[clang-cl] Enable -march

[aganea]

Summary

Currently, -march does not work in the clang-cl driver.

We are currently migrating from MSVC. The project size makes that we have to stick with clang-cl for a little while. However, we don't want to loose optimisation oportunities in the runtime code, and the MSVC flag /arch only supports a limited set of options.

[hans]

All flags in the m_x86_Features_Group, i.e. -msse, -mavx and so on are all supported. I'm just curious to hear what it is that you need from -march?

[aganea]

@hans I want to target precisely a given CPU when using clang-cl. The options in the m_x86_Features_Group are too granular. It is easier to just add -march=skylake on the cmd-line. Is there any other way?

[hans]

Okay, that sounds good to me.

Please also add a test for this in test/Driver/cl-options.c in the "Accept "core" clang options" section.

[aganea]

Updated diff with coverage test as requested.

[hans]

lgtm

[aganea]

Thank you!

[aganea]

@hans Just an after thought: maybe we should prevent usage of -march= and /arch: at the same time. What do you think? I can add another patch for that purpose.

[hans]

In D51806#1228893, @aganea wrote:

@hans Just an after thought: maybe we should prevent usage of -march= and /arch: at the same time. What do you think? I can add another patch for that purpose.

Hmm, yes, at least we should warn or do something smart. Currently it doesn't look like they'd interact well together in x86::getX86TargetCPU

[aganea]

Yes, indeed. The code in clang/lib/Driver/ToolChains/Arch/X86.cpp makes that –march is always parsed, leaving out /arch unused, no matter in which order they appear.

De : Nico Weber <[e-mail]>
Envoyé : 10 septembre 2018 12:14
À : [e-mail]; Senthil Kumar Selvaraj via Phabricator <[e-mail]>
Cc : Alexandre Ganea <[e-mail]>; cfe-commits <[e-mail]>
Objet : Re: [PATCH] D51806: [clang-cl] Enable -march

include/clang/Driver/Options.td

 def mno_pure_code : Flag<["-"], "mno-pure-code">, Alias<mno_execute_only>;
 def mtvos_version_min_EQ : Joined<["-"], "mtvos-version-min=">, Group<m_Group>;
 def mappletvos_version_min_EQ : Joined<["-"], "mappletvos-version-min=">, Alias<mtvos_version_min_EQ>;
 def mtvos_simulator_version_min_EQ : Joined<["-"], "mtvos-simulator-version-min=">;
 def mappletvsimulator_version_min_EQ : Joined<["-"], "mappletvsimulator-version-min=">, Alias<mtvos_simulator_version_min_EQ>;
 def mwatchos_version_min_EQ : Joined<["-"], "mwatchos-version-min=">, Group<m_Group>;
 def mwatchos_simulator_version_min_EQ : Joined<["-"], "mwatchos-simulator-version-min=">;
 def mwatchsimulator_version_min_EQ : Joined<["-"], "mwatchsimulator-version-min=">, Alias<mwatchos_simulator_version_min_EQ>;
-def march_EQ : Joined<["-"], "march=">, Group<m_Group>;
+def march_EQ : Joined<["-"], "march=">, Group<m_Group>, Flags<[CoreOption]>;
 def masm_EQ : Joined<["-"], "masm=">, Group<m_Group>, Flags<[DriverOption]>;
 def mcmodel_EQ : Joined<["-"], "mcmodel=">, Group<m_Group>;
 def mimplicit_it_EQ : Joined<["-"], "mimplicit-it=">, Group<m_Group>;
 def mdefault_build_attributes : Joined<["-"], "mdefault-build-attributes">, Group<m_Group>;
 def mno_default_build_attributes : Joined<["-"], "mno-default-build-attributes">, Group<m_Group>;
 def mconstant_cfstrings : Flag<["-"], "mconstant-cfstrings">, Group<clang_ignored_m_Group>;
 def mconsole : Joined<["-"], "mconsole">, Group<m_Group>, Flags<[DriverOption]>;
 def mwindows : Joined<["-"], "mwindows">, Group<m_Group>, Flags<[DriverOption]>;

test/Driver/cl-options.c

 // RUN:     -resource-dir=asdf \
 // RUN:     -Wunused-variable \
 // RUN:     -fmacro-backtrace-limit=0 \
 // RUN:     -fstandalone-debug \
 // RUN:     -flimit-debug-info \
 // RUN:     -flto \
 // RUN:     -fmerge-all-constants \
 // RUN:     -no-canonical-prefixes \
+// RUN:     -march=skylake \
 // RUN:     --version \
 // RUN:     -Werror /Zs -- %s 2>&1
 
 
 void f() { }