[llvm] [MemCpyOpt] Calculate the offset value to forward `memcpy` (PR #87190)

[Quentin Dian via llvm-commits]

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@@ -1152,11 +1188,12 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
   //
   // TODO: If the code between M and MDep is transparent to the destination "c",
   // then we could still perform the xform by moving M up to the first memcpy.
-  // TODO: It would be sufficient to check the MDep source up to the memcpy
-  // size of M, rather than MDep.
-  if (writtenBetween(MSSA, BAA, MemoryLocation::getForSource(MDep),
-                     MSSA->getMemoryAccess(MDep), MSSA->getMemoryAccess(M)))
+  if (writtenBetween(MSSA, BAA, MCopyLoc, MSSA->getMemoryAccess(MDep),
+                     MSSA->getMemoryAccess(M))) {
+    if (MForwardOffset > 0 && CopySource->use_empty())
+      cast<Instruction>(CopySource)->eraseFromParent();
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DianQK wrote:

Yes. :)

https://github.com/llvm/llvm-project/pull/87190