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llvm-project/mlir/lib/Conversion/SCFToStandard/SCFToStandard.cpp

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//===- SCFToStandard.cpp - ControlFlow to CFG 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
//
//===----------------------------------------------------------------------===//
//
// This file implements a pass to convert scf.for, scf.if and loop.terminator
// ops into standard CFG ops.
//
//===----------------------------------------------------------------------===//
#include "mlir/Conversion/SCFToStandard/SCFToStandard.h"
#include "../PassDetail.h"
#include "mlir/Dialect/SCF/SCF.h"
#include "mlir/Dialect/StandardOps/IR/Ops.h"
#include "mlir/IR/BlockAndValueMapping.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/MLIRContext.h"
#include "mlir/IR/Module.h"
#include "mlir/IR/PatternMatch.h"
#include "mlir/Transforms/DialectConversion.h"
#include "mlir/Transforms/Passes.h"
#include "mlir/Transforms/Utils.h"
using namespace mlir;
using namespace mlir::scf;
namespace {
struct SCFToStandardPass : public SCFToStandardBase<SCFToStandardPass> {
void runOnOperation() override;
};
// Create a CFG subgraph for the loop around its body blocks (if the body
// contained other loops, they have been already lowered to a flow of blocks).
// Maintain the invariants that a CFG subgraph created for any loop has a single
// entry and a single exit, and that the entry/exit blocks are respectively
// first/last blocks in the parent region. The original loop operation is
// replaced by the initialization operations that set up the initial value of
// the loop induction variable (%iv) and computes the loop bounds that are loop-
// invariant for affine loops. The operations following the original scf.for
// are split out into a separate continuation (exit) block. A condition block is
// created before the continuation block. It checks the exit condition of the
// loop and branches either to the continuation block, or to the first block of
// the body. The condition block takes as arguments the values of the induction
// variable followed by loop-carried values. Since it dominates both the body
// blocks and the continuation block, loop-carried values are visible in all of
// those blocks. Induction variable modification is appended to the last block
// of the body (which is the exit block from the body subgraph thanks to the
// invariant we maintain) along with a branch that loops back to the condition
// block. Loop-carried values are the loop terminator operands, which are
// forwarded to the branch.
//
// +---------------------------------+
// | <code before the ForOp> |
// | <definitions of %init...> |
// | <compute initial %iv value> |
// | br cond(%iv, %init...) |
// +---------------------------------+
// |
// -------| |
// | v v
// | +--------------------------------+
// | | cond(%iv, %init...): |
// | | <compare %iv to upper bound> |
// | | cond_br %r, body, end |
// | +--------------------------------+
// | | |
// | | -------------|
// | v |
// | +--------------------------------+ |
// | | body-first: | |
// | | <%init visible by dominance> | |
// | | <body contents> | |
// | +--------------------------------+ |
// | | |
// | ... |
// | | |
// | +--------------------------------+ |
// | | body-last: | |
// | | <body contents> | |
// | | <operands of yield = %yields>| |
// | | %new_iv =<add step to %iv> | |
// | | br cond(%new_iv, %yields) | |
// | +--------------------------------+ |
// | | |
// |----------- |--------------------
// v
// +--------------------------------+
// | end: |
// | <code after the ForOp> |
// | <%init visible by dominance> |
// +--------------------------------+
//
struct ForLowering : public OpRewritePattern<ForOp> {
using OpRewritePattern<ForOp>::OpRewritePattern;
LogicalResult matchAndRewrite(ForOp forOp,
PatternRewriter &rewriter) const override;
};
// Create a CFG subgraph for the scf.if operation (including its "then" and
// optional "else" operation blocks). We maintain the invariants that the
// subgraph has a single entry and a single exit point, and that the entry/exit
// blocks are respectively the first/last block of the enclosing region. The
// operations following the scf.if are split into a continuation (subgraph
// exit) block. The condition is lowered to a chain of blocks that implement the
// short-circuit scheme. The "scf.if" operation is replaced with a conditional
// branch to either the first block of the "then" region, or to the first block
// of the "else" region. In these blocks, "scf.yield" is unconditional branches
// to the post-dominating block. When the "scf.if" does not return values, the
// post-dominating block is the same as the continuation block. When it returns
// values, the post-dominating block is a new block with arguments that
// correspond to the values returned by the "scf.if" that unconditionally
// branches to the continuation block. This allows block arguments to dominate
// any uses of the hitherto "scf.if" results that they replaced. (Inserting a
// new block allows us to avoid modifying the argument list of an existing
// block, which is illegal in a conversion pattern). When the "else" region is
// empty, which is only allowed for "scf.if"s that don't return values, the
// condition branches directly to the continuation block.
//
// CFG for a scf.if with else and without results.
//
// +--------------------------------+
// | <code before the IfOp> |
// | cond_br %cond, %then, %else |
// +--------------------------------+
// | |
// | --------------|
// v |
// +--------------------------------+ |
// | then: | |
// | <then contents> | |
// | br continue | |
// +--------------------------------+ |
// | |
// |---------- |-------------
// | V
// | +--------------------------------+
// | | else: |
// | | <else contents> |
// | | br continue |
// | +--------------------------------+
// | |
// ------| |
// v v
// +--------------------------------+
// | continue: |
// | <code after the IfOp> |
// +--------------------------------+
//
// CFG for a scf.if with results.
//
// +--------------------------------+
// | <code before the IfOp> |
// | cond_br %cond, %then, %else |
// +--------------------------------+
// | |
// | --------------|
// v |
// +--------------------------------+ |
// | then: | |
// | <then contents> | |
// | br dom(%args...) | |
// +--------------------------------+ |
// | |
// |---------- |-------------
// | V
// | +--------------------------------+
// | | else: |
// | | <else contents> |
// | | br dom(%args...) |
// | +--------------------------------+
// | |
// ------| |
// v v
// +--------------------------------+
// | dom(%args...): |
// | br continue |
// +--------------------------------+
// |
// v
// +--------------------------------+
// | continue: |
// | <code after the IfOp> |
// +--------------------------------+
//
struct IfLowering : public OpRewritePattern<IfOp> {
using OpRewritePattern<IfOp>::OpRewritePattern;
LogicalResult matchAndRewrite(IfOp ifOp,
PatternRewriter &rewriter) const override;
};
struct ParallelLowering : public OpRewritePattern<mlir::scf::ParallelOp> {
using OpRewritePattern<mlir::scf::ParallelOp>::OpRewritePattern;
LogicalResult matchAndRewrite(mlir::scf::ParallelOp parallelOp,
PatternRewriter &rewriter) const override;
};
} // namespace
LogicalResult ForLowering::matchAndRewrite(ForOp forOp,
PatternRewriter &rewriter) const {
Location loc = forOp.getLoc();
// Start by splitting the block containing the 'scf.for' into two parts.
// The part before will get the init code, the part after will be the end
// point.
auto *initBlock = rewriter.getInsertionBlock();
auto initPosition = rewriter.getInsertionPoint();
auto *endBlock = rewriter.splitBlock(initBlock, initPosition);
// Use the first block of the loop body as the condition block since it is the
// block that has the induction variable and loop-carried values as arguments.
// Split out all operations from the first block into a new block. Move all
// body blocks from the loop body region to the region containing the loop.
auto *conditionBlock = &forOp.region().front();
auto *firstBodyBlock =
rewriter.splitBlock(conditionBlock, conditionBlock->begin());
auto *lastBodyBlock = &forOp.region().back();
rewriter.inlineRegionBefore(forOp.region(), endBlock);
auto iv = conditionBlock->getArgument(0);
// Append the induction variable stepping logic to the last body block and
// branch back to the condition block. Loop-carried values are taken from
// operands of the loop terminator.
Operation *terminator = lastBodyBlock->getTerminator();
rewriter.setInsertionPointToEnd(lastBodyBlock);
auto step = forOp.step();
auto stepped = rewriter.create<AddIOp>(loc, iv, step).getResult();
if (!stepped)
return failure();
SmallVector<Value, 8> loopCarried;
loopCarried.push_back(stepped);
loopCarried.append(terminator->operand_begin(), terminator->operand_end());
rewriter.create<BranchOp>(loc, conditionBlock, loopCarried);
rewriter.eraseOp(terminator);
// Compute loop bounds before branching to the condition.
rewriter.setInsertionPointToEnd(initBlock);
Value lowerBound = forOp.lowerBound();
Value upperBound = forOp.upperBound();
if (!lowerBound || !upperBound)
return failure();
// The initial values of loop-carried values is obtained from the operands
// of the loop operation.
SmallVector<Value, 8> destOperands;
destOperands.push_back(lowerBound);
auto iterOperands = forOp.getIterOperands();
destOperands.append(iterOperands.begin(), iterOperands.end());
rewriter.create<BranchOp>(loc, conditionBlock, destOperands);
// With the body block done, we can fill in the condition block.
rewriter.setInsertionPointToEnd(conditionBlock);
auto comparison =
rewriter.create<CmpIOp>(loc, CmpIPredicate::slt, iv, upperBound);
rewriter.create<CondBranchOp>(loc, comparison, firstBodyBlock,
ArrayRef<Value>(), endBlock, ArrayRef<Value>());
// The result of the loop operation is the values of the condition block
// arguments except the induction variable on the last iteration.
rewriter.replaceOp(forOp, conditionBlock->getArguments().drop_front());
return success();
}
LogicalResult IfLowering::matchAndRewrite(IfOp ifOp,
PatternRewriter &rewriter) const {
auto loc = ifOp.getLoc();
// Start by splitting the block containing the 'scf.if' into two parts.
// The part before will contain the condition, the part after will be the
// continuation point.
auto *condBlock = rewriter.getInsertionBlock();
auto opPosition = rewriter.getInsertionPoint();
auto *remainingOpsBlock = rewriter.splitBlock(condBlock, opPosition);
Block *continueBlock;
if (ifOp.getNumResults() == 0) {
continueBlock = remainingOpsBlock;
} else {
continueBlock =
rewriter.createBlock(remainingOpsBlock, ifOp.getResultTypes());
rewriter.create<BranchOp>(loc, remainingOpsBlock);
}
// Move blocks from the "then" region to the region containing 'scf.if',
// place it before the continuation block, and branch to it.
auto &thenRegion = ifOp.thenRegion();
auto *thenBlock = &thenRegion.front();
Operation *thenTerminator = thenRegion.back().getTerminator();
ValueRange thenTerminatorOperands = thenTerminator->getOperands();
rewriter.setInsertionPointToEnd(&thenRegion.back());
rewriter.create<BranchOp>(loc, continueBlock, thenTerminatorOperands);
rewriter.eraseOp(thenTerminator);
rewriter.inlineRegionBefore(thenRegion, continueBlock);
// Move blocks from the "else" region (if present) to the region containing
// 'scf.if', place it before the continuation block and branch to it. It
// will be placed after the "then" regions.
auto *elseBlock = continueBlock;
auto &elseRegion = ifOp.elseRegion();
if (!elseRegion.empty()) {
elseBlock = &elseRegion.front();
Operation *elseTerminator = elseRegion.back().getTerminator();
ValueRange elseTerminatorOperands = elseTerminator->getOperands();
rewriter.setInsertionPointToEnd(&elseRegion.back());
rewriter.create<BranchOp>(loc, continueBlock, elseTerminatorOperands);
rewriter.eraseOp(elseTerminator);
rewriter.inlineRegionBefore(elseRegion, continueBlock);
}
rewriter.setInsertionPointToEnd(condBlock);
rewriter.create<CondBranchOp>(loc, ifOp.condition(), thenBlock,
/*trueArgs=*/ArrayRef<Value>(), elseBlock,
/*falseArgs=*/ArrayRef<Value>());
// Ok, we're done!
rewriter.replaceOp(ifOp, continueBlock->getArguments());
return success();
}
LogicalResult
ParallelLowering::matchAndRewrite(ParallelOp parallelOp,
PatternRewriter &rewriter) const {
Location loc = parallelOp.getLoc();
BlockAndValueMapping mapping;
// For a parallel loop, we essentially need to create an n-dimensional loop
// nest. We do this by translating to scf.for ops and have those lowered in
// a further rewrite. If a parallel loop contains reductions (and thus returns
// values), forward the initial values for the reductions down the loop
// hierarchy and bubble up the results by modifying the "yield" terminator.
SmallVector<Value, 4> iterArgs = llvm::to_vector<4>(parallelOp.initVals());
bool first = true;
SmallVector<Value, 4> loopResults(iterArgs);
for (auto loop_operands :
llvm::zip(parallelOp.getInductionVars(), parallelOp.lowerBound(),
parallelOp.upperBound(), parallelOp.step())) {
Value iv, lower, upper, step;
std::tie(iv, lower, upper, step) = loop_operands;
ForOp forOp = rewriter.create<ForOp>(loc, lower, upper, step, iterArgs);
mapping.map(iv, forOp.getInductionVar());
auto iterRange = forOp.getRegionIterArgs();
iterArgs.assign(iterRange.begin(), iterRange.end());
if (first) {
// Store the results of the outermost loop that will be used to replace
// the results of the parallel loop when it is fully rewritten.
loopResults.assign(forOp.result_begin(), forOp.result_end());
first = false;
} else if (!forOp.getResults().empty()) {
// A loop is constructed with an empty "yield" terminator if there are
// no results.
rewriter.setInsertionPointToEnd(rewriter.getInsertionBlock());
rewriter.create<YieldOp>(loc, forOp.getResults());
}
rewriter.setInsertionPointToStart(forOp.getBody());
}
// Now copy over the contents of the body.
SmallVector<Value, 4> yieldOperands;
yieldOperands.reserve(parallelOp.getNumResults());
for (auto &op : parallelOp.getBody()->without_terminator()) {
// Reduction blocks are handled differently.
auto reduce = dyn_cast<ReduceOp>(op);
if (!reduce) {
rewriter.clone(op, mapping);
continue;
}
// Clone the body of the reduction operation into the body of the loop,
// using operands of "scf.reduce" and iteration arguments corresponding
// to the reduction value to replace arguments of the reduction block.
// Collect operands of "scf.reduce.return" to be returned by a final
// "scf.yield" instead.
Value arg = iterArgs[yieldOperands.size()];
Block &reduceBlock = reduce.reductionOperator().front();
mapping.map(reduceBlock.getArgument(0), mapping.lookupOrDefault(arg));
mapping.map(reduceBlock.getArgument(1),
mapping.lookupOrDefault(reduce.operand()));
for (auto &nested : reduceBlock.without_terminator())
rewriter.clone(nested, mapping);
yieldOperands.push_back(
mapping.lookup(reduceBlock.getTerminator()->getOperand(0)));
}
if (!yieldOperands.empty()) {
rewriter.setInsertionPointToEnd(rewriter.getInsertionBlock());
rewriter.create<YieldOp>(loc, yieldOperands);
}
rewriter.replaceOp(parallelOp, loopResults);
return success();
}
void mlir::populateLoopToStdConversionPatterns(
OwningRewritePatternList &patterns, MLIRContext *ctx) {
patterns.insert<ForLowering, IfLowering, ParallelLowering>(ctx);
}
void SCFToStandardPass::runOnOperation() {
OwningRewritePatternList patterns;
populateLoopToStdConversionPatterns(patterns, &getContext());
// Configure conversion to lower out scf.for, scf.if and scf.parallel.
// Anything else is fine.
ConversionTarget target(getContext());
target.addIllegalOp<scf::ForOp, scf::IfOp, scf::ParallelOp>();
target.markUnknownOpDynamicallyLegal([](Operation *) { return true; });
if (failed(applyPartialConversion(getOperation(), target, patterns)))
signalPassFailure();
}
std::unique_ptr<Pass> mlir::createLowerToCFGPass() {
return std::make_unique<SCFToStandardPass>();
}
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