[llvm] e80da0f - [RISCV] Disable combineBinOpToReduce if the reduction AVL might be 0.

[Craig Topper via llvm-commits]

Author: Craig Topper
Date: 2023-04-25T18:46:54-07:00
New Revision: e80da0f8409aaf70d18b12c5d5d884f2c9cbddcc

URL: https://github.com/llvm/llvm-project/commit/e80da0f8409aaf70d18b12c5d5d884f2c9cbddcc
DIFF: https://github.com/llvm/llvm-project/commit/e80da0f8409aaf70d18b12c5d5d884f2c9cbddcc.diff

LOG: [RISCV] Disable combineBinOpToReduce if the reduction AVL might be 0.

If the reduction AVL is 0, operand 0 of the reduction will be
returned rather than the scalar input.

To make the fold legal, we would need to fold the new scalar value
with whatever operand 0 is which may require a new scalar operation
before the reduction.

Block the combine if we can't prove AVL is non-zero.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D149174

Added: 
    

Modified: 
    llvm/lib/Target/RISCV/RISCVISelLowering.cpp

Removed: 
    


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diff  --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 6d2b442b0c2a6..22bca318c897b 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -9170,6 +9170,11 @@ static SDValue combineBinOpToReduce(SDNode *N, SelectionDAG &DAG,
   if (!ScalarV.hasOneUse())
     return SDValue();
 
+  // If the AVL is zero, operand 0 will be returned. So it's not safe to fold.
+  // FIXME: We might be able to improve this if operand 0 is undef.
+  if (!isNonZeroAVL(Reduce.getOperand(5)))
+    return SDValue();
+
   SDValue NewStart = N->getOperand(1 - ReduceIdx);
 
   SDLoc DL(N);