Subzero: Improve register availability peephole for function return values.

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index fcfe6ed8114f5630c838ee5f9c4e9bcbe6d8b072..b28c00944af1149af8191627cae5a3e7079dd687 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -2688,7 +2688,7 @@ void TargetX86Base<TraitsType>::lowerCall(const InstCall *Instr) {
   // Mark the call as killing all the caller-save registers.
   Context.insert<InstFakeKill>(NewCall);
   // Handle x86-32 floating point returns.
-  if (Dest != nullptr && isScalarFloatingType(Dest->getType()) &&
+  if (Dest != nullptr && isScalarFloatingType(DestTy) &&
       !Traits::X86_PASS_SCALAR_FP_IN_XMM) {
     // Special treatment for an FP function which returns its result in st(0).
     // If Dest ends up being a physical xmm register, the fstp emit code will
@@ -2706,14 +2706,19 @@ void TargetX86Base<TraitsType>::lowerCall(const InstCall *Instr) {
   // Process the return value, if any.
   if (Dest == nullptr)
     return;
-  // Assign the result of the call to Dest.
+  // Assign the result of the call to Dest.  Route it through a temporary so
+  // that the local register availability peephole can be subsequently used.
+  Variable *Tmp = nullptr;
   if (isVectorType(DestTy)) {
     assert(ReturnReg && "Vector type requires a return register");
-    _movp(Dest, ReturnReg);
+    Tmp = makeReg(DestTy);
+    _movp(Tmp, ReturnReg);
+    _movp(Dest, Tmp);
   } else if (isScalarFloatingType(DestTy)) {
     if (Traits::X86_PASS_SCALAR_FP_IN_XMM) {
       assert(ReturnReg && "FP type requires a return register");
-      _mov(Dest, ReturnReg);
+      _mov(Tmp, ReturnReg);
+      _mov(Dest, Tmp);
     }
   } else {
     assert(isScalarIntegerType(DestTy));
@@ -2723,10 +2728,14 @@ void TargetX86Base<TraitsType>::lowerCall(const InstCall *Instr) {
       auto *Dest64On32 = llvm::cast<Variable64On32>(Dest);
       Variable *DestLo = Dest64On32->getLo();
       Variable *DestHi = Dest64On32->getHi();
-      _mov(DestLo, ReturnReg);
-      _mov(DestHi, ReturnRegHi);
+      _mov(Tmp, ReturnReg);
+      _mov(DestLo, Tmp);
+      Variable *TmpHi = nullptr;

[Eric Holk, 2016/06/15 00:14:38]

Why is it okay for TmpHi to be NULL?

[Jim Stichnoth, 2016/06/15 00:35:31]

The _mov() method is "special" in that if the dest arg is nullptr it
automatically creates a new temporary of the appropriate type via makeReg(). 
The same is being done for "Variable *Tmp = nullptr;" above.

+      _mov(TmpHi, ReturnRegHi);
+      _mov(DestHi, TmpHi);
     } else {
-      _mov(Dest, ReturnReg);
+      _mov(Tmp, ReturnReg);
+      _mov(Dest, Tmp);
     }
   }
 }