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[DAG] visitFREEZE - generalize freeze(op()) -> op(freeze()) to any number of operands 

canCreateUndefOrPoison currently only handles unary ops, but we intend to change that soon - this more closely matches the pushFreezeToPreventPoisonFromPropagating behaviour where the freeze is pushed up to a single operand value, as long as all others are guaranteed not to be poison/undef.

However, pushFreezeToPreventPoisonFromPropagating would freeze all uses of the value - whilst this variant requires the frozen value to be only used in the op - we can look at generalize multiple uses later if the need arises.
This commit is contained in:
Simon Pilgrim 2022-08-10 13:12:04 +01:00
parent 1f02ad7131
commit 8623da5f74
1 changed files with 34 additions and 7 deletions
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41
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -13868,13 +13868,40 @@ SDValue DAGCombiner::visitFREEZE(SDNode *N) {
if (DAG.isGuaranteedNotToBeUndefOrPoison(N0, /*PoisonOnly*/ false))
return N0;
// Fold freeze(unaryop(x)) -> unaryop(freeze(x)).
// TODO: Replace with pushFreezeToPreventPoisonFromPropagating fold and
// support getNumOperands() >= 1.
if (N0.getNumOperands() == 1 &&
!DAG.canCreateUndefOrPoison(N0, /*PoisonOnly*/ false) && N0->hasOneUse())
return DAG.getNode(N0.getOpcode(), SDLoc(N0), N->getValueType(0),
DAG.getFreeze(N0.getOperand(0)));
// Fold freeze(op(x,y,z)) -> op(freeze(x),y,z).
// Try to push freeze through instructions that propagate but don't produce
// poison as far as possible. If an operand of freeze follows three
// conditions 1) one-use, 2) does not produce poison, and 3) has all but one
// guaranteed-non-poison operands then push the freeze through to the one
// operand that is not guaranteed non-poison.
if (!DAG.canCreateUndefOrPoison(N0, /*PoisonOnly*/ false) &&
N0->getNumValues() == 1 && N0->hasOneUse()) {
SDValue MaybePoisonOperand;
for (SDValue Op : N0->ops()) {
if (DAG.isGuaranteedNotToBeUndefOrPoison(Op, /*PoisonOnly*/ false))
continue;
if ((!MaybePoisonOperand && N0->isOnlyUserOf(Op.getNode())) ||
MaybePoisonOperand == Op) {
MaybePoisonOperand = Op;
continue;
}
// Multiple maybe-poison ops - bail out.
MaybePoisonOperand = SDValue();
break;
}
if (MaybePoisonOperand) {
// Recreate the node with the frozen maybe-poison operand.
// TODO: Drop the isOnlyUserOf constraint and replace all users of
// MaybePoisonOperand with FrozenMaybePoisonOperand
// to match pushFreezeToPreventPoisonFromPropagating behavior.
SDValue FrozenMaybePoisonOperand = DAG.getFreeze(MaybePoisonOperand);
SmallVector<SDValue> Ops(N0->op_begin(), N0->op_end());
for (SDValue &Op : Ops)
if (Op == MaybePoisonOperand)
Op = FrozenMaybePoisonOperand;
return DAG.getNode(N0.getOpcode(), SDLoc(N0), N0->getVTList(), Ops);
}
}
return SDValue();
}