[PATCH] D105339: [InstCombine] visitUnreachableInst(): iteratively erase instructions leading to unreachable

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.
lebedev.ri requested review of this revision.

In the original review D87149 <https://reviews.llvm.org/D87149> it was mentioned that this approach was tried,
and it lead to infinite combine loops, but i'm not seeing anything like that now,
neither in the `check-llvm`, nor on some codebases i tried.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D105339

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp


Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2874,23 +2874,24 @@
   // Try to remove the previous instruction if it must lead to unreachable.
   // This includes instructions like stores and "llvm.assume" that may not get
   // removed by simple dead code elimination.
-  Instruction *Prev = I.getPrevNonDebugInstruction();
-  if (Prev && !Prev->isEHPad() &&
-      isGuaranteedToTransferExecutionToSuccessor(Prev)) {
+  while (Instruction *Prev = I.getPrevNonDebugInstruction()) {
+    if (Prev->isEHPad() || !isGuaranteedToTransferExecutionToSuccessor(Prev))
+      return nullptr; // Can not drop any more instructions. We're done here.
     // Temporarily disable removal of volatile stores preceding unreachable,
     // pending a potential LangRef change permitting volatile stores to trap.
     // TODO: Either remove this code, or properly integrate the check into
     // isGuaranteedToTransferExecutionToSuccessor().
     if (auto *SI = dyn_cast<StoreInst>(Prev))
       if (SI->isVolatile())
-        return nullptr;
+        return nullptr; // Can not drop this instruction. We're done here.
 
     // A value may still have uses before we process it here (for example, in
     // another unreachable block), so convert those to poison.
     replaceInstUsesWith(*Prev, PoisonValue::get(Prev->getType()));
     eraseInstFromFunction(*Prev);
-    return &I;
   }
+  assert(I.getParent()->sizeWithoutDebug() == 1 && "The block is now empty.");
+  // FIXME: recurse into unconditional predecessors?
   return nullptr;
 }
 


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