1//===- LoopSpecialization.cpp - scf.parallel/SCR.for specialization -------===//
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// Specializes parallel loops and for loops for easier unrolling and
10// vectorization.
11//
12//===----------------------------------------------------------------------===//
13
14#include "mlir/Dialect/SCF/Transforms/Passes.h"
15
16#include "mlir/Dialect/Affine/Analysis/AffineStructures.h"
17#include "mlir/Dialect/Affine/IR/AffineOps.h"
18#include "mlir/Dialect/Arith/IR/Arith.h"
19#include "mlir/Dialect/SCF/IR/SCF.h"
20#include "mlir/Dialect/SCF/Transforms/Transforms.h"
21#include "mlir/Dialect/SCF/Utils/AffineCanonicalizationUtils.h"
22#include "mlir/Dialect/Utils/StaticValueUtils.h"
23#include "mlir/IR/AffineExpr.h"
24#include "mlir/IR/IRMapping.h"
25#include "mlir/IR/PatternMatch.h"
26#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
27#include "llvm/ADT/DenseMap.h"
28
29namespace mlir {
30#define GEN_PASS_DEF_SCFFORLOOPPEELING
31#define GEN_PASS_DEF_SCFFORLOOPSPECIALIZATION
32#define GEN_PASS_DEF_SCFPARALLELLOOPSPECIALIZATION
33#include "mlir/Dialect/SCF/Transforms/Passes.h.inc"
34} // namespace mlir
35
36using namespace mlir;
37using namespace mlir::affine;
38using scf::ForOp;
39using scf::ParallelOp;
40
41/// Rewrite a parallel loop with bounds defined by an affine.min with a constant
42/// into 2 loops after checking if the bounds are equal to that constant. This
43/// is beneficial if the loop will almost always have the constant bound and
44/// that version can be fully unrolled and vectorized.
45static void specializeParallelLoopForUnrolling(ParallelOp op) {
46 SmallVector<int64_t, 2> constantIndices;
47 constantIndices.reserve(op.getUpperBound().size());
48 for (auto bound : op.getUpperBound()) {
49 auto minOp = bound.getDefiningOp<AffineMinOp>();
50 if (!minOp)
51 return;
52 int64_t minConstant = std::numeric_limits<int64_t>::max();
53 for (AffineExpr expr : minOp.getMap().getResults()) {
54 if (auto constantIndex = dyn_cast<AffineConstantExpr>(expr))
55 minConstant = std::min(minConstant, constantIndex.getValue());
56 }
57 if (minConstant == std::numeric_limits<int64_t>::max())
58 return;
59 constantIndices.push_back(minConstant);
60 }
61
62 OpBuilder b(op);
63 IRMapping map;
64 Value cond;
65 for (auto bound : llvm::zip(op.getUpperBound(), constantIndices)) {
66 Value constant =
67 b.create<arith::ConstantIndexOp>(op.getLoc(), std::get<1>(bound));
68 Value cmp = b.create<arith::CmpIOp>(op.getLoc(), arith::CmpIPredicate::eq,
69 std::get<0>(bound), constant);
70 cond = cond ? b.create<arith::AndIOp>(op.getLoc(), cond, cmp) : cmp;
71 map.map(std::get<0>(bound), constant);
72 }
73 auto ifOp = b.create<scf::IfOp>(op.getLoc(), cond, /*withElseRegion=*/true);
74 ifOp.getThenBodyBuilder().clone(*op.getOperation(), map);
75 ifOp.getElseBodyBuilder().clone(*op.getOperation());
76 op.erase();
77}
78
79/// Rewrite a for loop with bounds defined by an affine.min with a constant into
80/// 2 loops after checking if the bounds are equal to that constant. This is
81/// beneficial if the loop will almost always have the constant bound and that
82/// version can be fully unrolled and vectorized.
83static void specializeForLoopForUnrolling(ForOp op) {
84 auto bound = op.getUpperBound();
85 auto minOp = bound.getDefiningOp<AffineMinOp>();
86 if (!minOp)
87 return;
88 int64_t minConstant = std::numeric_limits<int64_t>::max();
89 for (AffineExpr expr : minOp.getMap().getResults()) {
90 if (auto constantIndex = dyn_cast<AffineConstantExpr>(expr))
91 minConstant = std::min(minConstant, constantIndex.getValue());
92 }
93 if (minConstant == std::numeric_limits<int64_t>::max())
94 return;
95
96 OpBuilder b(op);
97 IRMapping map;
98 Value constant = b.create<arith::ConstantIndexOp>(op.getLoc(), minConstant);
99 Value cond = b.create<arith::CmpIOp>(op.getLoc(), arith::CmpIPredicate::eq,
100 bound, constant);
101 map.map(bound, constant);
102 auto ifOp = b.create<scf::IfOp>(op.getLoc(), cond, /*withElseRegion=*/true);
103 ifOp.getThenBodyBuilder().clone(*op.getOperation(), map);
104 ifOp.getElseBodyBuilder().clone(*op.getOperation());
105 op.erase();
106}
107
108/// Rewrite a for loop with bounds/step that potentially do not divide evenly
109/// into a for loop where the step divides the iteration space evenly, followed
110/// by an scf.if for the last (partial) iteration (if any).
111///
112/// This function rewrites the given scf.for loop in-place and creates a new
113/// scf.if operation for the last iteration. It replaces all uses of the
114/// unpeeled loop with the results of the newly generated scf.if.
115///
116/// The newly generated scf.if operation is returned via `ifOp`. The boundary
117/// at which the loop is split (new upper bound) is returned via `splitBound`.
118/// The return value indicates whether the loop was rewritten or not.
119static LogicalResult peelForLoop(RewriterBase &b, ForOp forOp,
120 ForOp &partialIteration, Value &splitBound) {
121 RewriterBase::InsertionGuard guard(b);
122 auto lbInt = getConstantIntValue(forOp.getLowerBound());
123 auto ubInt = getConstantIntValue(forOp.getUpperBound());
124 auto stepInt = getConstantIntValue(forOp.getStep());
125
126 // No specialization necessary if step size is 1. Also bail out in case of an
127 // invalid zero or negative step which might have happened during folding.
128 if (stepInt && *stepInt <= 1)
129 return failure();
130
131 // No specialization necessary if step already divides upper bound evenly.
132 // Fast path: lb, ub and step are constants.
133 if (lbInt && ubInt && stepInt && (*ubInt - *lbInt) % *stepInt == 0)
134 return failure();
135 // Slow path: Examine the ops that define lb, ub and step.
136 AffineExpr sym0, sym1, sym2;
137 bindSymbols(ctx: b.getContext(), exprs&: sym0, exprs&: sym1, exprs&: sym2);
138 SmallVector<Value> operands{forOp.getLowerBound(), forOp.getUpperBound(),
139 forOp.getStep()};
140 AffineMap map = AffineMap::get(dimCount: 0, symbolCount: 3, result: {(sym1 - sym0) % sym2});
141 affine::fullyComposeAffineMapAndOperands(map: &map, operands: &operands);
142 if (auto constExpr = dyn_cast<AffineConstantExpr>(Val: map.getResult(idx: 0)))
143 if (constExpr.getValue() == 0)
144 return failure();
145
146 // New upper bound: %ub - (%ub - %lb) mod %step
147 auto modMap = AffineMap::get(dimCount: 0, symbolCount: 3, result: {sym1 - ((sym1 - sym0) % sym2)});
148 b.setInsertionPoint(forOp);
149 auto loc = forOp.getLoc();
150 splitBound = b.createOrFold<AffineApplyOp>(loc, modMap,
151 ValueRange{forOp.getLowerBound(),
152 forOp.getUpperBound(),
153 forOp.getStep()});
154
155 // Create ForOp for partial iteration.
156 b.setInsertionPointAfter(forOp);
157 partialIteration = cast<ForOp>(b.clone(*forOp.getOperation()));
158 partialIteration.getLowerBoundMutable().assign(splitBound);
159 b.replaceAllUsesWith(forOp.getResults(), partialIteration->getResults());
160 partialIteration.getInitArgsMutable().assign(forOp->getResults());
161
162 // Set new upper loop bound.
163 b.modifyOpInPlace(forOp,
164 [&]() { forOp.getUpperBoundMutable().assign(splitBound); });
165
166 return success();
167}
168
169static void rewriteAffineOpAfterPeeling(RewriterBase &rewriter, ForOp forOp,
170 ForOp partialIteration,
171 Value previousUb) {
172 Value mainIv = forOp.getInductionVar();
173 Value partialIv = partialIteration.getInductionVar();
174 assert(forOp.getStep() == partialIteration.getStep() &&
175 "expected same step in main and partial loop");
176 Value step = forOp.getStep();
177
178 forOp.walk([&](Operation *affineOp) {
179 if (!isa<AffineMinOp, AffineMaxOp>(Val: affineOp))
180 return WalkResult::advance();
181 (void)scf::rewritePeeledMinMaxOp(rewriter, op: affineOp, iv: mainIv, ub: previousUb,
182 step,
183 /*insideLoop=*/true);
184 return WalkResult::advance();
185 });
186 partialIteration.walk([&](Operation *affineOp) {
187 if (!isa<AffineMinOp, AffineMaxOp>(Val: affineOp))
188 return WalkResult::advance();
189 (void)scf::rewritePeeledMinMaxOp(rewriter, op: affineOp, iv: partialIv, ub: previousUb,
190 step, /*insideLoop=*/false);
191 return WalkResult::advance();
192 });
193}
194
195LogicalResult mlir::scf::peelForLoopAndSimplifyBounds(RewriterBase &rewriter,
196 ForOp forOp,
197 ForOp &partialIteration) {
198 Value previousUb = forOp.getUpperBound();
199 Value splitBound;
200 if (failed(peelForLoop(rewriter, forOp, partialIteration, splitBound)))
201 return failure();
202
203 // Rewrite affine.min and affine.max ops.
204 rewriteAffineOpAfterPeeling(rewriter, forOp, partialIteration, previousUb);
205
206 return success();
207}
208
209/// Rewrites the original scf::ForOp as two scf::ForOp Ops, the first
210/// scf::ForOp corresponds to the first iteration of the loop which can be
211/// canonicalized away in the following optimizations. The second loop Op
212/// contains the remaining iterations, with a lower bound updated as the
213/// original lower bound plus the step (i.e. skips the first iteration).
214LogicalResult mlir::scf::peelForLoopFirstIteration(RewriterBase &b, ForOp forOp,
215 ForOp &firstIteration) {
216 RewriterBase::InsertionGuard guard(b);
217 auto lbInt = getConstantIntValue(forOp.getLowerBound());
218 auto ubInt = getConstantIntValue(forOp.getUpperBound());
219 auto stepInt = getConstantIntValue(forOp.getStep());
220
221 // Peeling is not needed if there is one or less iteration.
222 if (lbInt && ubInt && stepInt && ceil(float(*ubInt - *lbInt) / *stepInt) <= 1)
223 return failure();
224
225 AffineExpr lbSymbol, stepSymbol;
226 bindSymbols(ctx: b.getContext(), exprs&: lbSymbol, exprs&: stepSymbol);
227
228 // New lower bound for main loop: %lb + %step
229 auto ubMap = AffineMap::get(dimCount: 0, symbolCount: 2, result: {lbSymbol + stepSymbol});
230 b.setInsertionPoint(forOp);
231 auto loc = forOp.getLoc();
232 Value splitBound = b.createOrFold<AffineApplyOp>(
233 loc, ubMap, ValueRange{forOp.getLowerBound(), forOp.getStep()});
234
235 // Peel the first iteration.
236 IRMapping map;
237 map.map(forOp.getUpperBound(), splitBound);
238 firstIteration = cast<ForOp>(b.clone(*forOp.getOperation(), map));
239
240 // Update main loop with new lower bound.
241 b.modifyOpInPlace(forOp, [&]() {
242 forOp.getInitArgsMutable().assign(firstIteration->getResults());
243 forOp.getLowerBoundMutable().assign(splitBound);
244 });
245
246 return success();
247}
248
249static constexpr char kPeeledLoopLabel[] = "__peeled_loop__";
250static constexpr char kPartialIterationLabel[] = "__partial_iteration__";
251
252namespace {
253struct ForLoopPeelingPattern : public OpRewritePattern<ForOp> {
254 ForLoopPeelingPattern(MLIRContext *ctx, bool peelFront, bool skipPartial)
255 : OpRewritePattern<ForOp>(ctx), peelFront(peelFront),
256 skipPartial(skipPartial) {}
257
258 LogicalResult matchAndRewrite(ForOp forOp,
259 PatternRewriter &rewriter) const override {
260 // Do not peel already peeled loops.
261 if (forOp->hasAttr(kPeeledLoopLabel))
262 return failure();
263
264 scf::ForOp partialIteration;
265 // The case for peeling the first iteration of the loop.
266 if (peelFront) {
267 if (failed(
268 peelForLoopFirstIteration(rewriter, forOp, partialIteration))) {
269 return failure();
270 }
271 } else {
272 if (skipPartial) {
273 // No peeling of loops inside the partial iteration of another peeled
274 // loop.
275 Operation *op = forOp.getOperation();
276 while ((op = op->getParentOfType<scf::ForOp>())) {
277 if (op->hasAttr(name: kPartialIterationLabel))
278 return failure();
279 }
280 }
281 // Apply loop peeling.
282 if (failed(
283 peelForLoopAndSimplifyBounds(rewriter, forOp, partialIteration)))
284 return failure();
285 }
286
287 // Apply label, so that the same loop is not rewritten a second time.
288 rewriter.modifyOpInPlace(partialIteration, [&]() {
289 partialIteration->setAttr(kPeeledLoopLabel, rewriter.getUnitAttr());
290 partialIteration->setAttr(kPartialIterationLabel, rewriter.getUnitAttr());
291 });
292 rewriter.modifyOpInPlace(forOp, [&]() {
293 forOp->setAttr(kPeeledLoopLabel, rewriter.getUnitAttr());
294 });
295 return success();
296 }
297
298 // If set to true, the first iteration of the loop will be peeled. Otherwise,
299 // the unevenly divisible loop will be peeled at the end.
300 bool peelFront;
301
302 /// If set to true, loops inside partial iterations of another peeled loop
303 /// are not peeled. This reduces the size of the generated code. Partial
304 /// iterations are not usually performance critical.
305 /// Note: Takes into account the entire chain of parent operations, not just
306 /// the direct parent.
307 bool skipPartial;
308};
309} // namespace
310
311namespace {
312struct ParallelLoopSpecialization
313 : public impl::SCFParallelLoopSpecializationBase<
314 ParallelLoopSpecialization> {
315 void runOnOperation() override {
316 getOperation()->walk(
317 [](ParallelOp op) { specializeParallelLoopForUnrolling(op); });
318 }
319};
320
321struct ForLoopSpecialization
322 : public impl::SCFForLoopSpecializationBase<ForLoopSpecialization> {
323 void runOnOperation() override {
324 getOperation()->walk([](ForOp op) { specializeForLoopForUnrolling(op); });
325 }
326};
327
328struct ForLoopPeeling : public impl::SCFForLoopPeelingBase<ForLoopPeeling> {
329 void runOnOperation() override {
330 auto *parentOp = getOperation();
331 MLIRContext *ctx = parentOp->getContext();
332 RewritePatternSet patterns(ctx);
333 patterns.add<ForLoopPeelingPattern>(ctx, peelFront, skipPartial);
334 (void)applyPatternsGreedily(parentOp, std::move(patterns));
335
336 // Drop the markers.
337 parentOp->walk([](Operation *op) {
338 op->removeAttr(name: kPeeledLoopLabel);
339 op->removeAttr(name: kPartialIterationLabel);
340 });
341 }
342};
343} // namespace
344
345std::unique_ptr<Pass> mlir::createParallelLoopSpecializationPass() {
346 return std::make_unique<ParallelLoopSpecialization>();
347}
348
349std::unique_ptr<Pass> mlir::createForLoopSpecializationPass() {
350 return std::make_unique<ForLoopSpecialization>();
351}
352
353std::unique_ptr<Pass> mlir::createForLoopPeelingPass() {
354 return std::make_unique<ForLoopPeeling>();
355}
356

source code of mlir/lib/Dialect/SCF/Transforms/LoopSpecialization.cpp