[mlir][bufferization] Ownership-based deallocation: Allow manual (de)allocs (#68648)

Add a new attribute `bufferization.manual_deallocation` that can be
attached to allocation and deallocation ops. Buffers that are allocated
with this attribute are assigned an ownership of "false". Such buffers
can be deallocated manually (e.g., with `memref.dealloc`) if the
deallocation op also has the attribute set. Previously, the
ownership-based buffer deallocation pass used to reject IR with existing
deallocation ops. This is no longer the case if such ops have this new
attribute.

This change is useful for the sparse compiler, which currently
deallocates the sparse tensor buffers by itself.

Author: matthias-springer

mlir/include/mlir/Dialect/Bufferization/IR/BufferizationBase.td

@@ -60,6 +60,17 @@ def Bufferization_Dialect : Dialect {
     /// arguments during One-Shot Module Bufferize.
     constexpr const static ::llvm::StringLiteral
         kBufferLayoutAttrName = "bufferization.buffer_layout";
+
+    /// An attribute that can be attached to ops with an allocation and/or
+    /// deallocation side effect. It indicates that the op is under a "manual
+    /// deallocation" scheme. In the case of an allocation op, the returned
+    /// value is *not* an automatically managed allocation and assigned an
+    /// ownership of "false". Furthermore, only deallocation ops that are
+    /// guaranteed to deallocate a buffer under "manual deallocation" are
+    /// allowed to have this attribute. (Deallocation ops without this
+    /// attribute are rejected by the ownership-based buffer deallocation pass.)
+    constexpr const static ::llvm::StringLiteral
+        kManualDeallocation = "bufferization.manual_deallocation";
   }];
   let hasOperationAttrVerify = 1;
 }

mlir/lib/Dialect/Bufferization/IR/BufferizationDialect.cpp

@@ -28,6 +28,16 @@ constexpr const ::llvm::StringLiteral BufferizationDialect::kWritableAttrName;
 constexpr const ::llvm::StringLiteral
     BufferizationDialect::kBufferLayoutAttrName;
 
+/// An attribute that can be attached to ops with an allocation and/or
+/// deallocation side effect. It indicates that the op is under a "manual
+/// deallocation" scheme. In the case of an allocation op, the returned
+/// value is *not* an automatically managed allocation and assigned an
+/// ownership of "false". Furthermore, only deallocation ops that are
+/// guaranteed to deallocate a buffer under "manual deallocation" are
+/// allowed to have this attribute. (Deallocation ops without this
+/// attribute are rejected by the ownership-based buffer deallocation pass.)
+constexpr const ::llvm::StringLiteral BufferizationDialect::kManualDeallocation;
+
 //===----------------------------------------------------------------------===//
 // Bufferization Dialect Interfaces
 //===----------------------------------------------------------------------===//
@@ -105,6 +115,16 @@ BufferizationDialect::verifyOperationAttribute(Operation *op,
                                                NamedAttribute attr) {
   using bufferization::BufferizableOpInterface;
 
+  if (attr.getName() == kManualDeallocation) {
+    if (!mlir::hasEffect<MemoryEffects::Allocate>(op) &&
+        !mlir::hasEffect<MemoryEffects::Free>(op))
+      return op->emitOpError("attribute '")
+             << kManualDeallocation
+             << "' can be used only on ops that have an allocation and/or free "
+                "side effect";
+    return success();
+  }
+
   return op->emitError()
          << "attribute '" << attr.getName()
          << "' not supported as an op attribute by the bufferization dialect";

mlir/lib/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation.cpp

@@ -21,6 +21,7 @@
 #include "mlir/Dialect/Bufferization/IR/BufferDeallocationOpInterface.h"
 #include "mlir/Dialect/Bufferization/IR/Bufferization.h"
 #include "mlir/Dialect/Bufferization/Transforms/Passes.h"
+#include "mlir/Dialect/ControlFlow/IR/ControlFlowOps.h"
 #include "mlir/Dialect/Func/IR/FuncOps.h"
 #include "mlir/Dialect/MemRef/IR/MemRef.h"
 #include "mlir/Dialect/SCF/IR/SCF.h"
@@ -856,13 +857,32 @@ FailureOr<Operation *> BufferDeallocation::handleInterface(CallOpInterface op) {
 FailureOr<Operation *>
 BufferDeallocation::handleInterface(MemoryEffectOpInterface op) {
   auto *block = op->getBlock();
+  OpBuilder builder = OpBuilder::atBlockBegin(block);
 
-  for (auto operand : llvm::make_filter_range(op->getOperands(), isMemref))
-    if (op.getEffectOnValue<MemoryEffects::Free>(operand).has_value())
-      return op->emitError(
-          "memory free side-effect on MemRef value not supported!");
+  for (auto operand : llvm::make_filter_range(op->getOperands(), isMemref)) {
+    if (op.getEffectOnValue<MemoryEffects::Free>(operand).has_value()) {
+      if (!op->hasAttr(BufferizationDialect::kManualDeallocation))
+        return op->emitError(
+            "memory free side-effect on MemRef value not supported!");
+
+      // Buffers that were allocated under "manual deallocation" may be
+      // manually deallocated. We insert a runtime assertion to cover certain
+      // cases of invalid IR where an automatically managed buffer allocation
+      // is manually deallocated. This is not a bulletproof check!
+      OpBuilder::InsertionGuard g(builder);
+      builder.setInsertionPoint(op);
+      Ownership ownership = state.getOwnership(operand, block);
+      if (ownership.isUnique()) {
+        Value ownershipInverted = builder.create<arith::XOrIOp>(
+            op.getLoc(), ownership.getIndicator(),
+            buildBoolValue(builder, op.getLoc(), true));
+        builder.create<cf::AssertOp>(
+            op.getLoc(), ownershipInverted,
+            "expected that the block does not have ownership");
+      }
+    }
+  }
 
-  OpBuilder builder = OpBuilder::atBlockBegin(block);
   for (auto res : llvm::make_filter_range(op->getResults(), isMemref)) {
     auto allocEffect = op.getEffectOnValue<MemoryEffects::Allocate>(res);
     if (allocEffect.has_value()) {
@@ -880,6 +900,15 @@ BufferDeallocation::handleInterface(MemoryEffectOpInterface op) {
         continue;
       }
 
+      if (op->hasAttr(BufferizationDialect::kManualDeallocation)) {
+        // This allocation will be deallocated manually. Assign an ownership of
+        // "false", so that it will never be deallocated by the buffer
+        // deallocation pass.
+        state.resetOwnerships(res, block);
+        state.updateOwnership(res, buildBoolValue(builder, op.getLoc(), false));
+        continue;
+      }
+
       state.updateOwnership(res, buildBoolValue(builder, op.getLoc(), true));
       state.addMemrefToDeallocate(res, block);
     }

mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-memoryeffect-interface.mlir

@@ -124,3 +124,35 @@ func.func @op_without_aliasing_and_allocation() -> memref<4xf32> {
 //       CHECK:     [[CLONE:%.+]] = bufferization.clone [[GLOBAL]]
 //       CHECK:     scf.yield [[CLONE]] :
 //       CHECK:   return [[RES]] :
+
+// -----
+
+// Allocations with "bufferization.manual_deallocation" are assigned an
+// ownership of "false".
+
+func.func @manual_deallocation(%c: i1, %f: f32, %idx: index) -> f32 {
+  %0 = memref.alloc() {bufferization.manual_deallocation} : memref<5xf32>
+  linalg.fill ins(%f : f32) outs(%0 : memref<5xf32>)
+  %1 = memref.alloc() : memref<5xf32>
+  linalg.fill ins(%f : f32) outs(%1 : memref<5xf32>)
+  %2 = arith.select %c, %0, %1 : memref<5xf32>
+  %3 = memref.load %2[%idx] : memref<5xf32>
+
+  // Only buffers that are under "manual deallocation" are allowed to be
+  // deallocated with memref.dealloc. For consistency reasons, the
+  // manual_deallocation attribute must also be specified. A runtime insertion
+  // is inserted to ensure that we do not have ownership. (This is not a
+  // bulletproof check, but covers some cases of invalid IR.)
+  memref.dealloc %0 {bufferization.manual_deallocation} : memref<5xf32>
+
+  return %3 : f32
+}
+
+// CHECK-LABEL: func @manual_deallocation(
+//       CHECK:   %[[true:.*]] = arith.constant true
+//       CHECK:   %[[manual_alloc:.*]] = memref.alloc() {bufferization.manual_deallocation} : memref<5xf32>
+//       CHECK:   %[[managed_alloc:.*]] = memref.alloc() : memref<5xf32>
+//       CHECK:   %[[selected:.*]] = arith.select
+//       CHECK:   cf.assert %[[true]], "expected that the block does not have ownership"
+//       CHECK:   memref.dealloc %[[manual_alloc]]
+//       CHECK:   bufferization.dealloc (%[[managed_alloc]] : memref<5xf32>) if (%[[true]])

mlir/test/Dialect/Bufferization/invalid.mlir

@@ -136,3 +136,10 @@ func.func @invalid_dealloc_wrong_number_of_results(%arg0: memref<2xf32>, %arg1:
   %0:3 = "bufferization.dealloc"(%arg0, %arg1, %arg2, %arg2, %arg1) <{operandSegmentSizes = array<i32: 2, 2, 1>}> : (memref<2xf32>, memref<4xi32>, i1, i1, memref<4xi32>) -> (i1, i1, i1)
   return %0#0 : i1
 }
+
+// -----
+
+func.func @invalid_manual_deallocation() {
+  // expected-error @below{{op attribute 'bufferization.manual_deallocation' can be used only on ops that have an allocation and/or free side effect}}
+  arith.constant {bufferization.manual_deallocation} 0  : index
+}