1 //===- ForToWhile.cpp - scf.for to scf.while loop conversion --------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Transforms SCF.ForOp's into SCF.WhileOp's.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "PassDetail.h"
14 #include "mlir/Dialect/Arithmetic/IR/Arithmetic.h"
15 #include "mlir/Dialect/SCF/Passes.h"
16 #include "mlir/Dialect/SCF/SCF.h"
17 #include "mlir/Dialect/SCF/Transforms.h"
18 #include "mlir/IR/PatternMatch.h"
19 #include "mlir/Transforms/GreedyPatternRewriteDriver.h"
20 
21 using namespace llvm;
22 using namespace mlir;
23 using scf::ForOp;
24 using scf::WhileOp;
25 
26 namespace {
27 
28 struct ForLoopLoweringPattern : public OpRewritePattern<ForOp> {
29   using OpRewritePattern<ForOp>::OpRewritePattern;
30 
31   LogicalResult matchAndRewrite(ForOp forOp,
32                                 PatternRewriter &rewriter) const override {
33     // Generate type signature for the loop-carried values. The induction
34     // variable is placed first, followed by the forOp.iterArgs.
35     SmallVector<Type> lcvTypes;
36     SmallVector<Location> lcvLocs;
37     lcvTypes.push_back(forOp.getInductionVar().getType());
38     lcvLocs.push_back(forOp.getInductionVar().getLoc());
39     for (Value value : forOp.getInitArgs()) {
40       lcvTypes.push_back(value.getType());
41       lcvLocs.push_back(value.getLoc());
42     }
43 
44     // Build scf.WhileOp
45     SmallVector<Value> initArgs;
46     initArgs.push_back(forOp.getLowerBound());
47     llvm::append_range(initArgs, forOp.getInitArgs());
48     auto whileOp = rewriter.create<WhileOp>(forOp.getLoc(), lcvTypes, initArgs,
49                                             forOp->getAttrs());
50 
51     // 'before' region contains the loop condition and forwarding of iteration
52     // arguments to the 'after' region.
53     auto *beforeBlock = rewriter.createBlock(
54         &whileOp.getBefore(), whileOp.getBefore().begin(), lcvTypes, lcvLocs);
55     rewriter.setInsertionPointToStart(&whileOp.getBefore().front());
56     auto cmpOp = rewriter.create<arith::CmpIOp>(
57         whileOp.getLoc(), arith::CmpIPredicate::slt,
58         beforeBlock->getArgument(0), forOp.getUpperBound());
59     rewriter.create<scf::ConditionOp>(whileOp.getLoc(), cmpOp.getResult(),
60                                       beforeBlock->getArguments());
61 
62     // Inline for-loop body into an executeRegion operation in the "after"
63     // region. The return type of the execRegionOp does not contain the
64     // iv - yields in the source for-loop contain only iterArgs.
65     auto *afterBlock = rewriter.createBlock(
66         &whileOp.getAfter(), whileOp.getAfter().begin(), lcvTypes, lcvLocs);
67 
68     // Add induction variable incrementation
69     rewriter.setInsertionPointToEnd(afterBlock);
70     auto ivIncOp = rewriter.create<arith::AddIOp>(
71         whileOp.getLoc(), afterBlock->getArgument(0), forOp.getStep());
72 
73     // Rewrite uses of the for-loop block arguments to the new while-loop
74     // "after" arguments
75     for (const auto &barg : enumerate(forOp.getBody(0)->getArguments()))
76       barg.value().replaceAllUsesWith(afterBlock->getArgument(barg.index()));
77 
78     // Inline for-loop body operations into 'after' region.
79     for (auto &arg : llvm::make_early_inc_range(*forOp.getBody()))
80       arg.moveBefore(afterBlock, afterBlock->end());
81 
82     // Add incremented IV to yield operations
83     for (auto yieldOp : afterBlock->getOps<scf::YieldOp>()) {
84       SmallVector<Value> yieldOperands = yieldOp.getOperands();
85       yieldOperands.insert(yieldOperands.begin(), ivIncOp.getResult());
86       yieldOp->setOperands(yieldOperands);
87     }
88 
89     // We cannot do a direct replacement of the forOp since the while op returns
90     // an extra value (the induction variable escapes the loop through being
91     // carried in the set of iterargs). Instead, rewrite uses of the forOp
92     // results.
93     for (const auto &arg : llvm::enumerate(forOp.getResults()))
94       arg.value().replaceAllUsesWith(whileOp.getResult(arg.index() + 1));
95 
96     rewriter.eraseOp(forOp);
97     return success();
98   }
99 };
100 
101 struct ForToWhileLoop : public SCFForToWhileLoopBase<ForToWhileLoop> {
102   void runOnOperation() override {
103     FuncOp funcOp = getOperation();
104     MLIRContext *ctx = funcOp.getContext();
105     RewritePatternSet patterns(ctx);
106     patterns.add<ForLoopLoweringPattern>(ctx);
107     (void)applyPatternsAndFoldGreedily(funcOp, std::move(patterns));
108   }
109 };
110 } // namespace
111 
112 std::unique_ptr<Pass> mlir::createForToWhileLoopPass() {
113   return std::make_unique<ForToWhileLoop>();
114 }
115