[MC] Add support for -mcpu=native. (llvm#159414)

Support -mcpu=native by querying the Host CPU Name and Feature flags.

---------

Co-authored-by: Cameron McInally <[email protected]>

Author: mcinally

llvm/test/MC/AsmParser/native.s

@@ -0,0 +1,4 @@
+# RUN: llvm-mc -filetype=obj -o %t -mcpu=native %s 2> %t.stderr
+# RUN: FileCheck --allow-empty %s < %t.stderr
+
+# CHECK-NOT: {{.+}}

llvm/tools/llvm-mc/llvm-mc.cpp

@@ -459,14 +459,23 @@ int main(int argc, char **argv) {
   MAI->setCommentColumn(CommentColumn);
 
   // Package up features to be passed to target/subtarget
+  SubtargetFeatures Features;
   std::string FeaturesStr;
-  if (MAttrs.size()) {
-    SubtargetFeatures Features;
-    for (unsigned i = 0; i != MAttrs.size(); ++i)
-      Features.AddFeature(MAttrs[i]);
-    FeaturesStr = Features.getString();
+
+  // Replace -mcpu=native with Host CPU and features.
+  if (MCPU == "native") {
+    MCPU = std::string(llvm::sys::getHostCPUName());
+
+    llvm::StringMap<bool> TargetFeatures = llvm::sys::getHostCPUFeatures();
+    for (auto const &[FeatureName, IsSupported] : TargetFeatures)
+      Features.AddFeature(FeatureName, IsSupported);
   }
 
+  // Handle features passed to target/subtarget.
+  for (unsigned i = 0; i != MAttrs.size(); ++i)
+    Features.AddFeature(MAttrs[i]);
+  FeaturesStr = Features.getString();
+
   std::unique_ptr<MCSubtargetInfo> STI(
       TheTarget->createMCSubtargetInfo(TheTriple, MCPU, FeaturesStr));
   if (!STI) {