[llvm] [MemCpyOpt] Check `MDep` aliases to avoid infinite loops (NFC) (PR #140376)

[via llvm-commits]

https://github.com/dianqk created https://github.com/llvm/llvm-project/pull/140376

cc #103218.

>From aa1c9bc06db868e64516c845ec46b5867c1d8d23 Mon Sep 17 00:00:00 2001
From: dianqk <dianqk at dianqk.net>
Date: Sat, 17 May 2025 20:00:16 +0800
Subject: [PATCH] [MemCpyOpt] Check `MDep` aliases to avoid infinite loops
 (NFC)

---
 llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 9c8cd53d56b56..a78e3770384ae 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1104,16 +1104,17 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
 bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
                                                   MemCpyInst *MDep,
                                                   BatchAAResults &BAA) {
+  // We can only optimize non-volatile memcpy's.
+  if (MDep->isVolatile())
+    return false;
+
   // If dep instruction is reading from our current input, then it is a noop
   // transfer and substituting the input won't change this instruction. Just
   // ignore the input and let someone else zap MDep. This handles cases like:
   //    memcpy(a <- a)
   //    memcpy(b <- a)
-  if (M->getSource() == MDep->getSource())
-    return false;
-
-  // We can only optimize non-volatile memcpy's.
-  if (MDep->isVolatile())
+  // This also avoids infinite loops.
+  if (BAA.isMustAlias(MDep->getDest(), MDep->getSource()))
     return false;
 
   int64_t MForwardOffset = 0;
@@ -1177,10 +1178,6 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
       CopySourceAlign = commonAlignment(*CopySourceAlign, MForwardOffset);
   }
 
-  // Avoid infinite loops
-  if (BAA.isMustAlias(M->getSource(), CopySource))
-    return false;
-
   // Verify that the copied-from memory doesn't change in between the two
   // transfers.  For example, in:
   //    memcpy(a <- b)