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

[craig.topper]

Summary

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.

[• rafael]

This is fine by me, but I am not sure if I should be the one to LGTM it.

Does this make passing -target-cpu for native redundant?

[craig.topper]

The target-cpu is still used for LLVM "features" that don't have a real CPUID bit. For instance, slow-bt-mem and slow-incdec. It would also still be used to pick the scheduling model.

[chandlerc]

Looks good, go for it.

[echristo]

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

[craig.topper]

r233672

lib/Driver/Tools.cpp

static void AddGoldPlugin(const ToolChain &ToolChain, const ArgList &Args,
   std::string CPU = getCPUName(Args, ToolChain.getTriple());
   if (!CPU.empty())
     CmdArgs.push_back(Args.MakeArgString(Twine("-plugin-opt=mcpu=") + CPU));
 }
 
 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.
     Features.push_back("-rdrnd");
     Features.push_back("-aes");
     Features.push_back("-pclmul");
     Features.push_back("-rtm");
     Features.push_back("-hle");