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

[Henry Jiang 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
+///     start = str;
+///     do {
+///       str++;
+///     } while(*str != '\0');
+/// \endcode
+///
+/// The transformed output is similar to below c-code:
+/// \code
+///     str = start + strlen(start)
+///     len = str - start
+/// \endcode
+///
+/// Later the pointer subtraction will be folded by InstCombine
+bool LoopIdiomRecognize::recognizeAndInsertStrLen() {
+  if (DisableLIRPStrlen)
+    return false;
+
+  // Give up if the loop has multiple blocks or multiple backedges.
+  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
+    return false;
+
+  // It should have a preheader containing nothing but an unconditional branch.
+  auto *Preheader = CurLoop->getLoopPreheader();
+  if (!Preheader || &Preheader->front() != Preheader->getTerminator())
+    return false;
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mustartt wrote:

I agree that this was unnecessary 

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