Re-enable elimination of unnecessary SUBREG_TO_REG instructions in

LowerSubregs, and fix an x86-64 isel bug that this exposed.

SUBREG_TO_REG for x86-64 implicit zero extension is only safe for
isel to generate when the source is known to always have zeros in
the high 32 bits. The EXTRACT_SUBREG instruction does not clear
the high 32 bits.

llvm-svn: 54444

1 files changed, 9 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/LowerSubregs.cpp b/llvm/lib/CodeGen/LowerSubregs.cpp
index 1cdd34b77f4..30894ad6ad7 100644
--- a/llvm/lib/CodeGen/LowerSubregs.cpp
+++ b/llvm/lib/CodeGen/LowerSubregs.cpp
@@ -108,15 +108,20 @@ bool LowerSubregsInstructionPass::LowerSubregToReg(MachineInstr *MI) {
 
   DOUT << "subreg: CONVERTING: " << *MI;
 
-  // Insert sub-register copy
-  const TargetRegisterClass *TRC0= TRI.getPhysicalRegisterRegClass(DstSubReg);
-  const TargetRegisterClass *TRC1= TRI.getPhysicalRegisterRegClass(InsReg);
-  TII.copyRegToReg(*MBB, MI, DstSubReg, InsReg, TRC0, TRC1);
+  if (DstSubReg == InsReg) {
+    // No need to insert an identify copy instruction.
+    DOUT << "subreg: eliminated!";
+  } else {
+    // Insert sub-register copy
+    const TargetRegisterClass *TRC0= TRI.getPhysicalRegisterRegClass(DstSubReg);
+    const TargetRegisterClass *TRC1= TRI.getPhysicalRegisterRegClass(InsReg);
+    TII.copyRegToReg(*MBB, MI, DstSubReg, InsReg, TRC0, TRC1);
 
 #ifndef NDEBUG
   MachineBasicBlock::iterator dMI = MI;
   DOUT << "subreg: " << *(--dMI);
 #endif
+  }
 
   DOUT << "\n";
   MBB->erase(MI);