[llvm] [MemCpyOpt] Drop dead `memmove` calls on `memset`'d source data (PR #101930)

[Antonio Frighetto via llvm-commits]

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@@ -1853,12 +1854,54 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
   return false;
 }
 
+/// Memmove calls with overlapping src/dest buffers that come after a memset may
+/// be removed.
+bool MemCpyOptPass::isMemMoveMemSetDependency(MemMoveInst *M) {
+  MemoryUseOrDef *MemMoveAccess = MSSA->getMemoryAccess(M);
+  if (!MemMoveAccess)
+    return false;
+
+  BatchAAResults BAA(*AA);
+  MemoryAccess *FirstDef = MemMoveAccess->getDefiningAccess();
+  MemoryLocation SourceLoc = MemoryLocation::getForSource(M);
+  MemoryAccess *SourceClobber =
+      MSSA->getWalker()->getClobberingMemoryAccess(FirstDef, SourceLoc, BAA);
+
+  MemSetInst *MS = nullptr;
+  if (auto *Def = dyn_cast<MemoryDef>(SourceClobber))
+    MS = dyn_cast_or_null<MemSetInst>(Def->getMemoryInst());
+
+  // Limit the memset to be within the same basic block.
+  if (!MS || MS->getParent() != M->getParent())
+    return false;
+
+  // The destination buffer must have been memset'd.
+  if (!BAA.isMustAlias(MS->getDest(), M->getDest()))
+    return false;
+
+  // No clobbering writes in between.
+  if (writtenBetween(MSSA, BAA, SourceLoc, MSSA->getMemoryAccess(MS),
+                     MemMoveAccess))
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antoniofrighetto wrote:

I see, indeed the first dominating clobbering access for the destination location should be the memset too. Could you please check that now this is more aligned with what you thought? Though, shouldn't the LocationSize be just B to check the source region (and not A+B)?

https://github.com/llvm/llvm-project/pull/101930