[llvm] [MemCpyOpt] Don't perform stack-move opt for out-of-bounds copy (PR #217607)

[Antonio Frighetto via llvm-commits]

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@@ -1572,6 +1572,13 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
     return false;
   }
 
+  if (*SrcOffset) {
+    // Make sure that the copied offset is actually part of the alloca. There
+    // might be an out-of-bounds copy in dead code.
+    if (!Size.isFixed() || *SrcOffset + Size > *SrcSize)
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antoniofrighetto wrote:

I guess you would need to memcpy at least `2^64 - SrcOffset` bytes to make `*SrcOffset + Size` wrap around, which I don't think it should matter in practice (and wouldn't probably reach this code anyways).

https://github.com/llvm/llvm-project/pull/217607