[llvm] [SimplifyLibCalls] Merge sqrt into the power of exp (PR #79146)

[Matt Arsenault via llvm-commits]

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@@ -2538,6 +2538,70 @@ Value *LibCallSimplifier::optimizeLog(CallInst *Log, IRBuilderBase &B) {
   return Ret;
 }
 
+// sqrt(exp(X)) -> exp(X * 0.5)
+Value *LibCallSimplifier::mergeSqrtToExp(CallInst *CI, IRBuilderBase &B) {
+  if (!CI->isFast())
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arsenm wrote:

I did some experiments and this is net error reducing. This reassociation avoids some intermediate overflows to infinity and overall reduces worst case ulp. I think just reassoc is adequate

https://github.com/llvm/llvm-project/pull/79146