Enable RegisterScavenging for X86

Only enable register scavenging if we will need it to handle huge
frames.

Author: wesleywiser

llvm/lib/CodeGen/PrologEpilogInserter.cpp

@@ -1545,7 +1545,7 @@ void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
       // If this instruction has a FrameIndex operand, we need to
       // use that target machine register info object to eliminate
       // it.
-      TRI.eliminateFrameIndex(MI, SPAdj, i);
+      TRI.eliminateFrameIndex(MI, SPAdj, i, RS);
 
       // Reset the iterator if we were at the beginning of the BB.
       if (AtBeginning) {

llvm/lib/Target/X86/X86RegisterInfo.h

@@ -13,6 +13,8 @@
 #ifndef LLVM_LIB_TARGET_X86_X86REGISTERINFO_H
 #define LLVM_LIB_TARGET_X86_X86REGISTERINFO_H
 
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 
 #define GET_REGINFO_HEADER
@@ -176,6 +178,13 @@ class X86RegisterInfo final : public X86GenRegisterInfo {
                              SmallVectorImpl<MCPhysReg> &Hints,
                              const MachineFunction &MF, const VirtRegMap *VRM,
                              const LiveRegMatrix *Matrix) const override;
+
+  bool requiresRegisterScavenging(const MachineFunction &MF) const override {
+    const MachineFrameInfo &MFI = MF.getFrameInfo();
+
+    // We need to register scavenge if the frame is very large.
+    return Is64Bit && !isInt<32>(MFI.estimateStackSize(MF));
+  }
 };
 
 } // End llvm namespace