[DebugInfo][InstrRef] Avoid crash when values optimised out late in sdag

It appears that we can emit all the instructions for a function, including
debug instructions, and then optimise some of the values out late.
Specifically, in the attached test case, an argument gets optimised out
after DBG_VALUE / DBG_INSTR_REFs are created. This confuses
MachineFunction::finalizeDebugInstrRefs, which expects to be able to find a
defining instruction, and crashes instead.

Fix this by identifying when there's no defining instruction, and
translating that instead into a DBG_VALUE $noreg.

Differential Revision: https://reviews.llvm.org/D114476

1 files changed, 6 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index 28c645d..310c272 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -1170,9 +1170,10 @@ auto MachineFunction::salvageCopySSA(MachineInstr &MI)
 void MachineFunction::finalizeDebugInstrRefs() {
   auto *TII = getSubtarget().getInstrInfo();
 
-  auto MakeDbgValue = [&](MachineInstr &MI) {
+  auto MakeUndefDbgValue = [&](MachineInstr &MI) {
     const MCInstrDesc &RefII = TII->get(TargetOpcode::DBG_VALUE);
     MI.setDesc(RefII);
+    MI.getOperand(0).setReg(0);
     MI.getOperand(1).ChangeToRegister(0, false);
   };
 
@@ -1187,15 +1188,15 @@ void MachineFunction::finalizeDebugInstrRefs() {
       Register Reg = MI.getOperand(0).getReg();
 
       // Some vregs can be deleted as redundant in the meantime. Mark those
-      // as DBG_VALUE $noreg.
-      if (Reg == 0) {
-        MakeDbgValue(MI);
+      // as DBG_VALUE $noreg. Additionally, some normal instructions are
+      // quickly deleted, leaving dangling references to vregs with no def.
+      if (Reg == 0 || !RegInfo->hasOneDef(Reg)) {
+        MakeUndefDbgValue(MI);
         continue;
       }
 
       assert(Reg.isVirtual());
       MachineInstr &DefMI = *RegInfo->def_instr_begin(Reg);
-      assert(RegInfo->hasOneDef(Reg));
 
       // If we've found a copy-like instruction, follow it back to the
       // instruction that defines the source value, see salvageCopySSA docs