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

[Jameson Nash via llvm-commits]

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@@ -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.
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vtjnash wrote:

It appears that request would eliminate this TODO comment and replace isUsedWithInAlloca with isStaticAlloca, but the logic is the same to do the relevant size computations whether at runtime or compile time, where most of the added code is present

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