2069190 - Configure tests for e.g. -mavxvnni don't guarantee the flag actually works

Status: RESOLVED FIXED

(Firefox Build System :: General, defect)

Description

[Mike Hommey [:glandium]]

There's a gap between compiler support for those flags and them actually working... with GCC. Because GCC produces assembly and calls GAS to generate the machine code, actual support for the corresponding instructions depend on what GAS supports, and the GCC and GAS versions are not necessarily lined up properly for that.
GCC does disable support at build time when it detects GAS doesn't support the flags at GCC build time, but the GAS used at GCC build time and Firefox build time can be different. And thus we can end up in a situation where configure says we can use -mavxvnni, but the build fails because GAS doesn't understand the operands produced by GCC.

Comment 1

[Mike Hommey [:glandium]]

Attachment: Bug 2069190 - Check target feature flags enabling instruction sets actually work.

There's a gap between compiler support for those flags and them actually
working... with GCC. Because GCC produces assembly and calls GAS to
generate the machine code, actual support for the corresponding
instructions depend on what GAS supports, and the GCC and GAS versions
are not necessarily lined up properly for that.

GCC does disable support at build time when it detects GAS doesn't
support the flags at GCC build time, but the GAS used at GCC build time
and Firefox build time can be different. And thus we can end up in a
situation where configure says we can use -mavxvnni, but the build fails
because GAS doesn't understand the operands produced by GCC.

Comment 2

[Mike Hommey [:glandium]]

Attachment: Bug 2069190 - Don't build neon i8mm code when the corresponding compiler flag is not supported.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/da3f96042f38
https://hg.mozilla.org/integration/autoland/rev/740716cdb8dd
Check target feature flags enabling instruction sets actually work. r=firefox-build-system-reviewers,sergesanspaille
https://github.com/mozilla-firefox/firefox/commit/38922f5e93d5
https://hg.mozilla.org/integration/autoland/rev/7d97c162e9be
Don't build neon i8mm code when the corresponding compiler flag is not supported. r=firefox-build-system-reviewers,sergesanspaille

Comment 4

[amarc]

https://hg.mozilla.org/mozilla-central/rev/740716cdb8dd
https://hg.mozilla.org/mozilla-central/rev/7d97c162e9be