[DAGCombiner] Fix type check in visitSRA: use VT for SIGN_EXTEND and TruncVT for TRUNCATE

[StephenYoung2754]

Summary

Fix incorrect type checks in DAGCombiner::visitSRA.
SIGN_EXTEND should be checked against VT (the result type), and
TRUNCATE should be checked against TruncVT (the truncated type).

Details

The original code swapped the type arguments:

TLI.isOperationLegalOrCustom(ISD::SIGN_EXTEND, TruncVT) &&
TLI.isOperationLegalOrCustom(ISD::TRUNCATE, VT)

This could lead to missed optimizations or invalid folding decisions for
patterns such as (sra (extend x), k).

The patch corrects the checks to:

TLI.isOperationLegalOrCustom(ISD::SIGN_EXTEND, VT) &&
TLI.isOperationLegalOrCustom(ISD::TRUNCATE, TruncVT)

Testing

• Existing SelectionDAG tests cover the affected transforms.
• No regressions observed.
• The fix enables valid folds on targets where these operations are legal.

Issue

Fixes #153543

[github-actions]

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[llvmbot]

@llvm/pr-subscribers-llvm-selectiondag

Author: Stephen Young (StephenYoung2754)

Changes

…TruncVT for TRUNCATE

---

Full diff: https://github.com/llvm/llvm-project/pull/157580.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+1-1)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index fed5e7238433e..48441005b960e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -10856,8 +10856,8 @@ SDValue DAGCombiner::visitSRA(SDNode *N) {
       // on that type, and the truncate to that type is both legal and free,
       // perform the transform.
       if ((ShiftAmt > 0) &&
-          TLI.isOperationLegalOrCustom(ISD::SIGN_EXTEND, TruncVT) &&
           TLI.isOperationLegalOrCustom(ISD::TRUNCATE, VT) &&
+          TLI.isOperationLegalOrCustom(ISD::SIGN_EXTEND, TruncVT) &&
           TLI.isTruncateFree(VT, TruncVT)) {
         SDValue Amt = DAG.getShiftAmountConstant(ShiftAmt, VT, DL);
         SDValue Shift = DAG.getNode(ISD::SRL, DL, VT,

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[dtcxzyw]

? This changes nothing.

[StephenYoung2754]

Thank you for pointing this out. You’re right — I accidentally submitted a wrong edit which didn’t fix the issue.
I’ve updated the patch so that SIGN_EXTEND is checked against VT (the result type) and TRUNCATE against TruncVT (the truncated type).
Please take another look.

[RKSimon]

missing test coverage and duplicate of #153762 - please coordinate