[X86] Move the implicit enabling of sse2 for 64-bit mode from X86Subtarget::initSubtargetFeatures to X86_MC::ParseX86Triple.

ParseX86Triple already checks for 64-bit mode and produces a
static string. We can just add +sse2 to the end of that static
string. This avoids a potential reallocation when appending it
to the std::string at runtime.

This is a slight change to the behavior of tools that only use
MC layer which weren't implicitly enabling sse2 before, but will
now. I don't think we check for sse2 explicitly in any MC layer
components so this shouldn't matter in practice. And if it did
matter the new behavior is more correct.

Author: topperc

llvm/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp

@@ -44,8 +44,10 @@ using namespace llvm;
 
 std::string X86_MC::ParseX86Triple(const Triple &TT) {
   std::string FS;
-  if (TT.getArch() == Triple::x86_64)
-    FS = "+64bit-mode,-32bit-mode,-16bit-mode";
+  // SSE2 should default to enabled in 64-bit mode, but can be turned off
+  // explicitly.
+  if (TT.isArch64Bit())
+    FS = "+64bit-mode,-32bit-mode,-16bit-mode,+sse2";
   else if (TT.getEnvironment() != Triple::CODE16)
     FS = "-64bit-mode,+32bit-mode,-16bit-mode";
   else

llvm/lib/Target/X86/X86Subtarget.cpp

@@ -234,11 +234,6 @@ void X86Subtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   std::string FullFS = X86_MC::ParseX86Triple(TargetTriple);
   assert(!FullFS.empty() && "Failed to parse X86 triple");
 
-  // SSE2 should default to enabled in 64-bit mode, but can be turned off
-  // explicitly.
-  if (TargetTriple.isArch64Bit())
-    FullFS += ",+sse2";
-
   if (!FS.empty())
     FullFS = (Twine(FullFS) + "," + FS).str();