Handle <undef> operands in TwoAddressInstructionPass.

When the source register to a 2-addr instruction is undefined, there is
no need to attempt any transformations - simply replace the source
register with the destination register.

This also comes up when lowering IMPLICIT_DEF instructions - make sure
the <undef> flag is moved to the new partial register def operand:

  %vreg8<def> = INSERT_SUBREG %vreg9<undef>, %vreg0<kill>, sub_16bit
rewrite undef:
  %vreg8<def> = INSERT_SUBREG %vreg8<undef>, %vreg0<kill>, sub_16bit
convert to:
  %vreg8:sub_16bit<def,read-undef> = COPY %vreg0<kill>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159120 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 31 insertions, 12 deletions

diff --git a/lib/CodeGen/TwoAddressInstructionPass.cpp b/lib/CodeGen/TwoAddressInstructionPass.cpp
index ec2b577..c87ab64 100644
--- a/lib/CodeGen/TwoAddressInstructionPass.cpp
+++ b/lib/CodeGen/TwoAddressInstructionPass.cpp
@@ -1456,6 +1456,19 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
                "two address instruction invalid");
 
         unsigned regB = mi->getOperand(SrcIdx).getReg();
+
+        // Deal with <undef> uses immediately - simply rewrite the src operand.
+        if (mi->getOperand(SrcIdx).isUndef()) {
+          unsigned DstReg = mi->getOperand(DstIdx).getReg();
+          // Constrain the DstReg register class if required.
+          if (TargetRegisterInfo::isVirtualRegister(DstReg))
+            if (const TargetRegisterClass *RC = TII->getRegClass(MCID, SrcIdx,
+                                                                 TRI, MF))
+              MRI->constrainRegClass(DstReg, RC);
+          mi->getOperand(SrcIdx).setReg(DstReg);
+          DEBUG(dbgs() << "\t\trewrite undef:\t" << *mi);
+          continue;
+        }
         TiedOperands[regB].push_back(std::make_pair(SrcIdx, DstIdx));
       }
 
@@ -1609,19 +1622,20 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
         MadeChange = true;
 
         DEBUG(dbgs() << "\t\trewrite to:\t" << *mi);
+      }
 
-        // Rewrite INSERT_SUBREG as COPY now that we no longer need SSA form.
-        if (mi->isInsertSubreg()) {
-          // From %reg = INSERT_SUBREG %reg, %subreg, subidx
-          // To   %reg:subidx = COPY %subreg
-          unsigned SubIdx = mi->getOperand(3).getImm();
-          mi->RemoveOperand(3);
-          assert(mi->getOperand(0).getSubReg() == 0 && "Unexpected subreg idx");
-          mi->getOperand(0).setSubReg(SubIdx);
-          mi->RemoveOperand(1);
-          mi->setDesc(TII->get(TargetOpcode::COPY));
-          DEBUG(dbgs() << "\t\tconvert to:\t" << *mi);
-        }
+      // Rewrite INSERT_SUBREG as COPY now that we no longer need SSA form.
+      if (mi->isInsertSubreg()) {
+        // From %reg = INSERT_SUBREG %reg, %subreg, subidx
+        // To   %reg:subidx = COPY %subreg
+        unsigned SubIdx = mi->getOperand(3).getImm();
+        mi->RemoveOperand(3);
+        assert(mi->getOperand(0).getSubReg() == 0 && "Unexpected subreg idx");
+        mi->getOperand(0).setSubReg(SubIdx);
+        mi->getOperand(0).setIsUndef(mi->getOperand(1).isUndef());
+        mi->RemoveOperand(1);
+        mi->setDesc(TII->get(TargetOpcode::COPY));
+        DEBUG(dbgs() << "\t\tconvert to:\t" << *mi);
       }
 
       // Clear TiedOperands here instead of at the top of the loop
@@ -1846,6 +1860,11 @@ bool TwoAddressInstructionPass::EliminateRegSequences() {
     SmallVector<unsigned, 4> RealSrcs;
     SmallSet<unsigned, 4> Seen;
     for (unsigned i = 1, e = MI->getNumOperands(); i < e; i += 2) {
+      // Nothing needs to be inserted for <undef> operands.
+      if (MI->getOperand(i).isUndef()) {
+        MI->getOperand(i).setReg(0);
+        continue;
+      }
       unsigned SrcReg = MI->getOperand(i).getReg();
       unsigned SrcSubIdx = MI->getOperand(i).getSubReg();
       unsigned SubIdx = MI->getOperand(i+1).getImm();