d613f5ab65
from it. This is necessary progress to squaring away the parent relationship that a StmtBlock has with its enclosing if/for/fn, and makes room for functions to have more than one block in the future. This also removes IfClause and ForStmtBody. This is step 5/n towards merging instructions and statements, NFC. PiperOrigin-RevId: 226936541
443 lines
18 KiB
C++
443 lines
18 KiB
C++
//===- LoopUtils.cpp ---- Misc utilities for loop transformation ----------===//
|
|
//
|
|
// Copyright 2019 The MLIR Authors.
|
|
//
|
|
// Licensed under the Apache License, Version 2.0 (the "License");
|
|
// you may not use this file except in compliance with the License.
|
|
// You may obtain a copy of the License at
|
|
//
|
|
// http://www.apache.org/licenses/LICENSE-2.0
|
|
//
|
|
// Unless required by applicable law or agreed to in writing, software
|
|
// distributed under the License is distributed on an "AS IS" BASIS,
|
|
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
|
|
// See the License for the specific language governing permissions and
|
|
// limitations under the License.
|
|
// =============================================================================
|
|
//
|
|
// This file implements miscellaneous loop transformation routines.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#include "mlir/Transforms/LoopUtils.h"
|
|
|
|
#include "mlir/Analysis/LoopAnalysis.h"
|
|
#include "mlir/IR/AffineExpr.h"
|
|
#include "mlir/IR/AffineMap.h"
|
|
#include "mlir/IR/Builders.h"
|
|
#include "mlir/IR/BuiltinOps.h"
|
|
#include "mlir/IR/Statements.h"
|
|
#include "mlir/IR/StmtVisitor.h"
|
|
#include "mlir/StandardOps/StandardOps.h"
|
|
#include "llvm/ADT/DenseMap.h"
|
|
#include "llvm/Support/Debug.h"
|
|
|
|
#define DEBUG_TYPE "LoopUtils"
|
|
|
|
using namespace mlir;
|
|
|
|
/// Returns the upper bound of an unrolled loop with lower bound 'lb' and with
|
|
/// the specified trip count, stride, and unroll factor. Returns nullptr when
|
|
/// the trip count can't be expressed as an affine expression.
|
|
AffineMap mlir::getUnrolledLoopUpperBound(const ForStmt &forStmt,
|
|
unsigned unrollFactor,
|
|
MLFuncBuilder *builder) {
|
|
auto lbMap = forStmt.getLowerBoundMap();
|
|
|
|
// Single result lower bound map only.
|
|
if (lbMap.getNumResults() != 1)
|
|
return AffineMap::Null();
|
|
|
|
// Sometimes, the trip count cannot be expressed as an affine expression.
|
|
auto tripCount = getTripCountExpr(forStmt);
|
|
if (!tripCount)
|
|
return AffineMap::Null();
|
|
|
|
AffineExpr lb(lbMap.getResult(0));
|
|
unsigned step = forStmt.getStep();
|
|
auto newUb = lb + (tripCount - tripCount % unrollFactor - 1) * step;
|
|
|
|
return builder->getAffineMap(lbMap.getNumDims(), lbMap.getNumSymbols(),
|
|
{newUb}, {});
|
|
}
|
|
|
|
/// Returns the lower bound of the cleanup loop when unrolling a loop with lower
|
|
/// bound 'lb' and with the specified trip count, stride, and unroll factor.
|
|
/// Returns an AffinMap with nullptr storage (that evaluates to false)
|
|
/// when the trip count can't be expressed as an affine expression.
|
|
AffineMap mlir::getCleanupLoopLowerBound(const ForStmt &forStmt,
|
|
unsigned unrollFactor,
|
|
MLFuncBuilder *builder) {
|
|
auto lbMap = forStmt.getLowerBoundMap();
|
|
|
|
// Single result lower bound map only.
|
|
if (lbMap.getNumResults() != 1)
|
|
return AffineMap::Null();
|
|
|
|
// Sometimes the trip count cannot be expressed as an affine expression.
|
|
AffineExpr tripCount(getTripCountExpr(forStmt));
|
|
if (!tripCount)
|
|
return AffineMap::Null();
|
|
|
|
AffineExpr lb(lbMap.getResult(0));
|
|
unsigned step = forStmt.getStep();
|
|
auto newLb = lb + (tripCount - tripCount % unrollFactor) * step;
|
|
return builder->getAffineMap(lbMap.getNumDims(), lbMap.getNumSymbols(),
|
|
{newLb}, {});
|
|
}
|
|
|
|
/// Promotes the loop body of a forStmt to its containing block if the forStmt
|
|
/// was known to have a single iteration. Returns false otherwise.
|
|
// TODO(bondhugula): extend this for arbitrary affine bounds.
|
|
bool mlir::promoteIfSingleIteration(ForStmt *forStmt) {
|
|
Optional<uint64_t> tripCount = getConstantTripCount(*forStmt);
|
|
if (!tripCount.hasValue() || tripCount.getValue() != 1)
|
|
return false;
|
|
|
|
// TODO(mlir-team): there is no builder for a max.
|
|
if (forStmt->getLowerBoundMap().getNumResults() != 1)
|
|
return false;
|
|
|
|
// Replaces all IV uses to its single iteration value.
|
|
if (!forStmt->use_empty()) {
|
|
if (forStmt->hasConstantLowerBound()) {
|
|
auto *mlFunc = forStmt->findFunction();
|
|
MLFuncBuilder topBuilder(&mlFunc->getBody()->front());
|
|
auto constOp = topBuilder.create<ConstantIndexOp>(
|
|
forStmt->getLoc(), forStmt->getConstantLowerBound());
|
|
forStmt->replaceAllUsesWith(constOp);
|
|
} else {
|
|
const AffineBound lb = forStmt->getLowerBound();
|
|
SmallVector<SSAValue *, 4> lbOperands(lb.operand_begin(),
|
|
lb.operand_end());
|
|
MLFuncBuilder builder(forStmt->getBlock(), StmtBlock::iterator(forStmt));
|
|
auto affineApplyOp = builder.create<AffineApplyOp>(
|
|
forStmt->getLoc(), lb.getMap(), lbOperands);
|
|
forStmt->replaceAllUsesWith(affineApplyOp->getResult(0));
|
|
}
|
|
}
|
|
// Move the loop body statements to the loop's containing block.
|
|
auto *block = forStmt->getBlock();
|
|
block->getStatements().splice(StmtBlock::iterator(forStmt),
|
|
forStmt->getBody()->getStatements());
|
|
forStmt->erase();
|
|
return true;
|
|
}
|
|
|
|
/// Promotes all single iteration for stmt's in the MLFunction, i.e., moves
|
|
/// their body into the containing StmtBlock.
|
|
void mlir::promoteSingleIterationLoops(MLFunction *f) {
|
|
// Gathers all innermost loops through a post order pruned walk.
|
|
class LoopBodyPromoter : public StmtWalker<LoopBodyPromoter> {
|
|
public:
|
|
void visitForStmt(ForStmt *forStmt) { promoteIfSingleIteration(forStmt); }
|
|
};
|
|
|
|
LoopBodyPromoter fsw;
|
|
fsw.walkPostOrder(f);
|
|
}
|
|
|
|
/// Generates a 'for' stmt with the specified lower and upper bounds while
|
|
/// generating the right IV remappings for the shifted statements. The
|
|
/// statement blocks that go into the loop are specified in stmtGroupQueue
|
|
/// starting from the specified offset, and in that order; the first element of
|
|
/// the pair specifies the shift applied to that group of statements; note that
|
|
/// the shift is multiplied by the loop step before being applied. Returns
|
|
/// nullptr if the generated loop simplifies to a single iteration one.
|
|
static ForStmt *
|
|
generateLoop(AffineMap lbMap, AffineMap ubMap,
|
|
const std::vector<std::pair<uint64_t, ArrayRef<Statement *>>>
|
|
&stmtGroupQueue,
|
|
unsigned offset, ForStmt *srcForStmt, MLFuncBuilder *b) {
|
|
SmallVector<MLValue *, 4> lbOperands(srcForStmt->getLowerBoundOperands());
|
|
SmallVector<MLValue *, 4> ubOperands(srcForStmt->getUpperBoundOperands());
|
|
|
|
assert(lbMap.getNumInputs() == lbOperands.size());
|
|
assert(ubMap.getNumInputs() == ubOperands.size());
|
|
|
|
auto *loopChunk = b->createFor(srcForStmt->getLoc(), lbOperands, lbMap,
|
|
ubOperands, ubMap, srcForStmt->getStep());
|
|
|
|
OperationStmt::OperandMapTy operandMap;
|
|
|
|
for (auto it = stmtGroupQueue.begin() + offset, e = stmtGroupQueue.end();
|
|
it != e; ++it) {
|
|
uint64_t shift = it->first;
|
|
auto stmts = it->second;
|
|
// All 'same shift' statements get added with their operands being remapped
|
|
// to results of cloned statements, and their IV used remapped.
|
|
// Generate the remapping if the shift is not zero: remappedIV = newIV -
|
|
// shift.
|
|
if (!srcForStmt->use_empty() && shift != 0) {
|
|
auto b = MLFuncBuilder::getForStmtBodyBuilder(loopChunk);
|
|
auto *ivRemap = b.create<AffineApplyOp>(
|
|
srcForStmt->getLoc(),
|
|
b.getSingleDimShiftAffineMap(-static_cast<int64_t>(
|
|
srcForStmt->getStep() * shift)),
|
|
loopChunk)
|
|
->getResult(0);
|
|
operandMap[srcForStmt] = cast<MLValue>(ivRemap);
|
|
} else {
|
|
operandMap[srcForStmt] = loopChunk;
|
|
}
|
|
for (auto *stmt : stmts) {
|
|
loopChunk->getBody()->push_back(stmt->clone(operandMap, b->getContext()));
|
|
}
|
|
}
|
|
if (promoteIfSingleIteration(loopChunk))
|
|
return nullptr;
|
|
return loopChunk;
|
|
}
|
|
|
|
/// Skew the statements in the body of a 'for' statement with the specified
|
|
/// statement-wise shifts. The shifts are with respect to the original execution
|
|
/// order, and are multiplied by the loop 'step' before being applied. A shift
|
|
/// of zero for each statement will lead to no change.
|
|
// The skewing of statements with respect to one another can be used for example
|
|
// to allow overlap of asynchronous operations (such as DMA communication) with
|
|
// computation, or just relative shifting of statements for better register
|
|
// reuse, locality or parallelism. As such, the shifts are typically expected to
|
|
// be at most of the order of the number of statements. This method should not
|
|
// be used as a substitute for loop distribution/fission.
|
|
// This method uses an algorithm// in time linear in the number of statements in
|
|
// the body of the for loop - (using the 'sweep line' paradigm). This method
|
|
// asserts preservation of SSA dominance. A check for that as well as that for
|
|
// memory-based depedence preservation check rests with the users of this
|
|
// method.
|
|
UtilResult mlir::stmtBodySkew(ForStmt *forStmt, ArrayRef<uint64_t> shifts,
|
|
bool unrollPrologueEpilogue) {
|
|
if (forStmt->getBody()->empty())
|
|
return UtilResult::Success;
|
|
|
|
// If the trip counts aren't constant, we would need versioning and
|
|
// conditional guards (or context information to prevent such versioning). The
|
|
// better way to pipeline for such loops is to first tile them and extract
|
|
// constant trip count "full tiles" before applying this.
|
|
auto mayBeConstTripCount = getConstantTripCount(*forStmt);
|
|
if (!mayBeConstTripCount.hasValue()) {
|
|
LLVM_DEBUG(llvm::dbgs() << "non-constant trip count loop\n";);
|
|
return UtilResult::Success;
|
|
}
|
|
uint64_t tripCount = mayBeConstTripCount.getValue();
|
|
|
|
assert(isStmtwiseShiftValid(*forStmt, shifts) &&
|
|
"shifts will lead to an invalid transformation\n");
|
|
|
|
int64_t step = forStmt->getStep();
|
|
|
|
unsigned numChildStmts = forStmt->getBody()->getStatements().size();
|
|
|
|
// Do a linear time (counting) sort for the shifts.
|
|
uint64_t maxShift = 0;
|
|
for (unsigned i = 0; i < numChildStmts; i++) {
|
|
maxShift = std::max(maxShift, shifts[i]);
|
|
}
|
|
// Such large shifts are not the typical use case.
|
|
if (maxShift >= numChildStmts) {
|
|
LLVM_DEBUG(llvm::dbgs() << "stmt shifts too large - unexpected\n";);
|
|
return UtilResult::Success;
|
|
}
|
|
|
|
// An array of statement groups sorted by shift amount; each group has all
|
|
// statements with the same shift in the order in which they appear in the
|
|
// body of the 'for' stmt.
|
|
std::vector<std::vector<Statement *>> sortedStmtGroups(maxShift + 1);
|
|
unsigned pos = 0;
|
|
for (auto &stmt : *forStmt->getBody()) {
|
|
auto shift = shifts[pos++];
|
|
sortedStmtGroups[shift].push_back(&stmt);
|
|
}
|
|
|
|
// Unless the shifts have a specific pattern (which actually would be the
|
|
// common use case), prologue and epilogue are not meaningfully defined.
|
|
// Nevertheless, if 'unrollPrologueEpilogue' is set, we will treat the first
|
|
// loop generated as the prologue and the last as epilogue and unroll these
|
|
// fully.
|
|
ForStmt *prologue = nullptr;
|
|
ForStmt *epilogue = nullptr;
|
|
|
|
// Do a sweep over the sorted shifts while storing open groups in a
|
|
// vector, and generating loop portions as necessary during the sweep. A block
|
|
// of statements is paired with its shift.
|
|
std::vector<std::pair<uint64_t, ArrayRef<Statement *>>> stmtGroupQueue;
|
|
|
|
auto origLbMap = forStmt->getLowerBoundMap();
|
|
uint64_t lbShift = 0;
|
|
MLFuncBuilder b(forStmt);
|
|
for (uint64_t d = 0, e = sortedStmtGroups.size(); d < e; ++d) {
|
|
// If nothing is shifted by d, continue.
|
|
if (sortedStmtGroups[d].empty())
|
|
continue;
|
|
if (!stmtGroupQueue.empty()) {
|
|
assert(d >= 1 &&
|
|
"Queue expected to be empty when the first block is found");
|
|
// The interval for which the loop needs to be generated here is:
|
|
// [lbShift, min(lbShift + tripCount, d)) and the body of the
|
|
// loop needs to have all statements in stmtQueue in that order.
|
|
ForStmt *res;
|
|
if (lbShift + tripCount * step < d * step) {
|
|
res = generateLoop(
|
|
b.getShiftedAffineMap(origLbMap, lbShift),
|
|
b.getShiftedAffineMap(origLbMap, lbShift + tripCount * step),
|
|
stmtGroupQueue, 0, forStmt, &b);
|
|
// Entire loop for the queued stmt groups generated, empty it.
|
|
stmtGroupQueue.clear();
|
|
lbShift += tripCount * step;
|
|
} else {
|
|
res = generateLoop(b.getShiftedAffineMap(origLbMap, lbShift),
|
|
b.getShiftedAffineMap(origLbMap, d), stmtGroupQueue,
|
|
0, forStmt, &b);
|
|
lbShift = d * step;
|
|
}
|
|
if (!prologue && res)
|
|
prologue = res;
|
|
epilogue = res;
|
|
} else {
|
|
// Start of first interval.
|
|
lbShift = d * step;
|
|
}
|
|
// Augment the list of statements that get into the current open interval.
|
|
stmtGroupQueue.push_back({d, sortedStmtGroups[d]});
|
|
}
|
|
|
|
// Those statements groups left in the queue now need to be processed (FIFO)
|
|
// and their loops completed.
|
|
for (unsigned i = 0, e = stmtGroupQueue.size(); i < e; ++i) {
|
|
uint64_t ubShift = (stmtGroupQueue[i].first + tripCount) * step;
|
|
epilogue = generateLoop(b.getShiftedAffineMap(origLbMap, lbShift),
|
|
b.getShiftedAffineMap(origLbMap, ubShift),
|
|
stmtGroupQueue, i, forStmt, &b);
|
|
lbShift = ubShift;
|
|
if (!prologue)
|
|
prologue = epilogue;
|
|
}
|
|
|
|
// Erase the original for stmt.
|
|
forStmt->erase();
|
|
|
|
if (unrollPrologueEpilogue && prologue)
|
|
loopUnrollFull(prologue);
|
|
if (unrollPrologueEpilogue && !epilogue && epilogue != prologue)
|
|
loopUnrollFull(epilogue);
|
|
|
|
return UtilResult::Success;
|
|
}
|
|
|
|
/// Unrolls this loop completely.
|
|
bool mlir::loopUnrollFull(ForStmt *forStmt) {
|
|
Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(*forStmt);
|
|
if (mayBeConstantTripCount.hasValue()) {
|
|
uint64_t tripCount = mayBeConstantTripCount.getValue();
|
|
if (tripCount == 1) {
|
|
return promoteIfSingleIteration(forStmt);
|
|
}
|
|
return loopUnrollByFactor(forStmt, tripCount);
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/// Unrolls and jams this loop by the specified factor or by the trip count (if
|
|
/// constant) whichever is lower.
|
|
bool mlir::loopUnrollUpToFactor(ForStmt *forStmt, uint64_t unrollFactor) {
|
|
Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(*forStmt);
|
|
|
|
if (mayBeConstantTripCount.hasValue() &&
|
|
mayBeConstantTripCount.getValue() < unrollFactor)
|
|
return loopUnrollByFactor(forStmt, mayBeConstantTripCount.getValue());
|
|
return loopUnrollByFactor(forStmt, unrollFactor);
|
|
}
|
|
|
|
/// Unrolls this loop by the specified factor. Returns true if the loop
|
|
/// is successfully unrolled.
|
|
bool mlir::loopUnrollByFactor(ForStmt *forStmt, uint64_t unrollFactor) {
|
|
assert(unrollFactor >= 1 && "unroll factor should be >= 1");
|
|
|
|
if (unrollFactor == 1 || forStmt->getBody()->empty())
|
|
return false;
|
|
|
|
auto lbMap = forStmt->getLowerBoundMap();
|
|
auto ubMap = forStmt->getUpperBoundMap();
|
|
|
|
// Loops with max/min expressions won't be unrolled here (the output can't be
|
|
// expressed as an MLFunction in the general case). However, the right way to
|
|
// do such unrolling for an MLFunction would be to specialize the loop for the
|
|
// 'hotspot' case and unroll that hotspot.
|
|
if (lbMap.getNumResults() != 1 || ubMap.getNumResults() != 1)
|
|
return false;
|
|
|
|
// Same operand list for lower and upper bound for now.
|
|
// TODO(bondhugula): handle bounds with different operand lists.
|
|
if (!forStmt->matchingBoundOperandList())
|
|
return false;
|
|
|
|
Optional<uint64_t> mayBeConstantTripCount = getConstantTripCount(*forStmt);
|
|
|
|
// If the trip count is lower than the unroll factor, no unrolled body.
|
|
// TODO(bondhugula): option to specify cleanup loop unrolling.
|
|
if (mayBeConstantTripCount.hasValue() &&
|
|
mayBeConstantTripCount.getValue() < unrollFactor)
|
|
return false;
|
|
|
|
// Generate the cleanup loop if trip count isn't a multiple of unrollFactor.
|
|
if (getLargestDivisorOfTripCount(*forStmt) % unrollFactor != 0) {
|
|
DenseMap<const MLValue *, MLValue *> operandMap;
|
|
MLFuncBuilder builder(forStmt->getBlock(), ++StmtBlock::iterator(forStmt));
|
|
auto *cleanupForStmt = cast<ForStmt>(builder.clone(*forStmt, operandMap));
|
|
auto clLbMap = getCleanupLoopLowerBound(*forStmt, unrollFactor, &builder);
|
|
assert(clLbMap &&
|
|
"cleanup loop lower bound map for single result bound maps can "
|
|
"always be determined");
|
|
cleanupForStmt->setLowerBoundMap(clLbMap);
|
|
// Promote the loop body up if this has turned into a single iteration loop.
|
|
promoteIfSingleIteration(cleanupForStmt);
|
|
|
|
// Adjust upper bound.
|
|
auto unrolledUbMap =
|
|
getUnrolledLoopUpperBound(*forStmt, unrollFactor, &builder);
|
|
assert(unrolledUbMap &&
|
|
"upper bound map can alwayys be determined for an unrolled loop "
|
|
"with single result bounds");
|
|
forStmt->setUpperBoundMap(unrolledUbMap);
|
|
}
|
|
|
|
// Scale the step of loop being unrolled by unroll factor.
|
|
int64_t step = forStmt->getStep();
|
|
forStmt->setStep(step * unrollFactor);
|
|
|
|
// Builder to insert unrolled bodies right after the last statement in the
|
|
// body of 'forStmt'.
|
|
MLFuncBuilder builder(forStmt->getBody(), forStmt->getBody()->end());
|
|
|
|
// Keep a pointer to the last statement in the original block so that we know
|
|
// what to clone (since we are doing this in-place).
|
|
StmtBlock::iterator srcBlockEnd = std::prev(forStmt->getBody()->end());
|
|
|
|
// Unroll the contents of 'forStmt' (append unrollFactor-1 additional copies).
|
|
for (unsigned i = 1; i < unrollFactor; i++) {
|
|
DenseMap<const MLValue *, MLValue *> operandMap;
|
|
|
|
// If the induction variable is used, create a remapping to the value for
|
|
// this unrolled instance.
|
|
if (!forStmt->use_empty()) {
|
|
// iv' = iv + 1/2/3...unrollFactor-1;
|
|
auto d0 = builder.getAffineDimExpr(0);
|
|
auto bumpMap = builder.getAffineMap(1, 0, {d0 + i * step}, {});
|
|
auto *ivUnroll =
|
|
builder.create<AffineApplyOp>(forStmt->getLoc(), bumpMap, forStmt)
|
|
->getResult(0);
|
|
operandMap[forStmt] = cast<MLValue>(ivUnroll);
|
|
}
|
|
|
|
// Clone the original body of 'forStmt'.
|
|
for (auto it = forStmt->getBody()->begin(); it != std::next(srcBlockEnd);
|
|
it++) {
|
|
builder.clone(*it, operandMap);
|
|
}
|
|
}
|
|
|
|
// Promote the loop body up if this has turned into a single iteration loop.
|
|
promoteIfSingleIteration(forStmt);
|
|
|
|
return true;
|
|
}
|