[SelectionDAG] Avoid aliasing analysis if the object size is unknown.

[HsiangKai]

Summary

If the size of memory access is unknown, do not use it to analysis. One example of unknown size memory access is to load/store scalable vector objects on the stack.

[HsiangKai]

clang format.

[courbet]

Thanks, could you please add a unit test for this ?

[niravd]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

103

This is too aggressive. We can still determine that a two addresses are non-aliasing even if one is unknown size. (e.g. FrameIndex vs. Global)

[HsiangKai]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

103

The checking is under the condition that BasePtr0.equalBaseIndex(BasePtr1, ...) equals true. It would not be the case you considered.

[HsiangKai]

Add unit tests.

[niravd]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

103

What about the case where equalBaseIndex returns true, and the lowered addressed access has a known size smaller than the difference? In that case, we should be able to determine that they are do not alias.

[HsiangKai]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

103

I doubt there is a such scenario to share the same base between fixed and scalable types. I could add an assertion here.

[HsiangKai]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

103

I could not exclude the case that one fixed object is fixed length type and another fixed object is scalable vector type. Sorry for that. I will update the patch.

[niravd]

LGTM modulo minor nit. Thanks!

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

105

nit: These bools are single use and can be inlined.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGAddressAnalysis.cpp

 //==- llvm/CodeGen/SelectionDAGAddressAnalysis.cpp - DAG Address Analysis --==//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/CodeGen/SelectionDAGAddressAnalysis.h"
+#include "llvm/Analysis/MemoryLocation.h"
 #include "llvm/CodeGen/ISDOpcodes.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/SelectionDAG.h"
 #include "llvm/CodeGen/SelectionDAGNodes.h"
 #include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Debug.h"
bool BaseIndexOffset::computeAliasing(const SDNode *Op0,
   BaseIndexOffset BasePtr0 = match(Op0, DAG);
   BaseIndexOffset BasePtr1 = match(Op1, DAG);
 
   if (!(BasePtr0.getBase().getNode() && BasePtr1.getBase().getNode()))
     return false;
   int64_t PtrDiff;
   if (NumBytes0.hasValue() && NumBytes1.hasValue() &&
       BasePtr0.equalBaseIndex(BasePtr1, DAG, PtrDiff)) {
+    // If the size of memory access is unknown, do not use it to analysis.
+    // One example of unknown size memory access is to load/store scalable
+    // vector objects on the stack.
+    if (*NumBytes0 == static_cast<int64_t>(MemoryLocation::UnknownSize) ||
+        *NumBytes1 == static_cast<int64_t>(MemoryLocation::UnknownSize))
+      return false;
     // BasePtr1 is PtrDiff away from BasePtr0. They alias if none of the
     // following situations arise:
     IsAlias = !(
         // [----BasePtr0----]
         //                         [---BasePtr1--]
         // ========PtrDiff========>
         (*NumBytes0 <= PtrDiff) ||
         //                     [----BasePtr0----]