[llvm] [MemCpyOpt] allow some undef contents overread in processMemCpyMemCpyDependence (PR #143745)

[Nikita Popov via llvm-commits]

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@@ -1129,14 +1132,28 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
     MForwardOffset = *Offset;
   }
 
-  // The length of the memcpy's must be the same, or the preceding one
-  // must be larger than the following one.
-  if (MForwardOffset != 0 || MDep->getLength() != M->getLength()) {
+  Value *CopyLength = M->getLength();
+
+  // The length of the memcpy's must be the same, or the preceding one must be
+  // larger than the following one, or the contents of the overread must be
+  // undefined bytes of a defined size.
+  if (MForwardOffset != 0 || MDep->getLength() != CopyLength) {
     auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
-    auto *MLen = dyn_cast<ConstantInt>(M->getLength());
-    if (!MDepLen || !MLen ||
-        MDepLen->getZExtValue() < MLen->getZExtValue() + MForwardOffset)
-      return false;
+    auto *MLen = dyn_cast<ConstantInt>(CopyLength);
+    if (!MDepLen || !MLen)
+      return false; // This could be converted to a runtime test (%CopyLength =
+                    // min(max(0, MDepLen - MForwardOffset), MLen)), but it is
+                    // unclear if that is useful
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nikic wrote:

Please do not comment at the end of line, move this above the if.

https://github.com/llvm/llvm-project/pull/143745