[llvm] [InstCombine] Combine interleaved PHI reduction chains. (PR #143878)

[Nikita Popov via llvm-commits]

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@@ -996,6 +997,150 @@ Instruction *InstCombinerImpl::foldPHIArgOpIntoPHI(PHINode &PN) {
   return NewCI;
 }
 
+/// Try to fold reduction ops interleaved through two PHIs to a single PHI.
+///
+/// For example, combine:
+///   %phi1 = phi [init1, %BB1], [%op1, %BB2]
+///   %phi2 = phi [init2, %BB1], [%op2, %BB2]
+///   %op1 = binop %phi1, constant1
+///   %op2 = binop %phi2, constant2
+///   %rdx = binop %op1, %op2
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nikic wrote:

Why does the pattern start matching at one of the phi nodes rather than at %rdx? This is pretty unusual for InstCombine, and it's not immediately obvious to me why it is necessary.

https://github.com/llvm/llvm-project/pull/143878