1298470 - define the correct SSSE3_FLAGS when compiling with clang-cl

Status: RESOLVED FIXED

(Firefox Build System :: General, defect)

Description

[Nathan Froyd [:froydnj]]

clang-cl is more picky than MSVC when you use intrinsics: it only lets you use particular intrinsics if you have specifically enabled the relevant instruction sets on the command line (for the most part).  So when compiling things that expect to be able to use SSSE3, MSVC is happy to let -arch:SSE2 be enough, while clang-cl wants -mssse3.

Comment 1

[Nathan Froyd [:froydnj]]

Attachment: Bug 1298470 - use the correct SSSE3_FLAGS for clang-cl;

MSVC allows one to specify -arch:SSE2 and enable all SSE2+ instructions.
clang-cl is pickier and wants you to specify -mssse3.

Review commit: https://reviewboard.mozilla.org/r/74622/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/74622/

Comment 2

[(no longer active)]

This isn't the right thing to do.  This not only enables the SSSE3 intrinsics, but it *also* tells the clang code generator that it should use the SSSE3 instruction set where possible, which will make the resulting binary assume the availability of the said instruction set.

There's some existing places where in the moz.build file we pass -mssse3 to clang-cl only for those files that need it.  See <https://dxr.mozilla.org/mozilla-central/source/gfx/skia/generate_mozbuild.py#105> for an example.  We should use the same pattern for other code which requires SSSE3 instructions.

Comment 3

[Nathan Froyd [:froydnj]]

(In reply to :Ehsan Akhgari from comment #2)
> This isn't the right thing to do.  This not only enables the SSSE3
> intrinsics, but it *also* tells the clang code generator that it should use
> the SSSE3 instruction set where possible, which will make the resulting
> binary assume the availability of the said instruction set.
> 
> There's some existing places where in the moz.build file we pass -mssse3 to
> clang-cl only for those files that need it.  See
> <https://dxr.mozilla.org/mozilla-central/source/gfx/skia/generate_mozbuild.
> py#105> for an example.  We should use the same pattern for other code which
> requires SSSE3 instructions.

This is basically what SSSE3_FLAGS is for.  We're not enabling the use of SSSE3 globally.

Comment 4

[(no longer active)]

You're right, nevermind.  I misread the patch.  Sorry!

Comment 5

[Mike Hommey [:glandium]]

Comment on attachment 8785421 [details]
Bug 1298470 - use the correct SSSE3_FLAGS for clang-cl;

https://reviewboard.mozilla.org/r/74622/#review73074

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/987bda408af3
use the correct SSSE3_FLAGS for clang-cl; r=glandium

Comment 7

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/987bda408af3