[X86] combineGatherScatter - use FoldConstantArithmetic for truncation of constant build vectors

[RKSimon]

No need to explicitly check with BuildVectorSDNode::isConstant - FoldConstantArithmetic can handle this, including through bitcasts etc.

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Changes

No need to explicitly check with BuildVectorSDNode::isConstant - FoldConstantArithmetic can handle this, including through bitcasts etc.

---

Full diff: https://github.com/llvm/llvm-project/pull/136033.diff

1 Files Affected:

• (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+4-7)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 3adeb4628eabf..024b653f78cb5 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -56529,16 +56529,13 @@ static SDValue combineGatherScatter(SDNode *N, SelectionDAG &DAG,
     if (IndexWidth > 32 && DAG.ComputeNumSignBits(Index) > (IndexWidth - 32)) {
       EVT NewVT = IndexVT.changeVectorElementType(MVT::i32);
 
-      // FIXME: We could support more than just constant vectors, but we need to
+      // FIXME: We could support more than just constant fold, but we need to
       // careful with costing. A truncate that can be optimized out would be
       // fine. Otherwise we might only want to create a truncate if it avoids a
       // split.
-      if (auto *BV = dyn_cast<BuildVectorSDNode>(Index)) {
-        if (BV->isConstant()) {
-          Index = DAG.getNode(ISD::TRUNCATE, DL, NewVT, Index);
-          return rebuildGatherScatter(GorS, Index, Base, Scale, DAG);
-        }
-      }
+      if (SDValue TruncIndex =
+              DAG.FoldConstantArithmetic(ISD::TRUNCATE, DL, NewVT, Index))
+        return rebuildGatherScatter(GorS, TruncIndex, Base, Scale, DAG);
 
       // Shrink any sign/zero extends from 32 or smaller to larger than 32 if
       // there are sufficient sign bits. Only do this before legalize types to

[llvm-ci]

LLVM Buildbot has detected a new failure on builder arc-builder running on arc-worker while building llvm at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/3/builds/14683

Here is the relevant piece of the build log for the reference

Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
0.825 [60/18/1] Linking CXX executable tools/clang/unittests/Basic/BasicTests
0.888 [59/18/2] Linking CXX executable tools/clang/unittests/Parse/ParseTests
1.662 [58/18/3] Linking CXX executable tools/clang/unittests/Format/FormatTests
5.670 [57/18/4] Linking CXX executable tools/clang/unittests/Lex/LexTests
6.152 [56/18/5] Linking CXX executable tools/clang/unittests/libclang/libclangTests
10.206 [55/18/6] Linking CXX executable tools/clang/unittests/Analysis/ClangAnalysisTests
10.615 [54/18/7] Linking CXX executable tools/clang/unittests/AST/ByteCode/InterpTests
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1211.386719