[PATCH] D148710: [DAGCombiner] Limit 'hoist add/sub binop w/ constant op' to the same divergency property
chenglin.bi via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Apr 20 05:26:32 PDT 2023
bcl5980 added inline comments.
================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:3874
// Hoist one-use addition by non-opaque constant:
- // (x + C) - y -> (x - y) + C
- if (N0.getOpcode() == ISD::ADD && N0.hasOneUse() &&
- isConstantOrConstantVector(N0.getOperand(1), /*NoOpaques=*/true)) {
- SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), N1);
- return DAG.getNode(ISD::ADD, DL, VT, Sub, N0.getOperand(1));
- }
- // y - (x + C) -> (y - x) - C
- if (N1.getOpcode() == ISD::ADD && N1.hasOneUse() &&
- isConstantOrConstantVector(N1.getOperand(1), /*NoOpaques=*/true)) {
- SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0, N1.getOperand(0));
- return DAG.getNode(ISD::SUB, DL, VT, Sub, N1.getOperand(1));
- }
- // (x - C) - y -> (x - y) - C
- // This is necessary because SUB(X,C) -> ADD(X,-C) doesn't work for vectors.
- if (N0.getOpcode() == ISD::SUB && N0.hasOneUse() &&
- isConstantOrConstantVector(N0.getOperand(1), /*NoOpaques=*/true)) {
- SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), N1);
- return DAG.getNode(ISD::SUB, DL, VT, Sub, N0.getOperand(1));
- }
- // (C - x) - y -> C - (x + y)
- if (N0.getOpcode() == ISD::SUB && N0.hasOneUse() &&
- isConstantOrConstantVector(N0.getOperand(0), /*NoOpaques=*/true)) {
- SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(1), N1);
- return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), Add);
+ if (N0->isDivergent() == N1->isDivergent()) {
+ // (x + C) - y -> (x - y) + C
----------------
foad wrote:
> bcl5980 wrote:
> > foad wrote:
> > > We also want to do this if x is divergent and y is uniform.
> > If x is divergent and y is uniform, we can transform to
> > (x + C) - y --> x + (C - y)
> > y - (x + C) --> (y - C) - x
> > (x - C) -y --> x - (C + y) ;; this is speical case that AMDGPU doesnt' support op1 as sgpr, so this case we needn't optimize it. But this should be a TLI behaivor. Generally I think we still can't enable it.
> > (C - x) - y --> (C -y) -x
> >
> > So I believe in DAGCombiner we only need to enable it when the divergency property is the same.
> >
> >
> The rules we implement in DAGCombiner should make sense without having to understand exactly what any particular target can do.
>
> The original rule here was "pull constants out of nested adds/subs".
>
> The new rule is "pull constants out of nested adds/subs, unless that increases the number of divergent nodes in the dag".
Yeah, the rule is correct. But it means (N0->isDivergent() == N1->isDivergent()) not (N0->isDivergent() || !N1->isDivergent()) I think.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D148710/new/
https://reviews.llvm.org/D148710
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