[llvm] 858d1df - [DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignBits failures. NFC. (#154111)

[via llvm-commits]

Author: Simon Pilgrim
Date: 2025-08-18T14:55:09+01:00
New Revision: 858d1dfa2c4823422c8c6b0459130954cf89fb73

URL: https://github.com/llvm/llvm-project/commit/858d1dfa2c4823422c8c6b0459130954cf89fb73
DIFF: https://github.com/llvm/llvm-project/commit/858d1dfa2c4823422c8c6b0459130954cf89fb73.diff

LOG: [DAG] visitTRUNCATE - early out from computeKnownBits/ComputeNumSignBits failures. NFC. (#154111)

Avoid unnecessary (costly) computeKnownBits/ComputeNumSignBits calls - use MaskedValueIsZero instead of computeKnownBits directly to simplify code.

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 785245b2d9e74..43d4138df8b49 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(B, UpperBits) &&
+                  DAG.MaskedValueIsZero(A, UpperBits);
       } else {
-        unsigned SignBitsA = DAG.ComputeNumSignBits(A);
-        unsigned SignBitsB = DAG.ComputeNumSignBits(B);
-        CanFold = SignBitsA > NeededBits && SignBitsB > NeededBits;
+        unsigned NeededBits = SrcBits - TruncBits;
+        CanFold = DAG.ComputeNumSignBits(B) > NeededBits &&
+                  DAG.ComputeNumSignBits(A) > NeededBits;
       }
 
       if (CanFold) {