[mlir] Add transformation to wrap scf::while in zero-trip-check

mlir/include/mlir/Dialect/SCF/Transforms/Transforms.h

@@ -30,6 +30,7 @@ namespace scf {
 class IfOp;
 class ForOp;
 class ParallelOp;
+class WhileOp;
 
 /// Fuses all adjacent scf.parallel operations with identical bounds and step
 /// into one scf.parallel operations. Uses a naive aliasing and dependency
@@ -181,6 +182,41 @@ FailureOr<ForOp> pipelineForLoop(RewriterBase &rewriter, ForOp forOp,
                                  const PipeliningOption &options,
                                  bool *modifiedIR = nullptr);
 
+/// Create zero-trip-check around a `while` op and return the new loop op in the
+/// check. The while loop is rotated to avoid evaluating the condition twice. It
+/// turns:
+///
+///   scf.while (%arg0 = %init) : (i32) -> i64 {
+///     %val = .., %arg0 : i64
+///     %cond = arith.cmpi .., %arg0 : i32
+///     scf.condition(%cond) %val : i64
+///   } do {
+///   ^bb0(%arg1: i64):
+///     %next = .., %arg1 : i32
+///     scf.yield %next : i32
+///   }
+///
+///  into:
+///
+///   %pre_val = .., %init : i64
+///   %pre_cond = arith.cmpi .., %init : i32
+///   scf.if %pre_cond -> i64 {
+///     %res = scf.while (%arg1 = %va0) : (i64) -> i64 {
+///       %next = .., %arg1 : i32
+///       %val = .., %next : i64
+///       %cond = arith.cmpi .., %next : i32
+///       scf.condition(%cond) %val : i64
+///     } do {
+///     ^bb0(%arg2: i64):
+///       %scf.yield %arg2 : i32
+///     }
+///     scf.yield %res : i64
+///   } else {
+///     scf.yield %pre_val : i64
+///   }
+FailureOr<WhileOp> wrapWhileLoopInZeroTripCheck(WhileOp whileOp,
+                                                RewriterBase &rewriter);
+
 } // namespace scf
 } // namespace mlir
 

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

@@ -13,6 +13,7 @@ add_mlir_dialect_library(MLIRSCFTransforms
   ParallelLoopTiling.cpp
   StructuralTypeConversions.cpp
   TileUsingInterface.cpp
+  WrapInZeroTripCheck.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/SCF

mlir/lib/Dialect/SCF/Transforms/WrapInZeroTripCheck.cpp

@@ -0,0 +1,122 @@
+//===- WrapInZeroTripCheck.cpp - Loop transforms to add zero-trip-check ---===//
+//
+// 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 "mlir/Dialect/SCF/IR/SCF.h"
+#include "mlir/Dialect/SCF/Transforms/Transforms.h"
+#include "mlir/IR/IRMapping.h"
+#include "mlir/IR/PatternMatch.h"
+
+using namespace mlir;
+
+/// Create zero-trip-check around a `while` op and return the new loop op in the
+/// check. The while loop is rotated to avoid evaluating the condition twice.
+///
+/// Given an example below:
+///
+///   scf.while (%arg0 = %init) : (i32) -> i64 {
+///     %val = .., %arg0 : i64
+///     %cond = arith.cmpi .., %arg0 : i32
+///     scf.condition(%cond) %val : i64
+///   } do {
+///   ^bb0(%arg1: i64):
+///     %next = .., %arg1 : i32
+///     scf.yield %next : i32
+///   }
+///
+/// First clone before block to the front of the loop:
+///
+///   %pre_val = .., %init : i64
+///   %pre_cond = arith.cmpi .., %init : i32
+///   scf.while (%arg0 = %init) : (i32) -> i64 {
+///     %val = .., %arg0 : i64
+///     %cond = arith.cmpi .., %arg0 : i32
+///     scf.condition(%cond) %val : i64
+///   } do {
+///   ^bb0(%arg1: i64):
+///     %next = .., %arg1 : i32
+///     scf.yield %next : i32
+///   }
+///
+/// Create `if` op with the condition, rotate and move the loop into the else
+/// branch:
+///
+///   %pre_val = .., %init : i64
+///   %pre_cond = arith.cmpi .., %init : i32
+///   scf.if %pre_cond -> i64 {
+///     %res = scf.while (%arg1 = %va0) : (i64) -> i64 {
+///       // Original after block
+///       %next = .., %arg1 : i32
+///       // Original before block
+///       %val = .., %next : i64
+///       %cond = arith.cmpi .., %next : i32
+///       scf.condition(%cond) %val : i64
+///     } do {
+///     ^bb0(%arg2: i64):
+///       %scf.yield %arg2 : i32
+///     }
+///     scf.yield %res : i64
+///   } else {
+///     scf.yield %pre_val : i64
+///   }
+FailureOr<scf::WhileOp>
+mlir::scf::wrapWhileLoopInZeroTripCheck(scf::WhileOp whileOp,
+                                        RewriterBase &rewriter) {
+  IRMapping mapper;
+  Block *beforeBlock = whileOp.getBeforeBody();
+  // Clone before block before the loop for zero-trip-check.
+  for (auto [arg, init] :
+       llvm::zip_equal(beforeBlock->getArguments(), whileOp.getInits())) {
+    mapper.map(arg, init);
+  }
+  rewriter.setInsertionPoint(whileOp);
+  for (auto &op : *beforeBlock) {
+    if (isa<scf::ConditionOp>(op)) {
+      break;
+    }
+    // Safe to clone everything as in a single block all defs have been cloned
+    // and added to mapper in order.
+    rewriter.insert(op.clone(mapper));
+  }
+
+  auto condOp = whileOp.getConditionOp();
+  auto clonedCondition = mapper.lookupOrDefault(condOp.getCondition());
+  auto clonedCondArgs = llvm::map_to_vector(
+      condOp.getArgs(), [&](Value arg) { return mapper.lookupOrDefault(arg); });
+
+  // Create zero-trip-check and move the while loop in.
+  scf::WhileOp newLoopOp = nullptr;
+  auto ifOp = rewriter.create<scf::IfOp>(
+      whileOp->getLoc(), clonedCondition,
+      [&](OpBuilder &builder, Location loc) {
+        // Then runs the while loop.
+        newLoopOp = builder.create<scf::WhileOp>(
+            loc, whileOp.getResultTypes(), clonedCondArgs,
+            [&](OpBuilder &builder, Location loc, ValueRange args) {
+              // Rotate and move the loop body into before block.
+              auto newBlock = builder.getBlock();
+              rewriter.mergeBlocks(whileOp.getAfterBody(), newBlock, args);
+              auto yieldOp = cast<scf::YieldOp>(newBlock->getTerminator());
+              rewriter.mergeBlocks(whileOp.getBeforeBody(), newBlock,
+                                   yieldOp.getResults());
+              rewriter.eraseOp(yieldOp);
+            },
+            [&](OpBuilder &builder, Location loc, ValueRange args) {
+              // Pass-through values.
+              builder.create<scf::YieldOp>(loc, args);
+            });
+        builder.create<scf::YieldOp>(loc, newLoopOp.getResults());
+      },
+      [&](OpBuilder &builder, Location loc) {
+        // Else returns the results from zero-trip-check.
+        builder.create<scf::YieldOp>(loc, clonedCondArgs);
+      });
+
+  rewriter.replaceOp(whileOp, ifOp);
+
+  return newLoopOp;
+}

mlir/test/Dialect/SCF/wrap-while-loop-in-zero-trip-check.mlir

@@ -0,0 +1,40 @@
+// RUN: mlir-opt %s -test-wrap-scf-while-loop-in-zero-trip-check -split-input-file  | FileCheck %s
+
+func.func @wrap_while_loop_in_zero_trip_check(%bound : i32) -> i32 {
+  %cst0 = arith.constant 0 : i32
+  %cst5 = arith.constant 5 : i32
+  %res:2 = scf.while (%iter = %cst0) : (i32) -> (i32, i32) {
+    %cond = arith.cmpi slt, %iter, %bound : i32
+    %inv = arith.addi %bound, %cst5 : i32
+    scf.condition(%cond) %iter, %inv : i32, i32
+  } do {
+  ^bb0(%arg1: i32, %arg2: i32):
+    %next = arith.addi %arg1, %arg2 : i32
+    scf.yield %next : i32
+  }
+  return %res#0 : i32
+}
+
+// CHECK-LABEL: func.func @wrap_while_loop_in_zero_trip_check(
+// CHECK-SAME:      %[[ARG0:.*]]: i32) -> i32 {
+// CHECK-DAG:     %[[C0:.*]] = arith.constant 0 : i32
+// CHECK-DAG:     %[[C5:.*]] = arith.constant 5 : i32
+// CHECK-DAG:     %[[PRE_COND:.*]] = arith.cmpi slt, %[[C0]], %[[ARG0]] : i32
+// CHECK-DAG:     %[[PRE_INV:.*]] = arith.addi %[[ARG0]], %[[C5]] : i32
+// CHECK:         %[[IF:.*]]:2 = scf.if %[[PRE_COND]] -> (i32, i32) {
+// CHECK:           %[[WHILE:.*]]:2 = scf.while (
+// CHECK-SAME:          %[[ARG1:.*]] = %[[C0]], %[[ARG2:.*]] = %[[PRE_INV]]
+// CHECK-SAME:      ) : (i32, i32) -> (i32, i32) {
+// CHECK:             %[[NEXT:.*]] = arith.addi %[[ARG1]], %[[ARG2]] : i32
+// CHECK:             %[[COND:.*]] = arith.cmpi slt, %[[NEXT]], %[[ARG0]] : i32
+// CHECK:             %[[INV:.*]] = arith.addi %[[ARG0]], %[[C5]] : i32
+// CHECK:             scf.condition(%[[COND]]) %[[NEXT]], %[[INV]] : i32, i32
+// CHECK:           } do {
+// CHECK:           ^bb0(%[[ARG3:.*]]: i32, %[[ARG4:.*]]: i32):
+// CHECK:             scf.yield %[[ARG3]], %[[ARG4]] : i32, i32
+// CHECK:           }
+// CHECK:           scf.yield %[[WHILE]]#0, %[[WHILE]]#1 : i32, i32
+// CHECK:         } else {
+// CHECK:           scf.yield %[[C0]], %[[PRE_INV]] : i32, i32
+// CHECK:         }
+// CHECK:         return %[[IF]]#0 : i32

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

@@ -3,6 +3,7 @@ add_mlir_library(MLIRSCFTestPasses
   TestLoopParametricTiling.cpp
   TestLoopUnrolling.cpp
   TestSCFUtils.cpp
+  TestSCFWrapInZeroTripCheck.cpp
   TestWhileOpBuilder.cpp
 
   EXCLUDE_FROM_LIBMLIR

mlir/test/lib/Dialect/SCF/TestSCFWrapInZeroTripCheck.cpp

@@ -0,0 +1,58 @@
+//===- TestWrapInZeroTripCheck.cpp -- Passes to test SCF zero-trip-check --===//
+//
+// 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 the passes to test wrap-in-zero-trip-check transforms on
+// SCF loop ops.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/Dialect/SCF/IR/SCF.h"
+#include "mlir/Dialect/SCF/Transforms/Transforms.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/Pass/Pass.h"
+
+using namespace mlir;
+
+namespace {
+
+struct TestWrapWhileLoopInZeroTripCheck
+    : public PassWrapper<TestWrapWhileLoopInZeroTripCheck,
+                         OperationPass<func::FuncOp>> {
+  MLIR_DEFINE_EXPLICIT_INTERNAL_INLINE_TYPE_ID(TestWrapWhileLoopInZeroTripCheck)
+
+  StringRef getArgument() const final {
+    return "test-wrap-scf-while-loop-in-zero-trip-check";
+  }
+  StringRef getDescription() const final {
+    return "test scf::wrapWhileLoopInZeroTripCheck";
+  }
+
+  void runOnOperation() override {
+    func::FuncOp func = getOperation();
+    MLIRContext *context = &getContext();
+    IRRewriter rewriter(context);
+    func.walk([&](scf::WhileOp op) {
+      auto result = scf::wrapWhileLoopInZeroTripCheck(op, rewriter);
+      if (failed(result)) {
+        // Ignore not implemented failure in tests. The expected output should
+        // catch problems (e.g. transformation doesn't happen).
+      }
+    });
+  }
+};
+
+} // namespace
+
+namespace mlir {
+namespace test {
+void registerTestSCFWrapInZeroTripCheckPasses() {
+  PassRegistration<TestWrapWhileLoopInZeroTripCheck>();
+}
+} // namespace test
+} // namespace mlir

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

@@ -129,6 +129,7 @@ void registerTestPreparationPassWithAllowedMemrefResults();
 void registerTestRecursiveTypesPass();
 void registerTestSCFUtilsPass();
 void registerTestSCFWhileOpBuilderPass();
+void registerTestSCFWrapInZeroTripCheckPasses();
 void registerTestShapeMappingPass();
 void registerTestSliceAnalysisPass();
 void registerTestTensorCopyInsertionPass();
@@ -251,6 +252,7 @@ void registerTestPasses() {
   mlir::test::registerTestRecursiveTypesPass();
   mlir::test::registerTestSCFUtilsPass();
   mlir::test::registerTestSCFWhileOpBuilderPass();
+  mlir::test::registerTestSCFWrapInZeroTripCheckPasses();
   mlir::test::registerTestShapeMappingPass();
   mlir::test::registerTestSliceAnalysisPass();
   mlir::test::registerTestTensorCopyInsertionPass();