1 files changed, 47 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86FixupBWInsts.cpp b/llvm/lib/Target/X86/X86FixupBWInsts.cpp
index 045a4ed8dca..24d78b0b776 100644
--- a/llvm/lib/Target/X86/X86FixupBWInsts.cpp
+++ b/llvm/lib/Target/X86/X86FixupBWInsts.cpp
@@ -93,6 +93,11 @@ class FixupBWInstPass : public MachineFunctionPass {
   /// OK, otherwise return nullptr.
   MachineInstr *tryReplaceLoad(unsigned New32BitOpcode, MachineInstr *MI) const;
 
+  /// Change the MachineInstr \p MI into the equivalent 32-bit copy if it is
+  /// safe to do so.  Return the replacement instruction if OK, otherwise return
+  /// nullptr.
+  MachineInstr *tryReplaceCopy(MachineInstr *MI) const;
+
 public:
   FixupBWInstPass() : MachineFunctionPass(ID) {}
 
@@ -219,6 +224,39 @@ MachineInstr *FixupBWInstPass::tryReplaceLoad(unsigned New32BitOpcode,
   return MIB;
 }
 
+MachineInstr *FixupBWInstPass::tryReplaceCopy(MachineInstr *MI) const {
+  assert(MI->getNumExplicitOperands() == 2);
+  auto &OldDest = MI->getOperand(0);
+  auto &OldSrc = MI->getOperand(1);
+
+  unsigned NewDestReg;
+  if (!getSuperRegDestIfDead(MI, NewDestReg))
+    return nullptr;
+
+  unsigned NewSrcReg = getX86SubSuperRegister(OldSrc.getReg(), 32);
+
+  // This is only correct if we access the same subregister index: otherwise,
+  // we could try to replace "movb %ah, %al" with "movl %eax, %eax".
+  auto *TRI = &TII->getRegisterInfo();
+  if (TRI->getSubRegIndex(NewSrcReg, OldSrc.getReg()) !=
+      TRI->getSubRegIndex(NewDestReg, OldDest.getReg()))
+    return nullptr;
+
+  // Safe to change the instruction.
+  // Don't set src flags, as we don't know if we're also killing the superreg.
+  MachineInstrBuilder MIB =
+      BuildMI(*MF, MI->getDebugLoc(), TII->get(X86::MOV32rr), NewDestReg)
+          .addReg(NewSrcReg);
+
+#ifndef NDEBUG
+  // Make sure we didn't drop any other implicit operand.
+  for (auto &Op : MI->implicit_operands())
+    assert(Op.getReg() == NewDestReg && "Copy imp-defs unrelated reg?");
+#endif
+
+  return MIB;
+}
+
 void FixupBWInstPass::processBasicBlock(MachineFunction &MF,
                                         MachineBasicBlock &MBB) {
 
@@ -265,6 +303,15 @@ void FixupBWInstPass::processBasicBlock(MachineFunction &MF,
       NewMI = tryReplaceLoad(X86::MOVZX32rm16, MI);
       break;
 
+    case X86::MOV8rr:
+    case X86::MOV16rr:
+      // Always try to replace 8/16 bit copies with a 32 bit copy.
+      // Code size is either less (16) or equal (8), and there is sometimes a
+      // perf advantage from eliminating a false dependence on the upper portion
+      // of the register.
+      NewMI = tryReplaceCopy(MI);
+      break;
+
     default:
       // nothing to do here.
       break;