[llvm] [LoopVectorizer][AArch64] Add support for partial reduce subtraction (PR #123636)

[David Sherwood via llvm-commits]

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@@ -8800,6 +8800,10 @@ bool VPRecipeBuilder::getScaledReductions(
     return false;
 
   using namespace llvm::PatternMatch;
+  // Use the side-effect of match to replace BinOp only if the pattern is
+  // matched, we don't care at this point whether it actually matched.
+  match(BinOp, m_Neg(m_BinOp(BinOp)));
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david-arm wrote:

I don't understand how this works. We've already bailed out above if `BinOp` is not a BinaryOperator, which surely means that it cannot simultaneously be a unary operator, which was m_Neg represents?

https://github.com/llvm/llvm-project/pull/123636