prefer to avoid aliasing if reasonably possible.

"it looks like one but it isn't because you assume that the original pointer is a uint16_t"

That is not what the spec says and not how gcc and clang operate when -fstrict-aliasing is in effect. The rule is unambiguous: no pointer can alias another pointer unless the alias is of type char*.

Think of it like this: when strict aliasing is in effect, and when there are no char* pointers in scope, the compiler can assume that values it is trying to read or write through a pointer will not change underneath it.

With -fno-strict-aliasing, gcc and clang conservatively assume that every pointer is being aliased somewhere unless it is trivial to prove otherwise; it's infeasible to track aliasing program-wide.

(Incidentally, that is why the return values of malloc, calloc and operator new are marked as noalias. They logically can't alias other pointers but if the compiler didn't know that, it would have to assume the worst and emit significantly worse code around dynamic allocations. Apparently there are systems that violate that assumption because clang++ has a -fno-sane-operator-new switch, but I digress.)

bnoordhuis · 2016-09-23T06:52:44Z

Can you write this as sizeof(*data16)? EDIT: Here and in the other functions.

bnoordhuis · 2016-09-23T06:53:01Z

Can you use sizeof(temp) in this block and the blocks below? LGTM apart from that.

zbjornson · 2016-09-25T18:45:01Z

Reviewers: fyi, the one change since Ben gave his LGTM was moving dst up a scope (out of if (IsBigEndian()) {}, back where it was originally. (And adding the test in the 2nd commit.)

addaleax · 2016-09-25T20:17:10Z

LGTM, thanks for @mentioning me!

bnoordhuis

Still LGTM but the commit logs should conform to the style guide.

Did you check what code paths clang and gcc emit? In the other PR, I had to prove that src == dst and had proper alignment before the compiler generated the fast case.

zbjornson · 2016-09-26T07:03:03Z

Fixed commit text.

Did you check what code paths clang and gcc emit?

Surprisingly it doesn't appear necessary to give those hints to the compilers in this incarnation for clang or gcc to emit bswap and nothing detrimental around it (https://godbolt.org/g/DxMNmF). The linux benchmarks from this PR match the BMs for #7644 as well (see #7645 (comment)).

gibfahn · 2016-09-26T14:40:31Z

CI: https://ci.nodejs.org/job/node-test-pull-request/4286/

zbjornson · 2016-09-26T16:12:04Z

Is this CI situation normal or am I cursed?!

addaleax · 2016-09-26T16:17:55Z

Ahem, yes, let’s give this another shot: https://ci.nodejs.org/job/node-test-commit/5328/

(It is, unfortunately, normal and the act of complaining about the general brokenness of CI is part of what forms the common core collaborator identity. Welcome to the club! :b)

jasnell

LGTM

zbjornson · 2016-09-30T18:28:22Z

More CI attempts needed to land?

lpinca · 2016-09-30T18:34:58Z

@zbjornson started new CI run: https://ci.nodejs.org/job/node-test-pull-request/4337/

lpinca · 2016-09-30T18:38:51Z

Oh didn't notice that there is a conflict.

Removes use of builtins that are unavailable for older clang. Per benchmarks, only uses builtins on Windows, where speedup is significant. Fixes: nodejs#7618

Between nodejs#3410 and nodejs#7645, bytes were swapped twice on bigendian platforms if buffer was not two-byte aligned. See comment in nodejs#7645.

zbjornson · 2016-09-30T18:47:14Z

@lpinca rebased

lpinca · 2016-09-30T18:50:39Z

Restarted: https://ci.nodejs.org/job/node-test-pull-request/4339/

gibfahn · 2016-10-02T14:12:08Z

Another CI: https://ci.nodejs.org/job/node-test-pull-request/4355/

zbjornson · 2016-10-03T03:38:31Z

🎉 7th time's a charm I guess :)

gibfahn · 2016-10-04T07:55:07Z

Okay, I'm going to start landing this.

no objections
3 LGTMs
Lots of CI

Removes use of builtins that are unavailable for older clang. Per benchmarks, only uses builtins on Windows, where speedup is significant. Also adds test for unaligned ucs2 buffer write. Between #3410 and #7645, bytes were swapped twice on bigendian platforms if buffer was not two-byte aligned. See comment in #7645. PR-URL: #7645Fixes: #7618 Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl> Reviewed-By: James M Snell <jasnell@gmail.com>

gibfahn · 2016-10-04T08:30:12Z

landed in 7420835 , thanks a lot @zbjornson !

gibfahn · 2016-10-04T08:38:47Z

Concerning backporting, I guess this should be backported to wherever #7157 was backported, which is v6 but not v4. If anyone disagrees let me know.

Removes use of builtins that are unavailable for older clang. Per benchmarks, only uses builtins on Windows, where speedup is significant. Also adds test for unaligned ucs2 buffer write. Between #3410 and #7645, bytes were swapped twice on bigendian platforms if buffer was not two-byte aligned. See comment in #7645. PR-URL: #7645Fixes: #7618 Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl> Reviewed-By: James M Snell <jasnell@gmail.com>

Removes use of builtins that are unavailable for older clang. Per benchmarks, only uses builtins on Windows, where speedup is significant. Also adds test for unaligned ucs2 buffer write. Between #3410 and #7645, bytes were swapped twice on bigendian platforms if buffer was not two-byte aligned. See comment in #7645. PR-URL: #7645Fixes: #7618 Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl> Reviewed-By: James M Snell <jasnell@gmail.com> Conflicts: src/node_buffer.cc

nodejs-github-bot added the c++ Issues and PRs that require attention from people who are familiar with C++. label Jul 10, 2016

zbjornson mentioned this pull request Jul 10, 2016
osx, build: Node v6.3.0 won't build on Clang 3.2 (OSX 10.8 Darwin 12.5.0) #7618
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addaleax added the lib / src Issues and PRs related to general changes in the lib or src directory. label Jul 10, 2016

zbjornson mentioned this pull request Jul 10, 2016
src: don't use __builtin_bswap16() and friends #7644
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mscdex added buffer Issues and PRs related to the buffer subsystem. and removed lib / src Issues and PRs related to general changes in the lib or src directory. labels Jul 10, 2016

bnoordhuis reviewed Jul 11, 2016
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zbjornson changed the title ~~src: fix build failure for clang 3.2 by checking for builtin presence~~src: fix build failure for clang 3.2; consolidate byte swapping codeSep 22, 2016

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zbjornson force-pushed the 7618-old-clang-support branch from c8281e3 to 65cbc45Compare September 30, 2016 18:43

zbjornson added 2 commits September 30, 2016 11:46

src: fix build for older clang
62ba6c8
Removes use of builtins that are unavailable for older clang. Per benchmarks, only uses builtins on Windows, where speedup is significant. Fixes: nodejs#7618

test: add test for unaligned ucs2 buffer write
2b69933
Between nodejs#3410 and nodejs#7645, bytes were swapped twice on bigendian platforms if buffer was not two-byte aligned. See comment in nodejs#7645.

zbjornson force-pushed the 7618-old-clang-support branch from 65cbc45 to 2b69933Compare September 30, 2016 18:46

gibfahn closed this Oct 4, 2016

addaleax added the dont-land-on-v4.x label Oct 5, 2016

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