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

[Antonio Frighetto via llvm-commits]

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@@ -1843,12 +1844,68 @@ 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) {
+  const auto &DL = M->getDataLayout();
+  MemoryUseOrDef *MemMoveAccess = MSSA->getMemoryAccess(M);
+  if (!MemMoveAccess)
+    return false;
+
+  // The memmove is of form memmove(x, x + A, B).
+  MemoryLocation SourceLoc = MemoryLocation::getForSource(M);
+  auto *Source = dyn_cast<GetElementPtrInst>(M->getSource());
+  if (!Source)
+    return false;
+  APInt Offset(DL.getIndexTypeSizeInBits(Source->getType()), 0);
+  auto MemMoveSize = SourceLoc.Size;
+  if (!Source->accumulateConstantOffset(DL, Offset) || Offset.isNegative() ||
+      Source->getPointerOperand() != M->getDest() || !MemMoveSize.hasValue())
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antoniofrighetto wrote:

Should have done earlier.

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