[DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignBits failures. NFC. #154111
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…its failures. NFC. Avoid unnecessary (costly) computeKnownBits/ComputeNumSignBits calls - use MaskedValueIsZero instead of computeKnownBits directly to simplify code.
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@llvm/pr-subscribers-llvm-selectiondag Author: Simon Pilgrim (RKSimon) ChangesAvoid unnecessary (costly) computeKnownBits/ComputeNumSignBits calls - use MaskedValueIsZero instead of computeKnownBits directly to simplify code. Full diff: https://github.com/llvm/llvm-project/pull/154111.diff 1 Files Affected:
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 785245b2d9e74..0e63f051b1d65 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16332,25 +16332,22 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
// (trunc (abdu/abds a, b)) -> (abdu/abds (trunc a), (trunc b))
if ((!LegalOperations || N0.hasOneUse()) &&
TLI.isOperationLegal(N0.getOpcode(), VT)) {
- EVT SrcVT = N0.getValueType();
EVT TruncVT = VT;
unsigned SrcBits = SrcVT.getScalarSizeInBits();
unsigned TruncBits = TruncVT.getScalarSizeInBits();
- unsigned NeededBits = SrcBits - TruncBits;
SDValue A = N0.getOperand(0);
SDValue B = N0.getOperand(1);
bool CanFold = false;
if (N0.getOpcode() == ISD::ABDU) {
- KnownBits KnownA = DAG.computeKnownBits(A);
- KnownBits KnownB = DAG.computeKnownBits(B);
- CanFold = KnownA.countMinLeadingZeros() >= NeededBits &&
- KnownB.countMinLeadingZeros() >= NeededBits;
+ APInt UpperBits = APInt::getBitsSetFrom(SrcBits, TruncBits);
+ CanFold = DAG.MaskedValueIsZero(A, UpperBits) &&
+ DAG.MaskedValueIsZero(B, UpperBits);
} else {
- unsigned SignBitsA = DAG.ComputeNumSignBits(A);
- unsigned SignBitsB = DAG.ComputeNumSignBits(B);
- CanFold = SignBitsA > NeededBits && SignBitsB > NeededBits;
+ unsigned NeededBits = SrcBits - TruncBits;
+ CanFold = DAG.ComputeNumSignBits(A) > NeededBits &&
+ DAG.ComputeNumSignBits(B) > NeededBits;
}
if (CanFold) {
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arsenm
reviewed
Aug 18, 2025
Comment on lines
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| CanFold = DAG.MaskedValueIsZero(A, UpperBits) && | ||
| DAG.MaskedValueIsZero(B, UpperBits); |
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Suggested change
| CanFold = DAG.MaskedValueIsZero(A, UpperBits) && | |
| DAG.MaskedValueIsZero(B, UpperBits); | |
| CanFold = DAG.MaskedValueIsZero(B, UpperBits) && | |
| DAG.MaskedValueIsZero(A, UpperBits); |
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why to check the second oprand for ABDU/ABDS firstly, in my understadning, the second is smaller, the possibility of MaskedValueIsZero will be bigger.
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for commutable instructions we move constants to the RHS, in the middle-end the node is often canonicalized to the RHS being the simpler in other ways as well but we don't tend to bother as much in DAG.
Comment on lines
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| CanFold = DAG.ComputeNumSignBits(A) > NeededBits && | ||
| DAG.ComputeNumSignBits(B) > NeededBits; |
There was a problem hiding this comment.
Suggested change
| CanFold = DAG.ComputeNumSignBits(A) > NeededBits && | |
| DAG.ComputeNumSignBits(B) > NeededBits; | |
| CanFold = DAG.ComputeNumSignBits(B) > NeededBits && | |
| DAG.ComputeNumSignBits(A) > NeededBits; |
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Avoid unnecessary (costly) computeKnownBits/ComputeNumSignBits calls - use MaskedValueIsZero instead of computeKnownBits directly to simplify code.