[llvm] [Transforms] LoopIdiomRecognize recognize strlen and wcslen (PR #108985)

[via llvm-commits]

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@@ -1524,6 +1545,232 @@ static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry,
   return nullptr;
 }
 
+/// Recognizes a strlen idiom by checking for loops that increment
+/// a char pointer and then subtract with the base pointer.
+///
+/// If detected, transforms the relevant code to a strlen function
+/// call, and returns true; otherwise, returns false.
+///
+/// The core idiom we are trying to detect is:
+/// \code
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RolandF77 wrote:

I think a clearer explanation of what this function does and how it works is needed. The idiom detected is compute the end address of a string. An instance of a strlen computation that has an end address computation of the expected form will be end addr - start addr which will become strlen + start addr - start addr.

https://github.com/llvm/llvm-project/pull/108985