[llvm] [AggressiveInstCombine] Fold i64 x i64 -> i128 multiply-by-parts (PR #156879)

[David Green via llvm-commits]

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@@ -1457,6 +1457,268 @@ static bool foldLibCalls(Instruction &I, TargetTransformInfo &TTI,
   return false;
 }
 
+/// Match low part of 128-bit multiplication.
+///
+/// Use counts are checked to prevent total instruction count increase as per
+/// contributors guide:
+/// https://llvm.org/docs/InstCombineContributorGuide.html#multi-use-handling
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davemgreen wrote:

I don't think we need links to the ContributorGuide here. It might be useful to explain what it is matching instead.

https://github.com/llvm/llvm-project/pull/156879