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

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In the original review 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.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D105339
This commit is contained in:
Roman Lebedev 2021-07-02 17:16:33 +03:00
parent 9a7d1a946c
commit e7b7a69e07
1 changed files with 6 additions and 5 deletions
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11
lib/Transforms/InstCombine/InstructionCombining.cpp
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@ -2874,23 +2874,24 @@ Instruction *InstCombinerImpl::visitUnreachableInst(UnreachableInst &I) {
// 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;
}