[mips] Support micromips attribute passed by front-end

[atanasyan]

Summary

This patch adds handling of the micromips and nomicromips attributes passed by front-end. The patch depends on D33363.

[sdardis]

LGTM.

[sdardis]

You should also add a test case to test/CodeGen/Mips/ showing that this attribute sets the assembler into micromips mode for some trivial function, along with another function without micromips being generated as non micromips code.

llvm/trunk/lib/Target/Mips/MipsTargetMachine.cpp

MipsTargetMachine::getSubtargetImpl(const Function &F) const {
   std::string FS = !FSAttr.hasAttribute(Attribute::None)
                        ? FSAttr.getValueAsString().str()
                        : TargetFS;
   bool hasMips16Attr =
       !F.getFnAttribute("mips16").hasAttribute(Attribute::None);
   bool hasNoMips16Attr =
       !F.getFnAttribute("nomips16").hasAttribute(Attribute::None);
 
+  bool HasMicroMipsAttr =
+      !F.getFnAttribute("micromips").hasAttribute(Attribute::None);
+  bool HasNoMicroMipsAttr =
+      !F.getFnAttribute("nomicromips").hasAttribute(Attribute::None);
+
   // FIXME: This is related to the code below to reset the target options,
   // we need to know whether or not the soft float flag is set on the
   // function, so we can enable it as a subtarget feature.
   bool softFloat =
       F.hasFnAttribute("use-soft-float") &&
       F.getFnAttribute("use-soft-float").getValueAsString() == "true";
 
   if (hasMips16Attr)
     FS += FS.empty() ? "+mips16" : ",+mips16";
   else if (hasNoMips16Attr)
     FS += FS.empty() ? "-mips16" : ",-mips16";
+  if (HasMicroMipsAttr)
+    FS += FS.empty() ? "+micromips" : ",+micromips";
+  else if (HasNoMicroMipsAttr)
+    FS += FS.empty() ? "-micromips" : ",-micromips";
   if (softFloat)
     FS += FS.empty() ? "+soft-float" : ",+soft-float";
 
   auto &I = SubtargetMap[CPU + FS];
   if (!I) {
     // This needs to be done before we create a new subtarget since any
     // creation will depend on the TM and the code generation flags on the
     // function that reside in TargetOptions.

llvm/trunk/test/CodeGen/Mips/micromips-attr.ll

+; RUN: llc -march=mips -mcpu=mips32 --mattr=-micromips < %s | FileCheck %s 
+
+define void @foo() #0 {
+entry:
+  ret void
+}
+; CHECK:        .set    micromips
+; CHECK-NEXT:   .set    nomips16
+; CHECK-NEXT:   .ent    foo
+; CHECK-NEXT: foo:
+
+define void @bar() #1 {
+entry:
+  ret void
+}
+; CHECK:        .set    nomicromips
+; CHECK-NEXT:   .set    nomips16
+; CHECK-NEXT:   .ent    bar
+; CHECK-NEXT: bar:
+
+attributes #0 = {
+  nounwind "micromips"
+  "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false"
+  "less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
+  "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
+  "no-signed-zeros-fp-math"="false" "no-trapping-math"="false"
+  "stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
+  "use-soft-float"="false"
+}
+
+attributes #1 = {
+  nounwind
+  "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false"
+  "less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
+  "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
+  "no-signed-zeros-fp-math"="false" "no-trapping-math"="false"
+  "stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
+  "use-soft-float"="false"
+}