[NVPTX] Use correct mul.wide operand type when matching on shl in combineMulWide

[justinfargnoli]

The operand types of a mul.wide are half the size of the output type. combineMulWide incorrectly uses the output type of the mul.wide for the operand type.

[Copilot]

Pull Request Overview

This PR fixes a type mismatch in the NVPTX backend's combineMulWide function. The mul.wide instruction requires operands that are half the size of the output type, but the code was incorrectly using the output type (ToVT) instead of the operand type (FromVT) when creating constant operands for shift-to-multiply transformations.

Key Changes:

• Corrected the bit width used when creating APInt constants from ToVT.getSizeInBits() to FromVT.getSizeInBits()
• Updated the type parameter in getConstant() from ToVT to FromVT

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[llvmbot]

@llvm/pr-subscribers-backend-nvptx

Author: Justin Fargnoli (justinfargnoli)

Changes

The operand types of a mul.wide are half the size of the output type. combineMulWide incorrectly uses the output type of the mul.wide for the operand type.

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Full diff: https://github.com/llvm/llvm-project/pull/168986.diff

1 Files Affected:

• (modified) llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp (+2-2)

diff --git a/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp b/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
index a77eb0240e677..b843b8344d15c 100644
--- a/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
@@ -5695,8 +5695,8 @@ static SDValue combineMulWide(SDNode *N, TargetLowering::DAGCombinerInfo &DCI,
   SDValue RHS = Op.getOperand(1);
   if (Op.getOpcode() == ISD::SHL) {
     const auto ShiftAmt = Op.getConstantOperandVal(1);
-    const auto MulVal = APInt(ToVT.getSizeInBits(), 1) << ShiftAmt;
-    RHS = DCI.DAG.getConstant(MulVal, DL, ToVT);
+    const auto MulVal = APInt(FromVT.getSizeInBits(), 1) << ShiftAmt;
+    RHS = DCI.DAG.getConstant(MulVal, DL, FromVT);
   }
   return DCI.DAG.getNode(Opcode, DL, ToVT, Op.getOperand(0), RHS);
 }

[justinfargnoli]

I don't have a test case that exposes this bug. I discovered this while prototyping another change. Since I'm not sure when that prototype will be ready for review, or whether it makes sense to land at all, I wanted to put up a fix and expose that this bug exists.

[github-actions]

🐧 Linux x64 Test Results

• 186431 tests passed
• 4864 tests skipped

[AlexMaclean]

LGTM

[Artem-B]

It would be great to add a test that would catch such a mismatch.