[mlir][LocalAliasAnalysis] Check for memref.distinct_objects in LocalAliasAnalysis (#161533)

Continuation of llvm/llvm-project#156913, I'm
planning to use it to infer LLVM alias scope attributes.

* Introduce `DistinctObjectsTrait` so analysis won't need to depend on
memref dialect directly.
* Check the trairin `LocalAliasAnalysis::aliasImpl`.

Author: Hardcode84

mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td

@@ -184,6 +184,7 @@ def AssumeAlignmentOp : MemRef_Op<"assume_alignment", [
 
 def DistinctObjectsOp : MemRef_Op<"distinct_objects", [
       Pure,
+      DistinctObjectsTrait,
       DeclareOpInterfaceMethods<InferTypeOpInterface>
       // ViewLikeOpInterface TODO: ViewLikeOpInterface only supports a single argument
     ]> {

mlir/include/mlir/Interfaces/ViewLikeInterface.h

@@ -230,6 +230,22 @@ LogicalResult verifyListOfOperandsOrIntegers(Operation *op, StringRef name,
                                              ArrayRef<int64_t> attr,
                                              ValueRange values);
 
+namespace OpTrait {
+/// This trai indicates that pointer-like objects (such as memrefs) returned
+/// from this operation will never alias with each other. This provides a
+/// guarantee to optimization passes that accesses through different results
+/// of this operation can be safely reordered, as they will never reference
+/// overlapping memory locations.
+///
+/// Operations with this trait take multiple pointer-like operands
+/// and return the same operands with additional non-aliasing guarantees.
+/// If the access to the results of this operation aliases at runtime, the
+/// behavior of such access is undefined.
+template <typename ConcreteType>
+class DistinctObjectsTrait
+    : public TraitBase<ConcreteType, DistinctObjectsTrait> {};
+} // namespace OpTrait
+
 } // namespace mlir
 
 #endif // MLIR_INTERFACES_VIEWLIKEINTERFACE_H_

mlir/include/mlir/Interfaces/ViewLikeInterface.td

@@ -414,4 +414,16 @@ def OffsetSizeAndStrideOpInterface : OpInterface<"OffsetSizeAndStrideOpInterface
   }];
 }
 
+// This trai indicates that pointer-like objects (such as memrefs) returned
+// from this operation will never alias with each other. This provides a
+// guarantee to optimization passes that accesses through different results
+// of this operation can be safely reordered, as they will never reference
+// overlapping memory locations.
+//
+// Operations with this trait take multiple pointer-like operands
+// and return the same operands with additional non-aliasing guarantees.
+// If the access to the results of this operation aliases at runtime, the
+// behavior of such access is undefined.
+def DistinctObjectsTrait : NativeOpTrait<"DistinctObjectsTrait">;
+
 #endif // MLIR_INTERFACES_VIEWLIKEINTERFACE

mlir/lib/Analysis/AliasAnalysis/LocalAliasAnalysis.cpp

@@ -258,6 +258,39 @@ getAllocEffectFor(Value value,
   return success();
 }
 
+static Operation *isDistinctObjectsOp(Operation *op) {
+  if (op && op->hasTrait<OpTrait::DistinctObjectsTrait>())
+    return op;
+
+  return nullptr;
+}
+
+static Value getDistinctObjectsOperand(Operation *op, Value value) {
+  unsigned argNumber = cast<OpResult>(value).getResultNumber();
+  return op->getOperand(argNumber);
+}
+
+static std::optional<AliasResult> checkDistinctObjects(Value lhs, Value rhs) {
+  // We should already checked that lhs and rhs are different.
+  assert(lhs != rhs && "lhs and rhs must be different");
+
+  // Result and corresponding operand must alias.
+  auto lhsOp = isDistinctObjectsOp(lhs.getDefiningOp());
+  if (lhsOp && getDistinctObjectsOperand(lhsOp, lhs) == rhs)
+    return AliasResult::MustAlias;
+
+  auto rhsOp = isDistinctObjectsOp(rhs.getDefiningOp());
+  if (rhsOp && getDistinctObjectsOperand(rhsOp, rhs) == lhs)
+    return AliasResult::MustAlias;
+
+  // If two different values come from the same `DistinctObjects` operation,
+  // they don't alias.
+  if (lhsOp && lhsOp == rhsOp)
+    return AliasResult::NoAlias;
+
+  return std::nullopt;
+}
+
 /// Given the two values, return their aliasing behavior.
 AliasResult LocalAliasAnalysis::aliasImpl(Value lhs, Value rhs) {
   if (lhs == rhs)
@@ -289,6 +322,9 @@ AliasResult LocalAliasAnalysis::aliasImpl(Value lhs, Value rhs) {
                : AliasResult::MayAlias;
   }
 
+  if (std::optional<AliasResult> result = checkDistinctObjects(lhs, rhs))
+    return *result;
+
   // Otherwise, neither of the values are constant so check to see if either has
   // an allocation effect.
   bool lhsHasAlloc = succeeded(getAllocEffectFor(lhs, lhsAlloc, lhsAllocScope));

mlir/test/Analysis/test-alias-analysis.mlir

@@ -256,3 +256,19 @@ func.func @constants(%arg: memref<2xf32>) attributes {test.ptr = "func"} {
 
   return
 }
+
+// -----
+
+// CHECK-LABEL: Testing : "distinct_objects"
+// CHECK-DAG: func.region0#0 <-> func.region0#1: MayAlias
+
+// CHECK-DAG: distinct#0 <-> distinct#1: NoAlias
+// CHECK-DAG: distinct#0 <-> func.region0#0: MustAlias
+// CHECK-DAG: distinct#1 <-> func.region0#0: MayAlias
+// CHECK-DAG: distinct#0 <-> func.region0#1: MayAlias
+// CHECK-DAG: distinct#1 <-> func.region0#1: MustAlias
+
+func.func @distinct_objects(%arg: memref<?xf32>, %arg1: memref<?xf32>) attributes {test.ptr = "func"} {
+  %0, %1 = memref.distinct_objects %arg, %arg1 {test.ptr = "distinct"} : memref<?xf32>, memref<?xf32>
+  return
+}