[PATCH] [X86] Use sys::getHostCPUFeatures to improve -march=native

[Eric Christopher]

I think this should be ok. It'll turn some "hidden" features that would
have been "target-cpu" features into "target-features" features explicitly,
but that'll just mean wordier attribute descriptions (and will be more
accurate).

-eric

On Sun, Mar 29, 2015 at 9:16 PM Craig Topper <craig.topper at gmail.com> wrote:

> Hi chandlerc, echristo, rafael, grosbach,
>
> Some versions of Sandybridge, Ivybridge, Haswell, and Broadwell CPUs don't
> support AVX. Currently, getHostCPUName changes the CPU name to Nehalem when
> it detects one of the CPUs. This also ends up disabling other features that
> these CPUs might still have as well as changing the scheduling model, etc.
>
> This patch attempts to correct this by allowing the CPU name to work
> unaltered and then use getHostCPUFeatures to explicitly specify which
> features are enabled and disabled.
>
> Similar patch will also be made to llc.
>
> http://reviews.llvm.org/D8694
>
> Files:
>   lib/Driver/Tools.cpp
>
> Index: lib/Driver/Tools.cpp
> ===================================================================
> --- lib/Driver/Tools.cpp
> +++ lib/Driver/Tools.cpp
> @@ -1566,6 +1566,17 @@
>  static void getX86TargetFeatures(const Driver &D, const llvm::Triple
> &Triple,
>                                   const ArgList &Args,
>                                   std::vector<const char *> &Features) {
> +  // If -march=native, autodetect the feature list.
> +  if (const Arg *A = Args.getLastArg(options::OPT_march_EQ)) {
> +    if (StringRef(A->getValue()) == "native") {
> +      llvm::StringMap<bool> HostFeatures;
> +      if (llvm::sys::getHostCPUFeatures(HostFeatures))
> +        for (auto &F : HostFeatures)
> +          Features.push_back(Args.MakeArgString((F.second ? "+" : "-") +
> +                                                F.first()));
> +    }
> +  }
> +
>    if (Triple.getArchName() == "x86_64h") {
>      // x86_64h implies quite a few of the more modern subtarget features
>      // for Haswell class CPUs, but not all of them. Opt-out of a few.
>
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