[mlir][tensor] Add a pattern to split tensor.pad ops

This commit adds a pattern to wrap a tensor.pad op with
an scf.if op to separate the cases where we don't need padding
(all pad sizes are actually zeros) and where we indeed need
padding.

This pattern is meant to handle padding inside tiled loops.
Under such cases the padding sizes typically depend on the
loop induction variables. Splitting them would allow treating
perfect tiles and edge tiles separately.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D117018

mlir/include/mlir/Dialect/Tensor/Transforms/Transforms.h

@@ -0,0 +1,26 @@
+//===- Transforms.h - Tensor Transformation Patterns ------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_TENSOR_TRANSFORMS_TRANSFORMS_H
+#define MLIR_DIALECT_TENSOR_TRANSFORMS_TRANSFORMS_H
+
+#include "mlir/IR/PatternMatch.h"
+
+namespace mlir {
+namespace tensor {
+
+/// Populates `patterns` with patterns to wrap a tensor.pad op with an scf.if op
+/// to separate the cases where we don't need padding (all pad sizes are
+/// actually zeros) and where we indeed need padding.
+void populateSplitPaddingPatterns(RewritePatternSet &patterns,
+                                  PatternBenefit baseBenefit = 1);
+
+} // namespace tensor
+} // namespace mlir
+
+#endif // MLIR_DIALECT_TENSOR_TRANSFORMS_TRANSFORMS_H

mlir/lib/Dialect/Tensor/Transforms/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_mlir_dialect_library(MLIRTensorTransforms
   BufferizableOpInterfaceImpl.cpp
   Bufferize.cpp
+  SplitPadding.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/Tensor/Transforms

mlir/lib/Dialect/Tensor/Transforms/SplitPadding.cpp

@@ -0,0 +1,94 @@
+//===- SplitPadding.cpp - Splitting tensor.pad Op -------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements patterns to wrap a tensor.pad op with an scf.if op
+/// to separate the cases where we don't need padding (all pad sizes are
+/// actually zeros) and where we indeed need padding.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/SCF/SCF.h"
+#include "mlir/Dialect/Tensor/IR/Tensor.h"
+#include "mlir/Dialect/Tensor/Transforms/Transforms.h"
+#include "mlir/Dialect/Utils/StaticValueUtils.h"
+#include "mlir/IR/PatternMatch.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "mlir-tensor-split-padding"
+
+using namespace mlir;
+
+/// Returns true if the the given `attrOrValue` is a constant zero.
+static bool isZero(OpFoldResult attrOrValue) {
+  if (Optional<int64_t> val = getConstantIntValue(attrOrValue))
+    return val.getValue() == 0;
+  return false;
+}
+
+/// Gets the given `attrOrValue` as a Value by creating constant ops for
+/// attributes.
+static Value getAsValue(OpFoldResult attrOrValue, OpBuilder &builder,
+                        Location loc) {
+  if (Value val = attrOrValue.dyn_cast<Value>())
+    return val;
+  auto attr = attrOrValue.get<Attribute>().cast<IntegerAttr>();
+  return builder.create<arith::ConstantIndexOp>(loc, attr.getInt());
+}
+
+namespace {
+
+struct SplitPadding final : public OpRewritePattern<tensor::PadOp> {
+  using OpRewritePattern::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(tensor::PadOp padOp,
+                                PatternRewriter &rewriter) const override {
+    // Avoid infinitely applying this pattern.
+    if (padOp->getParentOfType<scf::IfOp>())
+      return failure();
+
+    // If all padding sizes are zero, we don't need to do anything.
+    SmallVector<OpFoldResult> lowPads = padOp.getMixedLowPad();
+    SmallVector<OpFoldResult> highPads = padOp.getMixedHighPad();
+    if (llvm::all_of(lowPads, isZero) && llvm::all_of(highPads, isZero))
+      return failure();
+
+    // Build the condition for the scf.if op: all pad sizes are zero.
+    Location loc = padOp.getLoc();
+    Value cstZero = rewriter.create<arith::ConstantIndexOp>(loc, 0);
+    SmallVector<Value> eqZeroCmpVals;
+    for (OpFoldResult pad : llvm::concat<OpFoldResult>(lowPads, highPads)) {
+      if (!isZero(pad))
+        eqZeroCmpVals.push_back(rewriter.create<arith::CmpIOp>(
+            loc, arith::CmpIPredicate::eq, getAsValue(pad, rewriter, loc),
+            cstZero));
+    }
+    Value ifCond = eqZeroCmpVals.front();
+    for (Value cmp : llvm::makeArrayRef(eqZeroCmpVals).drop_front())
+      ifCond = rewriter.create<arith::AndIOp>(loc, ifCond, cmp);
+
+    // Build the scf.if op itself. For the "then" branch, we can elide the
+    // padding. For the "else" branch, we retain the clone op.
+    auto thenBuilder = [&padOp](OpBuilder &builder, Location loc) {
+      builder.create<scf::YieldOp>(loc, padOp.source());
+    };
+    auto elseBuilder = [&padOp](OpBuilder &builder, Location loc) {
+      Operation *newOp = builder.clone(*padOp);
+      builder.create<scf::YieldOp>(loc, newOp->getResults());
+    };
+    rewriter.replaceOpWithNewOp<scf::IfOp>(padOp, padOp.getType(), ifCond,
+                                           thenBuilder, elseBuilder);
+    return success();
+  }
+};
+
+} // namespace
+
+void tensor::populateSplitPaddingPatterns(RewritePatternSet &patterns,
+                                          PatternBenefit baseBenefit) {
+  patterns.add<SplitPadding>(patterns.getContext(), baseBenefit);
+}

mlir/test/Dialect/Tensor/split-padding.mlir

@@ -0,0 +1,44 @@
+// RUN: mlir-opt -split-input-file -test-tensor-transform-patterns=test-split-padding-patterns %s | FileCheck %s
+
+// CHECK-LABEL: func @pad_all_zero_sizes
+func @pad_all_zero_sizes(%input: tensor<?x?x?xf32>) -> tensor<?x?x?xf32> {
+  %f0 = arith.constant 0.0 : f32
+  %c0 = arith.constant 0 : index
+  %0 = tensor.pad %input low[0, %c0, 0] high[%c0, 0, 0] {
+  ^bb0(%dim0: index, %dim1: index, %dim2: index):
+    tensor.yield %f0 : f32
+  } : tensor<?x?x?xf32> to tensor<?x?x?xf32>
+  return %0 : tensor<?x?x?xf32>
+}
+
+// CHECK-NOT: scf.if
+//     CHECK: tensor.pad
+
+// -----
+
+// CHECK-LABEL: func @pad_non_zero_sizes
+//  CHECK-SAME: (%[[INPUT:.+]]: tensor<?x?x8xf32>, %[[LOW0:.+]]: index, %[[HIGH1:.+]]: index)
+func @pad_non_zero_sizes(%input: tensor<?x?x8xf32>, %low0: index, %high1: index) -> tensor<?x?x8xf32> {
+  %f0 = arith.constant 0.0 : f32
+  %0 = tensor.pad %input low[%low0, 0, 0] high[0, %high1, 0] {
+  ^bb0(%dim0: index, %dim1: index, %dim2: index):
+    tensor.yield %f0 : f32
+  } : tensor<?x?x8xf32> to tensor<?x?x8xf32>
+  return %0 : tensor<?x?x8xf32>
+}
+
+// CHECK: %[[F0:.+]] = arith.constant 0.000000e+00 : f32
+// CHECK: %[[C0:.+]] = arith.constant 0 : index
+// CHECK: %[[EQ0:.+]] = arith.cmpi eq, %[[LOW0]], %[[C0]] : index
+// CHECK: %[[EQ1:.+]] = arith.cmpi eq, %[[HIGH1]], %[[C0]] : index
+// CHECK: %[[AND:.+]] = arith.andi %[[EQ0]], %[[EQ1]] : i1
+// CHECK: %[[IF:.+]] = scf.if %[[AND]] -> (tensor<?x?x8xf32>) {
+// CHECK:   scf.yield %[[INPUT]] : tensor<?x?x8xf32>
+// CHECK: } else {
+// CHECK:   %[[PAD:.+]] = tensor.pad %[[INPUT]] low[%[[LOW0]], 0, 0] high[0, %[[HIGH1]], 0]  {
+// CHECK:   ^bb0(%{{.+}}: index, %{{.+}}: index, %{{.+}}: index):
+// CHECK:     tensor.yield %[[F0]] : f32
+// CHECK:   } : tensor<?x?x8xf32> to tensor<?x?x8xf32>
+// CHECK:   scf.yield %[[PAD]] : tensor<?x?x8xf32>
+// CHECK: }
+// CHECK: return %[[IF]] : tensor<?x?x8xf32>

mlir/test/lib/Dialect/CMakeLists.txt

@@ -8,6 +8,7 @@ add_subdirectory(SCF)
 add_subdirectory(Shape)
 add_subdirectory(SPIRV)
 add_subdirectory(StandardOps)
+add_subdirectory(Tensor)
 add_subdirectory(Test)
 add_subdirectory(Tosa)
 add_subdirectory(Vector)

mlir/test/lib/Dialect/Tensor/CMakeLists.txt

@@ -0,0 +1,14 @@
+# Exclude tests from libMLIR.so
+add_mlir_library(MLIRTensorTestPasses
+  TestTensorTransforms.cpp
+
+  EXCLUDE_FROM_LIBMLIR
+
+  LINK_LIBS PUBLIC
+  MLIRArithmetic
+  MLIRPass
+  MLIRSCF
+  MLIRTensor
+  MLIRTensorTransforms
+  MLIRTransforms
+  )

mlir/test/lib/Dialect/Tensor/TestTensorTransforms.cpp

@@ -0,0 +1,65 @@
+//===- TestTensorTransforms.cpp - Test Tensor transformation patterns -----===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements logic for testing Tensor transformations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
+#include "mlir/Dialect/SCF/SCF.h"
+#include "mlir/Dialect/Tensor/Transforms/Transforms.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+
+using namespace mlir;
+
+namespace {
+struct TestTensorTransforms
+    : public PassWrapper<TestTensorTransforms, OperationPass<FuncOp>> {
+  TestTensorTransforms() = default;
+  TestTensorTransforms(const TestTensorTransforms &pass) : PassWrapper(pass) {}
+
+  void getDependentDialects(DialectRegistry &registry) const override {
+    registry.insert<arith::ArithmeticDialect, scf::SCFDialect>();
+  }
+
+  StringRef getArgument() const final {
+    return "test-tensor-transform-patterns";
+  }
+  StringRef getDescription() const final {
+    return "Test Tensor transformation patterns by applying them greedily.";
+  }
+
+  void runOnOperation() override;
+
+  Option<bool> testSplitPaddingPatterns{
+      *this, "test-split-padding-patterns",
+      llvm::cl::desc("Test patterns to split tensor.pad ops"),
+      llvm::cl::init(false)};
+};
+} // namespace
+
+static void applySplitPaddingPatterns(FuncOp funcOp) {
+  RewritePatternSet patterns(funcOp.getContext());
+  tensor::populateSplitPaddingPatterns(patterns);
+  (void)applyPatternsAndFoldGreedily(funcOp, std::move(patterns));
+}
+
+void TestTensorTransforms::runOnOperation() {
+  FuncOp func = getOperation();
+  if (testSplitPaddingPatterns)
+    applySplitPaddingPatterns(func);
+}
+
+namespace mlir {
+namespace test {
+void registerTestTensorTransforms() {
+  PassRegistration<TestTensorTransforms>();
+}
+} // namespace test
+} // namespace mlir

mlir/tools/mlir-opt/CMakeLists.txt

@@ -23,7 +23,7 @@ if(MLIR_INCLUDE_TESTS)
     MLIRShapeTestPasses
     MLIRSPIRVTestPasses
     MLIRStandardOpsTestPasses
-    MLIRVectorTestPasses
+    MLIRTensorTestPasses
     MLIRTestAnalysis
     MLIRTestDialect
     MLIRTestIR
@@ -31,6 +31,7 @@ if(MLIR_INCLUDE_TESTS)
     MLIRTestReducer
     MLIRTestRewrite
     MLIRTestTransforms
+    MLIRVectorTestPasses
     )
 endif()
 

mlir/tools/mlir-opt/mlir-opt.cpp

@@ -106,6 +106,7 @@ void registerTestPreparationPassWithAllowedMemrefResults();
 void registerTestRecursiveTypesPass();
 void registerTestSCFUtilsPass();
 void registerTestSliceAnalysisPass();
+void registerTestTensorTransforms();
 void registerTestVectorLowerings();
 } // namespace test
 } // namespace mlir
@@ -194,6 +195,7 @@ void registerTestPasses() {
   mlir::test::registerTestRecursiveTypesPass();
   mlir::test::registerTestSCFUtilsPass();
   mlir::test::registerTestSliceAnalysisPass();
+  mlir::test::registerTestTensorTransforms();
   mlir::test::registerTestVectorLowerings();
 }
 #endif

utils/bazel/llvm-project-overlay/mlir/BUILD.bazel

@@ -4645,6 +4645,7 @@ cc_library(
     hdrs = [
         "include/mlir/Dialect/Tensor/Transforms/BufferizableOpInterfaceImpl.h",
         "include/mlir/Dialect/Tensor/Transforms/Passes.h",
+        "include/mlir/Dialect/Tensor/Transforms/Transforms.h",
     ],
     includes = ["include"],
     deps = [
@@ -4652,6 +4653,7 @@ cc_library(
         ":Async",
         ":BufferizationDialect",
         ":BufferizationTransforms",
+        ":DialectUtils",
         ":IR",
         ":MemRefDialect",
         ":ParallelLoopMapperAttrGen",
@@ -5985,6 +5987,7 @@ cc_binary(
         "//mlir/test:TestShapeDialect",
         "//mlir/test:TestStandardOps",
         "//mlir/test:TestStandardToLLVM",
+        "//mlir/test:TestTensor",
         "//mlir/test:TestTosaDialect",
         "//mlir/test:TestTransforms",
         "//mlir/test:TestTypeDialect",

utils/bazel/llvm-project-overlay/mlir/test/BUILD.bazel

@@ -510,6 +510,21 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "TestTensor",
+    srcs = glob(["lib/Dialect/Tensor/*.cpp"]),
+    defines = ["MLIR_CUDA_CONVERSIONS_ENABLED"],
+    includes = ["lib/Dialect/Test"],
+    deps = [
+        "//mlir:ArithmeticDialect",
+        "//mlir:Pass",
+        "//mlir:SCFDialect",
+        "//mlir:TensorDialect",
+        "//mlir:TensorTransforms",
+        "//mlir:Transforms",
+    ],
+)
+
 cc_library(
     name = "TestVector",
     srcs = glob(["lib/Dialect/Vector/*.cpp"]),