[llvm] [CGP] Consider arguments and ret values in `dupRetToEnableTailCallOpts` (PR #76613)

[Nikita Popov via llvm-commits]

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@@ -2620,8 +2651,31 @@ bool CodeGenPrepare::dupRetToEnableTailCallOpts(BasicBlock *BB,
       // Make sure the phi value is indeed produced by the tail call.
       if (CI && CI->hasOneUse() && CI->getParent() == PredBB &&
           TLI->mayBeEmittedAsTailCall(CI) &&
-          attributesPermitTailCall(F, CI, RetI, *TLI))
+          attributesPermitTailCall(F, CI, RetI, *TLI)) {
         TailCallBBs.push_back(PredBB);
+      } else {
+        /// Consider the cases in which the phi value is indirectly produced by
+        /// the tail call, for example when encountering memset(), memmove(),
+        /// strcpy(), whose return value may have been optimized out. In such
+        /// cases, the value needs to be the first function argument.
+        /// @code
+        /// bb0:
+        ///   tail call void @llvm.memset.p0.i64(ptr %0, i8 0, i64 %1)
+        ///   br label %return
+        /// return:
+        ///   %phi = phi ptr [ %0, %bb0 ], [ %2, %entry ]
+        /// @endcode
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nikic wrote:

Use `//` instead of `///` here -- don't think inline doxygen comments make sense?

https://github.com/llvm/llvm-project/pull/76613