[llvm] [MemCpyOpt] allow more memcpy-to-memcpy optimization (PR #150792)

[Jameson Nash via llvm-commits]

================
@@ -1532,21 +1559,43 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
     return false;
   }
 
-  // Check that copy is full with static size.
-  const DataLayout &DL = DestAlloca->getDataLayout();
-  std::optional<TypeSize> SrcSize = SrcAlloca->getAllocationSize(DL);
-  if (!SrcSize || Size != *SrcSize) {
-    LLVM_DEBUG(dbgs() << "Stack Move: Source alloca size mismatch\n");
-    return false;
-  }
-  std::optional<TypeSize> DestSize = DestAlloca->getAllocationSize(DL);
-  if (!DestSize || Size != *DestSize) {
-    LLVM_DEBUG(dbgs() << "Stack Move: Destination alloca size mismatch\n");
+  if (SrcAlloca->isUsedWithInAlloca() || DestAlloca->isUsedWithInAlloca())
     return false;
-  }
 
-  if (!SrcAlloca->isStaticAlloca() || !DestAlloca->isStaticAlloca())
-    return false;
+  Type *SrcType = SrcAlloca->getAllocatedType();
+  Type *DestType = DestAlloca->getAllocatedType();
+  // If they don't have common type, then they will need to be converted to a
+  // common size at runtime
+  const auto &DL = SrcAlloca->getDataLayout();
+  TypeSize SrcSize = DL.getTypeAllocSize(SrcType);
+  TypeSize DestSize = DL.getTypeAllocSize(DestType);
+  if (SrcType != DestType)
+    if (SrcSize != DestSize)
+      if (!SrcSize.isFixed() || !DestSize.isFixed())
+        return false;
+
+  // Check that copy is full with dest size, either because it wrote every byte,
+  // or it was fresh.
+  std::optional<TypeSize> FullSize = DestAlloca->getAllocationSize(DL);
+  if (!FullSize || Size != *FullSize)
+    if (!allOverreadUndefContents(MSSA, Store, BAA)) {
+      LLVM_DEBUG(dbgs() << "Stack Move: Destination alloca size mismatch\n");
+      return false;
+    }
+
+  // Check if it will be legal to combine allocas without breaking dominator.
+  // TODO: Try to hoist the arguments (recursively) instead of giving up
+  // immediately.
----------------
vtjnash wrote:

Which size domination check goes away?

Writing `llvm.umax()` instrinsic vs `llvm::max()` call seem essentially the same amount of code here, but slightly more annoying to have to convert from an LLVM Value to a compile-time value and back again, when LLVM's already has APIs to do this directly on LLVM Values (InstSimplifyFolder) just as easily

https://github.com/llvm/llvm-project/pull/150792