[llvm] [MemCpyOpt] Forward `memcpy` based on the actual copy memory location. (PR #87190)

[Nikita Popov via llvm-commits]

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@@ -1124,28 +1125,76 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
 bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
                                                   MemCpyInst *MDep,
                                                   BatchAAResults &BAA) {
-  // We can only transforms memcpy's where the dest of one is the source of the
-  // other.
-  if (M->getSource() != MDep->getDest() || MDep->isVolatile())
-    return false;
-
   // If dep instruction is reading from our current input, then it is a noop
-  // transfer and substituting the input won't change this instruction.  Just
-  // ignore the input and let someone else zap MDep.  This handles cases like:
+  // transfer and substituting the input won't change this instruction. Just
+  // ignore the input and let someone else zap MDep. This handles cases like:
   //    memcpy(a <- a)
   //    memcpy(b <- a)
   if (M->getSource() == MDep->getSource())
     return false;
 
-  // Second, the length of the memcpy's must be the same, or the preceding one
+  // We can only optimize non-volatile memcpy's.
+  if (MDep->isVolatile())
+    return false;
+
+  int64_t MForwardOffset = 0;
+  const DataLayout &DL = M->getModule()->getDataLayout();
+  // We can only transforms memcpy's where the dest of one is the source of the
+  // other, or they have an offset in a range.
+  if (M->getSource() != MDep->getDest()) {
+    std::optional<int64_t> Offset =
+        M->getSource()->getPointerOffsetFrom(MDep->getDest(), DL);
+    if (!Offset || *Offset < 0)
+      return false;
+    MForwardOffset = *Offset;
+  }
+
+  // The length of the memcpy's must be the same, or the preceding one
   // must be larger than the following one.
-  if (MDep->getLength() != M->getLength()) {
+  if (MForwardOffset != 0 || MDep->getLength() != M->getLength()) {
     auto *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
     auto *MLen = dyn_cast<ConstantInt>(M->getLength());
-    if (!MDepLen || !MLen || MDepLen->getZExtValue() < MLen->getZExtValue())
+    if (!MDepLen || !MLen ||
+        MDepLen->getZExtValue() < MLen->getZExtValue() + MForwardOffset)
       return false;
   }
 
+  IRBuilder<> Builder(M);
+  auto *CopySource = MDep->getSource();
+  auto CleanupOnFailure = llvm::make_scope_exit([&CopySource] {
+    if (CopySource->use_empty())
----------------
nikic wrote:

Can we make this specific to only the case where we create the instruction? I think your current code will cause a crash if the destination of the first memcpy is a non-instruction (e.g. argument) and we fall into the same-offset special-case below.

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