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//===- ForToWhile.cpp - scf.for to scf.while loop conversion --------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Transforms SCF.ForOp's into SCF.WhileOp's.
//
//===----------------------------------------------------------------------===//
#include "mlir/Dialect/SCF/Transforms/Passes.h"
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/Dialect/SCF/IR/SCF.h"
#include "mlir/Dialect/SCF/Transforms/Transforms.h"
#include "mlir/IR/PatternMatch.h"
#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
namespace mlir {
#define GEN_PASS_DEF_SCFFORTOWHILELOOP
#include "mlir/Dialect/SCF/Transforms/Passes.h.inc"
} // namespace mlir
using namespace llvm;
using namespace mlir;
using scf::ForOp;
using scf::WhileOp;
namespace {
struct ForLoopLoweringPattern : public OpRewritePattern<ForOp> {
using OpRewritePattern<ForOp>::OpRewritePattern;
LogicalResult matchAndRewrite(ForOp forOp,
PatternRewriter &rewriter) const override {
// Generate type signature for the loop-carried values. The induction
// variable is placed first, followed by the forOp.iterArgs.
SmallVector<Type> lcvTypes;
SmallVector<Location> lcvLocs;
lcvTypes.push_back(forOp.getInductionVar().getType());
lcvLocs.push_back(forOp.getInductionVar().getLoc());
for (Value value : forOp.getInitArgs()) {
lcvTypes.push_back(value.getType());
lcvLocs.push_back(value.getLoc());
}
// Build scf.WhileOp
SmallVector<Value> initArgs;
initArgs.push_back(forOp.getLowerBound());
llvm::append_range(initArgs, forOp.getInitArgs());
auto whileOp = WhileOp::create(rewriter, forOp.getLoc(), lcvTypes, initArgs,
forOp->getAttrs());
// 'before' region contains the loop condition and forwarding of iteration
// arguments to the 'after' region.
auto *beforeBlock = rewriter.createBlock(
&whileOp.getBefore(), whileOp.getBefore().begin(), lcvTypes, lcvLocs);
rewriter.setInsertionPointToStart(whileOp.getBeforeBody());
arith::CmpIPredicate predicate = forOp.getUnsignedCmp()
? arith::CmpIPredicate::ult
: arith::CmpIPredicate::slt;
auto cmpOp = arith::CmpIOp::create(rewriter, whileOp.getLoc(), predicate,
beforeBlock->getArgument(0),
forOp.getUpperBound());
scf::ConditionOp::create(rewriter, whileOp.getLoc(), cmpOp.getResult(),
beforeBlock->getArguments());
// Inline for-loop body into an executeRegion operation in the "after"
// region. The return type of the execRegionOp does not contain the
// iv - yields in the source for-loop contain only iterArgs.
auto *afterBlock = rewriter.createBlock(
&whileOp.getAfter(), whileOp.getAfter().begin(), lcvTypes, lcvLocs);
// Add induction variable incrementation
rewriter.setInsertionPointToEnd(afterBlock);
auto ivIncOp =
arith::AddIOp::create(rewriter, whileOp.getLoc(),
afterBlock->getArgument(0), forOp.getStep());
// Rewrite uses of the for-loop block arguments to the new while-loop
// "after" arguments
for (const auto &barg : enumerate(forOp.getBody(0)->getArguments()))
rewriter.replaceAllUsesWith(barg.value(),
afterBlock->getArgument(barg.index()));
// Inline for-loop body operations into 'after' region.
for (auto &arg : llvm::make_early_inc_range(*forOp.getBody()))
rewriter.moveOpBefore(&arg, afterBlock, afterBlock->end());
// Add incremented IV to yield operations
for (auto yieldOp : afterBlock->getOps<scf::YieldOp>()) {
SmallVector<Value> yieldOperands = yieldOp.getOperands();
yieldOperands.insert(yieldOperands.begin(), ivIncOp.getResult());
rewriter.modifyOpInPlace(yieldOp,
[&]() { yieldOp->setOperands(yieldOperands); });
}
// We cannot do a direct replacement of the forOp since the while op returns
// an extra value (the induction variable escapes the loop through being
// carried in the set of iterargs). Instead, rewrite uses of the forOp
// results.
for (const auto &arg : llvm::enumerate(forOp.getResults()))
rewriter.replaceAllUsesWith(arg.value(),
whileOp.getResult(arg.index() + 1));
rewriter.eraseOp(forOp);
return success();
}
};
struct ForToWhileLoop : public impl::SCFForToWhileLoopBase<ForToWhileLoop> {
void runOnOperation() override {
auto *parentOp = getOperation();
MLIRContext *ctx = parentOp->getContext();
RewritePatternSet patterns(ctx);
patterns.add<ForLoopLoweringPattern>(ctx);
(void)applyPatternsGreedily(parentOp, std::move(patterns));
}
};
} // namespace
std::unique_ptr<Pass> mlir::createForToWhileLoopPass() {
return std::make_unique<ForToWhileLoop>();
}
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