Avoid creating dangling physreg live ranges during DCE.

LiveRangeEdit::eliminateDeadDefs() can delete a dead instruction that
reads unreserved physregs. This would leave the corresponding regunit
live interval dangling because we don't have shrinkToUses() for physical
registers.

Fix this problem by turning the instruction into a KILL instead of
deleting it. This happens in a landing pad in
test/CodeGen/X86/2012-05-19-CoalescerCrash.ll:

  %vreg27<def,dead> = COPY %EDX<kill>; GR32:%vreg27

becomes:

  KILL %EDX<kill>

An upcoming fix to the machine verifier will catch problems like this by
verifying regunit live intervals.

This fixes PR13498. I am not including the test case from the PR since
we already have one exposing the problem once the verifier is fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161182 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 30 insertions, 6 deletions

diff --git a/lib/CodeGen/LiveRangeEdit.cpp b/lib/CodeGen/LiveRangeEdit.cpp
index 896fdbfbe2..b4ce9aa8c1 100644
--- a/lib/CodeGen/LiveRangeEdit.cpp
+++ b/lib/CodeGen/LiveRangeEdit.cpp
@@ -239,6 +239,7 @@ void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr*> &Dead,
 
       // Collect virtual registers to be erased after MI is gone.
       SmallVector<unsigned, 8> RegsToErase;
+      bool ReadsPhysRegs = false;
 
       // Check for live intervals that may shrink
       for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
@@ -246,8 +247,12 @@ void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr*> &Dead,
         if (!MOI->isReg())
           continue;
         unsigned Reg = MOI->getReg();
-        if (!TargetRegisterInfo::isVirtualRegister(Reg))
+        if (!TargetRegisterInfo::isVirtualRegister(Reg)) {
+          // Check if MI reads any unreserved physregs.
+          if (Reg && MOI->readsReg() && !LIS.isReserved(Reg))
+            ReadsPhysRegs = true;
           continue;
+        }
         LiveInterval &LI = LIS.getInterval(Reg);
 
         // Shrink read registers, unless it is likely to be expensive and
@@ -271,11 +276,30 @@ void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr*> &Dead,
         }
       }
 
-      if (TheDelegate)
-        TheDelegate->LRE_WillEraseInstruction(MI);
-      LIS.RemoveMachineInstrFromMaps(MI);
-      MI->eraseFromParent();
-      ++NumDCEDeleted;
+      // Currently, we don't support DCE of physreg live ranges. If MI reads
+      // any unreserved physregs, don't erase the instruction, but turn it into
+      // a KILL instead. This way, the physreg live ranges don't end up
+      // dangling.
+      // FIXME: It would be better to have something like shrinkToUses() for
+      // physregs. That could potentially enable more DCE and it would free up
+      // the physreg. It would not happen often, though.
+      if (ReadsPhysRegs) {
+        MI->setDesc(TII.get(TargetOpcode::KILL));
+        // Remove all operands that aren't physregs.
+        for (unsigned i = MI->getNumOperands(); i; --i) {
+          const MachineOperand &MO = MI->getOperand(i-1);
+          if (MO.isReg() && TargetRegisterInfo::isPhysicalRegister(MO.getReg()))
+            continue;
+          MI->RemoveOperand(i-1);
+        }
+        DEBUG(dbgs() << "Converted physregs to:\t" << *MI);
+      } else {
+        if (TheDelegate)
+          TheDelegate->LRE_WillEraseInstruction(MI);
+        LIS.RemoveMachineInstrFromMaps(MI);
+        MI->eraseFromParent();
+        ++NumDCEDeleted;
+      }
 
       // Erase any virtregs that are now empty and unused. There may be <undef>
       // uses around. Keep the empty live range in that case.