[mlir][bufferization] Better analysis around allocs and block arguments

[matthias-springer]

Values that are the result of buffer allocation ops are guaranteed to not be the same allocation as block arguments of containing blocks. This fact can be used to allow for more aggressive simplification of bufferization.dealloc ops.

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-bufferization

Changes

Values that are the result of buffer allocation ops are guaranteed to not be the same allocation as block arguments of containing blocks. This fact can be used to allow for more aggressive simplification of bufferization.dealloc ops.

---

Full diff: https://github.com/llvm/llvm-project/pull/67923.diff

3 Files Affected:

• (modified) mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp (+42-8)
• (modified) mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-region-branchop-interface.mlir (+7-6)
• (modified) mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir (+27)

diff --git a/mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp b/mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp
index 5a6de372e2310dc..a834b7d8f89fb72 100644
--- a/mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp
+++ b/mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp
@@ -49,19 +49,53 @@ static LogicalResult updateDeallocIfChanged(DeallocOp deallocOp,
   return success();
 }
 
-/// Checks if 'memref' may or must alias a MemRef in 'memrefList'. It is often a
+/// Given a memref value, return the "base" value by skipping over all
+/// ViewLikeOpInterface ops (if any) in the reverse use-def chain.
+static Value getViewBase(Value value) {
+  while (value) {
+    auto viewLikeOp = value.getDefiningOp<ViewLikeOpInterface>();
+    if (!viewLikeOp)
+      return value;
+    value = viewLikeOp.getViewSource();
+  }
+  return value;
+}
+
+/// Return "true" if the given values are guaranteed to be different (and
+/// non-aliasing) allocations based on the fact that one value is the result
+/// of an allocation and the other value is a block argument of a parent block.
+/// Note: This is a best-effort analysis that will eventually be replaced by a
+/// proper "is same allocation" analysis. This function may return "false" even
+/// though the two values are distinct allocations.
+static bool distinctAllocAndBlockArgument(Value v1, Value v2) {
+  Value v1Base = getViewBase(v1);
+  Value v2Base = getViewBase(v2);
+  auto areDistinct = [](Value v1, Value v2) {
+    if (Operation *op = v1.getDefiningOp())
+      if (hasEffect<MemoryEffects::Allocate>(op, v1))
+        if (auto bbArg = dyn_cast<BlockArgument>(v2))
+          if (bbArg.getOwner()->findAncestorOpInBlock(*op))
+            return true;
+    return false;
+  };
+  return areDistinct(v1Base, v2Base) || areDistinct(v2Base, v1Base);
+}
+
+/// Checks if `memref` may or must alias a MemRef in `otherList`. It is often a
 /// requirement of optimization patterns that there cannot be any aliasing
-/// memref in order to perform the desired simplification. The 'allowSelfAlias'
-/// argument indicates whether 'memref' may be present in 'memrefList' which
+/// memref in order to perform the desired simplification. The `allowSelfAlias`
+/// argument indicates whether `memref` may be present in `otherList` which
 /// makes this helper function applicable to situations where we already know
-/// that 'memref' is in the list but also when we don't want it in the list.
+/// that `memref` is in the list but also when we don't want it in the list.
 static bool potentiallyAliasesMemref(AliasAnalysis &analysis,
-                                     ValueRange memrefList, Value memref,
+                                     ValueRange otherList, Value memref,
                                      bool allowSelfAlias) {
-  for (auto mr : memrefList) {
-    if (allowSelfAlias && mr == memref)
+  for (auto other : otherList) {
+    if (allowSelfAlias && other == memref)
+      continue;
+    if (distinctAllocAndBlockArgument(other, memref))
       continue;
-    if (!analysis.alias(mr, memref).isNo())
+    if (!analysis.alias(other, memref).isNo())
       return true;
   }
   return false;
diff --git a/mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-region-branchop-interface.mlir b/mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-region-branchop-interface.mlir
index dc372749fc074be..ad7c4c783e907f3 100644
--- a/mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-region-branchop-interface.mlir
+++ b/mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/dealloc-region-branchop-interface.mlir
@@ -270,7 +270,8 @@ func.func @loop_alloc(
 //       CHECK: [[V0:%.+]]:2 = scf.for {{.*}} iter_args([[ARG6:%.+]] = [[ARG3]], [[ARG7:%.+]] = %false
 //       CHECK:   [[ALLOC1:%.+]] = memref.alloc()
 //       CHECK:   [[BASE:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG6]]
-//       CHECK:   bufferization.dealloc ([[BASE]] :{{.*}}) if ([[ARG7]]) retain ([[ALLOC1]] :
+//       CHECK:   bufferization.dealloc ([[BASE]] :{{.*}}) if ([[ARG7]])
+//   CHECK-NOT:       retain
 //       CHECK:   scf.yield [[ALLOC1]], %true
 //       CHECK: test.copy
 //       CHECK: [[BASE:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[V0]]#0
@@ -563,8 +564,8 @@ func.func @while_two_arg(%arg0: index) {
 //       CHECK: ^bb0([[ARG1:%.+]]: memref<?xf32>, [[ARG2:%.+]]: memref<?xf32>, [[ARG3:%.+]]: i1, [[ARG4:%.+]]: i1):
 //       CHECK:   [[ALLOC1:%.+]] = memref.alloc(
 //       CHECK:   [[BASE:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG2]]
-//       CHECK:   [[OWN:%.+]]:2 = bufferization.dealloc ([[BASE]] :{{.*}}) if ([[ARG4]]) retain ([[ARG1]], [[ALLOC1]] :
-//       CHECK:   [[OWN_AGG:%.+]] = arith.ori [[OWN]]#0, [[ARG3]]
+//       CHECK:   [[OWN:%.+]] = bufferization.dealloc ([[BASE]] :{{.*}}) if ([[ARG4]]) retain ([[ARG1]] :
+//       CHECK:   [[OWN_AGG:%.+]] = arith.ori [[OWN]], [[ARG3]]
 //       CHECK:   scf.yield [[ARG1]], [[ALLOC1]], [[OWN_AGG]], %true
 //       CHECK: [[BASE0:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[V0]]#0
 //       CHECK: [[BASE1:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[V0]]#1
@@ -594,10 +595,10 @@ func.func @while_three_arg(%arg0: index) {
 //       CHECK:   [[ALLOC1:%.+]] = memref.alloc(
 //       CHECK:   [[ALLOC2:%.+]] = memref.alloc(
 //       CHECK:   [[BASE0:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG1]]
-//       CHECK:   [[BASE1:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG2]]
 //       CHECK:   [[BASE2:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG3]]
-//       CHECK:   [[OWN:%.+]]:3 = bufferization.dealloc ([[BASE0]], [[BASE1]], [[BASE2]], [[ALLOC1]] :{{.*}}) if ([[ARG4]], [[ARG5]], [[ARG6]], %true{{[0-9_]*}}) retain ([[ALLOC2]], [[ALLOC1]], [[ARG2]] :
-//       CHECK:   scf.yield [[ALLOC2]], [[ALLOC1]], [[ARG2]], %true{{[0-9_]*}}, %true{{[0-9_]*}}, [[OWN]]#2 :
+//       CHECK:   [[OWN:%.+]] = bufferization.dealloc ([[BASE0]], [[BASE2]] :{{.*}}) if ([[ARG4]], [[ARG6]]) retain ([[ARG2]] :
+//       CHECK:   [[OWN_AGG:%.+]] = arith.ori [[OWN]], [[ARG5]]
+//       CHECK:   scf.yield [[ALLOC2]], [[ALLOC1]], [[ARG2]], %true{{[0-9_]*}}, %true{{[0-9_]*}}, [[OWN_AGG]] :
 //       CHECK: }
 //       CHECK: [[BASE0:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[V0]]#0
 //       CHECK: [[BASE1:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[V0]]#1
diff --git a/mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir b/mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir
index 98eb038df30a3b9..e192e9870becdbc 100644
--- a/mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir
+++ b/mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir
@@ -133,3 +133,30 @@ func.func @dealloc_remove_dealloc_memref_contained_in_retained_with_const_true_c
 //       CHECK:   [[BASE:%[a-zA-Z0-9_]+]],{{.*}} = memref.extract_strided_metadata [[ARG2]]
 //       CHECK:   bufferization.dealloc ([[BASE]] :{{.*}}) if (%true{{[0-9_]*}})
 //  CHECK-NEXT:   return [[ARG0]], [[ARG1]], %true{{[0-9_]*}}, %true{{[0-9_]*}} :
+
+// -----
+
+func.func @alloc_and_bbarg(%arg0: memref<5xf32>, %arg1: index, %arg2: index, %arg3: index) -> f32 {
+  %true = arith.constant true
+  %false = arith.constant false
+  %0:2 = scf.for %arg4 = %arg1 to %arg2 step %arg3 iter_args(%arg5 = %arg0, %arg6 = %false) -> (memref<5xf32>, i1) {
+    %alloc = memref.alloc() : memref<5xf32>
+    memref.copy %arg5, %alloc : memref<5xf32> to memref<5xf32>
+    %base_buffer_0, %offset_1, %sizes_2, %strides_3 = memref.extract_strided_metadata %arg5 : memref<5xf32> -> memref<f32>, index, index, index
+    %2 = bufferization.dealloc (%base_buffer_0, %alloc : memref<f32>, memref<5xf32>) if (%arg6, %true) retain (%alloc : memref<5xf32>)
+    scf.yield %alloc, %2 : memref<5xf32>, i1
+  }
+  %1 = memref.load %0#0[%arg1] : memref<5xf32>
+  %base_buffer, %offset, %sizes, %strides = memref.extract_strided_metadata %0#0 : memref<5xf32> -> memref<f32>, index, index, index
+  bufferization.dealloc (%base_buffer : memref<f32>) if (%0#1)
+  return %1 : f32
+}
+
+// CHECK-LABEL: func @alloc_and_bbarg
+//       CHECK:   %[[true:.*]] = arith.constant true
+//       CHECK:   scf.for {{.*}} iter_args(%[[iter:.*]] = %{{.*}}, %{{.*}} = %{{.*}})
+//       CHECK:     %[[alloc:.*]] = memref.alloc
+//       CHECK:     %[[view:.*]], %{{.*}}, %{{.*}}, %{{.*}} =  memref.extract_strided_metadata %[[iter]]
+//       CHECK:     bufferization.dealloc (%[[view]] : memref<f32>)
+//   CHECK-NOT:     retain
+//       CHECK:     scf.yield %[[alloc]], %[[true]]

[maerhart]

Nice addition!