[InstCombine] If inst in unreachable refers to an inst change it to poison (#78444)

Instructions in unreachable basic blocks are removed, but terminators
are not. In this case, even instructions that are only referenced by
a terminator, such as a return instruction, cannot be processed
properly.

This patch changes the operand of a return instruction in an
unreachable basic block to poison if it refers to the instruction,
allowing the instruction to be properly processed.

Fixes #65107.

1 files changed, 20 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index c4a8843..d3bb8907 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -2762,6 +2762,23 @@ bool llvm::replaceAllDbgUsesWith(Instruction &From, Value &To,
   return false;
 }
 
+bool llvm::handleUnreachableTerminator(
+    Instruction *I, SmallVectorImpl<Value *> &PoisonedValues) {
+  bool Changed = false;
+  // RemoveDIs: erase debug-info on this instruction manually.
+  I->dropDbgValues();
+  for (Use &U : I->operands()) {
+    Value *Op = U.get();
+    if (isa<Instruction>(Op) && !Op->getType()->isTokenTy()) {
+      U.set(PoisonValue::get(Op->getType()));
+      PoisonedValues.push_back(Op);
+      Changed = true;
+    }
+  }
+
+  return Changed;
+}
+
 std::pair<unsigned, unsigned>
 llvm::removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB) {
   unsigned NumDeadInst = 0;
@@ -2769,8 +2786,9 @@ llvm::removeAllNonTerminatorAndEHPadInstructions(BasicBlock *BB) {
   // Delete the instructions backwards, as it has a reduced likelihood of
   // having to update as many def-use and use-def chains.
   Instruction *EndInst = BB->getTerminator(); // Last not to be deleted.
-  // RemoveDIs: erasing debug-info must be done manually.
-  EndInst->dropDbgValues();
+  SmallVector<Value *> Uses;
+  handleUnreachableTerminator(EndInst, Uses);
+
   while (EndInst != &BB->front()) {
     // Delete the next to last instruction.
     Instruction *Inst = &*--EndInst->getIterator();