Get rid of a pseudo instruction and replace it with subreg based operation on real instructions, ridding the asm printers of the hack used to do this previously. In the process, update LowerSubregs to be careful about eliminating copies that have side affects.

Note: the coalescer will have to be careful about this too, when it starts coalescing insert_subreg nodes.
llvm-svn: 48329

1 files changed, 14 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/LowerSubregs.cpp b/llvm/lib/CodeGen/LowerSubregs.cpp
index 531713e47fb..8945dd57b18 100644
--- a/llvm/lib/CodeGen/LowerSubregs.cpp
+++ b/llvm/lib/CodeGen/LowerSubregs.cpp
@@ -96,13 +96,10 @@ bool LowerSubregsInstructionPass::LowerInsert(MachineInstr *MI) {
          (MI->getOperand(2).isRegister() && MI->getOperand(2).isUse()) &&
           MI->getOperand(3).isImmediate() && "Invalid insert_subreg");
           
+  // Check if we're inserting into an implicit undef value.
+  bool isImplicit = MI->getOperand(1).isImmediate();
   unsigned DstReg = MI->getOperand(0).getReg();
-  unsigned SrcReg = 0;
-  // Check if we're inserting into an implicit value.
-  if (MI->getOperand(1).isImmediate())
-    SrcReg = DstReg;
-  else
-    SrcReg = MI->getOperand(1).getReg();
+  unsigned SrcReg = isImplicit ? DstReg : MI->getOperand(1).getReg();
   unsigned InsReg = MI->getOperand(2).getReg();
   unsigned SubIdx = MI->getOperand(3).getImm();     
 
@@ -118,13 +115,20 @@ bool LowerSubregsInstructionPass::LowerInsert(MachineInstr *MI) {
 
   DOUT << "subreg: CONVERTING: " << *MI;
        
+  // Check whether the implict subreg copy has side affects or not. Only copies
+  // into an undef value have no side affects, that is they can be eliminated
+  // without changing the semantics of the program.
+  bool copyHasSideAffects = isImplicit? 
+                  MI->getOperand(1).getImm() != TargetInstrInfo::IMPL_VAL_UNDEF
+                  : false; 
+       
   // If the inserted register is already allocated into a subregister
   // of the destination, we copy the subreg into the source
   // However, this is only safe if the insert instruction is the kill
   // of the source register
   bool revCopyOrder = TRI.isSubRegister(DstReg, InsReg);
-  if (revCopyOrder && InsReg != DstSubReg) {
-    if (MI->getOperand(1).isKill()) {
+  if (revCopyOrder && (InsReg != DstSubReg || copyHasSideAffects)) {
+    if (isImplicit || MI->getOperand(1).isKill()) {
       DstSubReg = TRI.getSubReg(SrcReg, SubIdx);
       // Insert sub-register copy
       const TargetRegisterClass *TRC1 = 0;
@@ -144,7 +148,7 @@ bool LowerSubregsInstructionPass::LowerInsert(MachineInstr *MI) {
     }
   }
 #ifndef NDEBUG
-  if (InsReg == DstSubReg) {
+  if (InsReg == DstSubReg && !copyHasSideAffects) {
      DOUT << "subreg: Eliminated subreg copy\n";
   }
 #endif
@@ -174,7 +178,7 @@ bool LowerSubregsInstructionPass::LowerInsert(MachineInstr *MI) {
   }
 #endif
 
-  if (!revCopyOrder && InsReg != DstSubReg) {
+  if (!revCopyOrder && (InsReg != DstSubReg || copyHasSideAffects)) {
     // Insert sub-register copy
     const TargetRegisterClass *TRC1 = 0;
     if (TargetRegisterInfo::isPhysicalRegister(InsReg)) {