[llvm] [SimplifyCFG] Treat umul + extract pattern as cheap single instruction (Approach 2) (PR #128021)

[Gábor Spaits via llvm-commits]

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@@ -3810,6 +3815,7 @@ static bool foldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
   // instructions.  While we are at it, keep track of the instructions
   // that need to be moved to the dominating block.
   SmallPtrSet<Instruction *, 4> AggressiveInsts;
+  SmallPtrSet<Instruction *, 2> ZeroCostInstructions;
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spaits wrote:

Even though I would need to carry only one instruction, I have decided to still use a set.
I want this to be somewhat future proof, so if someone adds another pattern, that is 5 instructions long they can just add those instruction to this Set.
Or it even works, when there is one pattern, but multiple times.

https://github.com/llvm/llvm-project/pull/128021