[llvm] [ValueTracking] Add `matchSimpleBinaryIntrinsicRecurrence` helper (PR #145964)

[Nikita Popov via llvm-commits]

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@@ -965,6 +966,21 @@ LLVM_ABI bool matchSimpleRecurrence(const PHINode *P, BinaryOperator *&BO,
 LLVM_ABI bool matchSimpleRecurrence(const BinaryOperator *I, PHINode *&P,
                                     Value *&Start, Value *&Step);
 
+/// Attempt to match a simple value-accumulating recurrence of the form:
+///   %llvm.intrinsic.acc = phi Ty [%Init, %Entry], [%llvm.intrinsic, %backedge]
+///   %llvm.intrinsic = call Ty @llvm.intrinsic(%Invariant, %llvm.intrinsic.acc)
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nikic wrote:

Don't call this Invariant, here and elsewhere. This matcher does not actually guarantee it's invariant (just like the one above doesn't guarantee Step is invariant).

The pattern you want to match has an invariant there, but a typical reduction pattern wouldn#t.

https://github.com/llvm/llvm-project/pull/145964