[clang] [llvm] [InstCombine] Reassociate nested add/sub with constant (PR #191022)

[Max Graey via cfe-commits]

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@@ -1523,6 +1523,34 @@ static Instruction *foldBoxMultiply(BinaryOperator &I) {
   return nullptr;
 }
 
+/// Canonicalize a nested add/sub with a constant on the inner RHS by
+/// sinking the constant to the outer RHS.
+/// (X +/- C) +/- Y  ->  (X +/- Y) +/- C
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MaxGraey wrote:

Another idea is to try not to rearrange when `Inner->getParent() != I.getParent()` (very pessimistic) or somehow to utilize info of back edges (very optimistic)

https://github.com/llvm/llvm-project/pull/191022