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[NFC][InstCombine] visitUnreachableInst(): enhance comments somewhat

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Roman Lebedev 2021-07-02 17:28:33 +03:00
parent 33ff8078ff
commit 13e35ac124
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1 changed files with 11 additions and 1 deletions
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12
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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@ -2875,8 +2875,18 @@ Instruction *InstCombinerImpl::visitUnreachableInst(UnreachableInst &I) {
// This includes instructions like stores and "llvm.assume" that may not get // This includes instructions like stores and "llvm.assume" that may not get
// removed by simple dead code elimination. // removed by simple dead code elimination.
while (Instruction *Prev = I.getPrevNonDebugInstruction()) { while (Instruction *Prev = I.getPrevNonDebugInstruction()) {
if (Prev->isEHPad() || !isGuaranteedToTransferExecutionToSuccessor(Prev)) // While we theoretically can erase EH, that would result in a block that
// used to start with an EH no longer starting with EH, which is invalid.
// To make it valid, we'd need to fixup predecessors to no longer refer to
// this block, but that changes CFG, which is not allowed in InstCombine.
if (Prev->isEHPad())
return nullptr; // Can not drop any more instructions. We're done here. return nullptr; // Can not drop any more instructions. We're done here.
if (!isGuaranteedToTransferExecutionToSuccessor(Prev))
return nullptr; // Can not drop any more instructions. We're done here.
// Otherwise, this instruction can be freely erased,
// even if it is not side-effect free.
// Temporarily disable removal of volatile stores preceding unreachable, // Temporarily disable removal of volatile stores preceding unreachable,
// pending a potential LangRef change permitting volatile stores to trap. // pending a potential LangRef change permitting volatile stores to trap.
// TODO: Either remove this code, or properly integrate the check into // TODO: Either remove this code, or properly integrate the check into