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Keep phis contiguous after inverting a phi's incoming values #3126

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Keep phis contiguous after inverting a phi's incoming values #3126

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18 changes: 18 additions & 0 deletions enzyme/Enzyme/GradientUtils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6725,6 +6725,24 @@ Value *GradientUtils::invertPointerM(Value *const oval, IRBuilder<> &BuilderM,
which->addIncoming(val, cast<BasicBlock>(getNewFromOriginal(
phi->getIncomingBlock(i))));
}
// Inverting an incoming value can emit instructions into this block
// above `which` (e.g. materializing the shadow of a value defined in
// the header), which leaves the phis no longer contiguous at the block
// start and produces IR the verifier rejects. Restore the invariant.
{
BasicBlock *PB = which->getParent();
SmallVector<PHINode *, 4> phis;
for (auto &I : *PB)
if (auto *P = dyn_cast<PHINode>(&I))
phis.push_back(P);
Instruction *at = &*PB->begin();
for (auto *P : phis) {
if (P != at)
P->moveBefore(at);
else
at = P->getNextNode();
}
}
return which;
}
}
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