[llvm] [profcheck] Add heuristical profile metadata for lowering table-based cttz. (PR #161898)

[Aiden Grossman via llvm-commits]

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@@ -599,6 +607,19 @@ static bool tryToRecognizeTableBasedCttz(Instruction &I, const DataLayout &DL) {
     auto Cmp = B.CreateICmpEQ(X1, ConstantInt::get(XType, 0));
     auto Select = B.CreateSelect(Cmp, B.CreateZExt(ZeroTableElem, XType), Cttz);
 
+    // Attach heuristic branch weigths to the newly 'select' instruction that
+    // handles the cttz(0) edge case The assumpltion is tht the input to a cttz
+    // operation is rarely 0, so we add a strong 100-to-1 bias weights to the
+    // 'false' path.
+    // The 'ProfcheckDisableMetadataFixes' flag is a testing utility to disable
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boomanaiden154 wrote:

I'm not sure we need a comment for the flag given it should be removed soonish once ablation testing is done.

https://github.com/llvm/llvm-project/pull/161898