1//===-- LoopSink.cpp - Loop Sink Pass -------------------------------------===//
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// This pass does the inverse transformation of what LICM does.
10// It traverses all of the instructions in the loop's preheader and sinks
11// them to the loop body where frequency is lower than the loop's preheader.
12// This pass is a reverse-transformation of LICM. It differs from the Sink
13// pass in the following ways:
14//
15// * It only handles sinking of instructions from the loop's preheader to the
16// loop's body
17// * It uses alias set tracker to get more accurate alias info
18// * It uses block frequency info to find the optimal sinking locations
19//
20// Overall algorithm:
21//
22// For I in Preheader:
23// InsertBBs = BBs that uses I
24// For BB in sorted(LoopBBs):
25// DomBBs = BBs in InsertBBs that are dominated by BB
26// if freq(DomBBs) > freq(BB)
27// InsertBBs = UseBBs - DomBBs + BB
28// For BB in InsertBBs:
29// Insert I at BB's beginning
30//
31//===----------------------------------------------------------------------===//
32
33#include "llvm/Transforms/Scalar/LoopSink.h"
34#include "llvm/ADT/SetOperations.h"
35#include "llvm/ADT/Statistic.h"
36#include "llvm/Analysis/AliasAnalysis.h"
37#include "llvm/Analysis/BlockFrequencyInfo.h"
38#include "llvm/Analysis/LoopInfo.h"
39#include "llvm/Analysis/MemorySSA.h"
40#include "llvm/Analysis/MemorySSAUpdater.h"
41#include "llvm/Analysis/ScalarEvolution.h"
42#include "llvm/IR/Dominators.h"
43#include "llvm/IR/Instructions.h"
44#include "llvm/Support/BranchProbability.h"
45#include "llvm/Support/CommandLine.h"
46#include "llvm/Transforms/Scalar.h"
47#include "llvm/Transforms/Utils/Local.h"
48#include "llvm/Transforms/Utils/LoopUtils.h"
49using namespace llvm;
50
51#define DEBUG_TYPE "loopsink"
52
53STATISTIC(NumLoopSunk, "Number of instructions sunk into loop");
54STATISTIC(NumLoopSunkCloned, "Number of cloned instructions sunk into loop");
55
56static cl::opt<unsigned> SinkFrequencyPercentThreshold(
57 "sink-freq-percent-threshold", cl::Hidden, cl::init(Val: 90),
58 cl::desc("Do not sink instructions that require cloning unless they "
59 "execute less than this percent of the time."));
60
61static cl::opt<unsigned> MaxNumberOfUseBBsForSinking(
62 "max-uses-for-sinking", cl::Hidden, cl::init(Val: 30),
63 cl::desc("Do not sink instructions that have too many uses."));
64
65/// Return adjusted total frequency of \p BBs.
66///
67/// * If there is only one BB, sinking instruction will not introduce code
68/// size increase. Thus there is no need to adjust the frequency.
69/// * If there are more than one BB, sinking would lead to code size increase.
70/// In this case, we add some "tax" to the total frequency to make it harder
71/// to sink. E.g.
72/// Freq(Preheader) = 100
73/// Freq(BBs) = sum(50, 49) = 99
74/// Even if Freq(BBs) < Freq(Preheader), we will not sink from Preheade to
75/// BBs as the difference is too small to justify the code size increase.
76/// To model this, The adjusted Freq(BBs) will be:
77/// AdjustedFreq(BBs) = 99 / SinkFrequencyPercentThreshold%
78static BlockFrequency adjustedSumFreq(SmallPtrSetImpl<BasicBlock *> &BBs,
79 BlockFrequencyInfo &BFI) {
80 BlockFrequency T(0);
81 for (BasicBlock *B : BBs)
82 T += BFI.getBlockFreq(BB: B);
83 if (BBs.size() > 1)
84 T /= BranchProbability(SinkFrequencyPercentThreshold, 100);
85 return T;
86}
87
88/// Return a set of basic blocks to insert sinked instructions.
89///
90/// The returned set of basic blocks (BBsToSinkInto) should satisfy:
91///
92/// * Inside the loop \p L
93/// * For each UseBB in \p UseBBs, there is at least one BB in BBsToSinkInto
94/// that domintates the UseBB
95/// * Has minimum total frequency that is no greater than preheader frequency
96///
97/// The purpose of the function is to find the optimal sinking points to
98/// minimize execution cost, which is defined as "sum of frequency of
99/// BBsToSinkInto".
100/// As a result, the returned BBsToSinkInto needs to have minimum total
101/// frequency.
102/// Additionally, if the total frequency of BBsToSinkInto exceeds preheader
103/// frequency, the optimal solution is not sinking (return empty set).
104///
105/// \p ColdLoopBBs is used to help find the optimal sinking locations.
106/// It stores a list of BBs that is:
107///
108/// * Inside the loop \p L
109/// * Has a frequency no larger than the loop's preheader
110/// * Sorted by BB frequency
111///
112/// The complexity of the function is O(UseBBs.size() * ColdLoopBBs.size()).
113/// To avoid expensive computation, we cap the maximum UseBBs.size() in its
114/// caller.
115static SmallPtrSet<BasicBlock *, 2>
116findBBsToSinkInto(const Loop &L, const SmallPtrSetImpl<BasicBlock *> &UseBBs,
117 const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
118 DominatorTree &DT, BlockFrequencyInfo &BFI) {
119 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto;
120 if (UseBBs.size() == 0)
121 return BBsToSinkInto;
122
123 BBsToSinkInto.insert(I: UseBBs.begin(), E: UseBBs.end());
124 SmallPtrSet<BasicBlock *, 2> BBsDominatedByColdestBB;
125
126 // For every iteration:
127 // * Pick the ColdestBB from ColdLoopBBs
128 // * Find the set BBsDominatedByColdestBB that satisfy:
129 // - BBsDominatedByColdestBB is a subset of BBsToSinkInto
130 // - Every BB in BBsDominatedByColdestBB is dominated by ColdestBB
131 // * If Freq(ColdestBB) < Freq(BBsDominatedByColdestBB), remove
132 // BBsDominatedByColdestBB from BBsToSinkInto, add ColdestBB to
133 // BBsToSinkInto
134 for (BasicBlock *ColdestBB : ColdLoopBBs) {
135 BBsDominatedByColdestBB.clear();
136 for (BasicBlock *SinkedBB : BBsToSinkInto)
137 if (DT.dominates(A: ColdestBB, B: SinkedBB))
138 BBsDominatedByColdestBB.insert(Ptr: SinkedBB);
139 if (BBsDominatedByColdestBB.size() == 0)
140 continue;
141 if (adjustedSumFreq(BBs&: BBsDominatedByColdestBB, BFI) >
142 BFI.getBlockFreq(BB: ColdestBB)) {
143 for (BasicBlock *DominatedBB : BBsDominatedByColdestBB) {
144 BBsToSinkInto.erase(Ptr: DominatedBB);
145 }
146 BBsToSinkInto.insert(Ptr: ColdestBB);
147 }
148 }
149
150 // Can't sink into blocks that have no valid insertion point.
151 for (BasicBlock *BB : BBsToSinkInto) {
152 if (BB->getFirstInsertionPt() == BB->end()) {
153 BBsToSinkInto.clear();
154 break;
155 }
156 }
157
158 // If the total frequency of BBsToSinkInto is larger than preheader frequency,
159 // do not sink.
160 if (adjustedSumFreq(BBs&: BBsToSinkInto, BFI) >
161 BFI.getBlockFreq(BB: L.getLoopPreheader()))
162 BBsToSinkInto.clear();
163 return BBsToSinkInto;
164}
165
166// Sinks \p I from the loop \p L's preheader to its uses. Returns true if
167// sinking is successful.
168// \p LoopBlockNumber is used to sort the insertion blocks to ensure
169// determinism.
170static bool sinkInstruction(
171 Loop &L, Instruction &I, const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
172 const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber, LoopInfo &LI,
173 DominatorTree &DT, BlockFrequencyInfo &BFI, MemorySSAUpdater *MSSAU) {
174 // Compute the set of blocks in loop L which contain a use of I.
175 SmallPtrSet<BasicBlock *, 2> BBs;
176 for (auto &U : I.uses()) {
177 Instruction *UI = cast<Instruction>(Val: U.getUser());
178
179 // We cannot sink I if it has uses outside of the loop.
180 if (!L.contains(L: LI.getLoopFor(BB: UI->getParent())))
181 return false;
182
183 if (!isa<PHINode>(Val: UI)) {
184 BBs.insert(Ptr: UI->getParent());
185 continue;
186 }
187
188 // We cannot sink I to PHI-uses, try to look through PHI to find the incoming
189 // block of the value being used.
190 PHINode *PN = dyn_cast<PHINode>(Val: UI);
191 BasicBlock *PhiBB = PN->getIncomingBlock(U);
192
193 // If value's incoming block is from loop preheader directly, there's no
194 // place to sink to, bailout.
195 if (L.getLoopPreheader() == PhiBB)
196 return false;
197
198 BBs.insert(Ptr: PhiBB);
199 }
200
201 // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
202 // BBs.size() to avoid expensive computation.
203 // FIXME: Handle code size growth for min_size and opt_size.
204 if (BBs.size() > MaxNumberOfUseBBsForSinking)
205 return false;
206
207 // Find the set of BBs that we should insert a copy of I.
208 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
209 findBBsToSinkInto(L, UseBBs: BBs, ColdLoopBBs, DT, BFI);
210 if (BBsToSinkInto.empty())
211 return false;
212
213 // Return if any of the candidate blocks to sink into is non-cold.
214 if (BBsToSinkInto.size() > 1 &&
215 !llvm::set_is_subset(S1: BBsToSinkInto, S2: LoopBlockNumber))
216 return false;
217
218 // Copy the final BBs into a vector and sort them using the total ordering
219 // of the loop block numbers as iterating the set doesn't give a useful
220 // order. No need to stable sort as the block numbers are a total ordering.
221 SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
222 llvm::append_range(C&: SortedBBsToSinkInto, R&: BBsToSinkInto);
223 if (SortedBBsToSinkInto.size() > 1) {
224 llvm::sort(C&: SortedBBsToSinkInto, Comp: [&](BasicBlock *A, BasicBlock *B) {
225 return LoopBlockNumber.find(Val: A)->second < LoopBlockNumber.find(Val: B)->second;
226 });
227 }
228
229 BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
230 // FIXME: Optimize the efficiency for cloned value replacement. The current
231 // implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
232 for (BasicBlock *N : ArrayRef(SortedBBsToSinkInto).drop_front(N: 1)) {
233 assert(LoopBlockNumber.find(N)->second >
234 LoopBlockNumber.find(MoveBB)->second &&
235 "BBs not sorted!");
236 // Clone I and replace its uses.
237 Instruction *IC = I.clone();
238 IC->setName(I.getName());
239 IC->insertBefore(InsertPos: &*N->getFirstInsertionPt());
240
241 if (MSSAU && MSSAU->getMemorySSA()->getMemoryAccess(I: &I)) {
242 // Create a new MemoryAccess and let MemorySSA set its defining access.
243 MemoryAccess *NewMemAcc =
244 MSSAU->createMemoryAccessInBB(I: IC, Definition: nullptr, BB: N, Point: MemorySSA::Beginning);
245 if (NewMemAcc) {
246 if (auto *MemDef = dyn_cast<MemoryDef>(Val: NewMemAcc))
247 MSSAU->insertDef(Def: MemDef, /*RenameUses=*/true);
248 else {
249 auto *MemUse = cast<MemoryUse>(Val: NewMemAcc);
250 MSSAU->insertUse(Use: MemUse, /*RenameUses=*/true);
251 }
252 }
253 }
254
255 // Replaces uses of I with IC in N, except PHI-use which is being taken
256 // care of by defs in PHI's incoming blocks.
257 I.replaceUsesWithIf(New: IC, ShouldReplace: [N](Use &U) {
258 Instruction *UIToReplace = cast<Instruction>(Val: U.getUser());
259 return UIToReplace->getParent() == N && !isa<PHINode>(Val: UIToReplace);
260 });
261 // Replaces uses of I with IC in blocks dominated by N
262 replaceDominatedUsesWith(From: &I, To: IC, DT, BB: N);
263 LLVM_DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
264 << '\n');
265 NumLoopSunkCloned++;
266 }
267 LLVM_DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
268 NumLoopSunk++;
269 I.moveBefore(MovePos: &*MoveBB->getFirstInsertionPt());
270
271 if (MSSAU)
272 if (MemoryUseOrDef *OldMemAcc = cast_or_null<MemoryUseOrDef>(
273 Val: MSSAU->getMemorySSA()->getMemoryAccess(I: &I)))
274 MSSAU->moveToPlace(What: OldMemAcc, BB: MoveBB, Where: MemorySSA::Beginning);
275
276 return true;
277}
278
279/// Sinks instructions from loop's preheader to the loop body if the
280/// sum frequency of inserted copy is smaller than preheader's frequency.
281static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
282 DominatorTree &DT,
283 BlockFrequencyInfo &BFI,
284 MemorySSA &MSSA,
285 ScalarEvolution *SE) {
286 BasicBlock *Preheader = L.getLoopPreheader();
287 assert(Preheader && "Expected loop to have preheader");
288
289 assert(Preheader->getParent()->hasProfileData() &&
290 "Unexpected call when profile data unavailable.");
291
292 const BlockFrequency PreheaderFreq = BFI.getBlockFreq(BB: Preheader);
293 // If there are no basic blocks with lower frequency than the preheader then
294 // we can avoid the detailed analysis as we will never find profitable sinking
295 // opportunities.
296 if (all_of(Range: L.blocks(), P: [&](const BasicBlock *BB) {
297 return BFI.getBlockFreq(BB) > PreheaderFreq;
298 }))
299 return false;
300
301 MemorySSAUpdater MSSAU(&MSSA);
302 SinkAndHoistLICMFlags LICMFlags(/*IsSink=*/true, L, MSSA);
303
304 bool Changed = false;
305
306 // Sort loop's basic blocks by frequency
307 SmallVector<BasicBlock *, 10> ColdLoopBBs;
308 SmallDenseMap<BasicBlock *, int, 16> LoopBlockNumber;
309 int i = 0;
310 for (BasicBlock *B : L.blocks())
311 if (BFI.getBlockFreq(BB: B) < BFI.getBlockFreq(BB: L.getLoopPreheader())) {
312 ColdLoopBBs.push_back(Elt: B);
313 LoopBlockNumber[B] = ++i;
314 }
315 llvm::stable_sort(Range&: ColdLoopBBs, C: [&](BasicBlock *A, BasicBlock *B) {
316 return BFI.getBlockFreq(BB: A) < BFI.getBlockFreq(BB: B);
317 });
318
319 // Traverse preheader's instructions in reverse order because if A depends
320 // on B (A appears after B), A needs to be sunk first before B can be
321 // sinked.
322 for (Instruction &I : llvm::make_early_inc_range(Range: llvm::reverse(C&: *Preheader))) {
323 if (isa<PHINode>(Val: &I))
324 continue;
325 // No need to check for instruction's operands are loop invariant.
326 assert(L.hasLoopInvariantOperands(&I) &&
327 "Insts in a loop's preheader should have loop invariant operands!");
328 if (!canSinkOrHoistInst(I, AA: &AA, DT: &DT, CurLoop: &L, MSSAU, TargetExecutesOncePerLoop: false, LICMFlags))
329 continue;
330 if (sinkInstruction(L, I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI,
331 MSSAU: &MSSAU)) {
332 Changed = true;
333 if (SE)
334 SE->forgetBlockAndLoopDispositions(V: &I);
335 }
336 }
337
338 return Changed;
339}
340
341PreservedAnalyses LoopSinkPass::run(Function &F, FunctionAnalysisManager &FAM) {
342 // Enable LoopSink only when runtime profile is available.
343 // With static profile, the sinking decision may be sub-optimal.
344 if (!F.hasProfileData())
345 return PreservedAnalyses::all();
346
347 LoopInfo &LI = FAM.getResult<LoopAnalysis>(IR&: F);
348 // Nothing to do if there are no loops.
349 if (LI.empty())
350 return PreservedAnalyses::all();
351
352 AAResults &AA = FAM.getResult<AAManager>(IR&: F);
353 DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(IR&: F);
354 BlockFrequencyInfo &BFI = FAM.getResult<BlockFrequencyAnalysis>(IR&: F);
355 MemorySSA &MSSA = FAM.getResult<MemorySSAAnalysis>(IR&: F).getMSSA();
356
357 // We want to do a postorder walk over the loops. Since loops are a tree this
358 // is equivalent to a reversed preorder walk and preorder is easy to compute
359 // without recursion. Since we reverse the preorder, we will visit siblings
360 // in reverse program order. This isn't expected to matter at all but is more
361 // consistent with sinking algorithms which generally work bottom-up.
362 SmallVector<Loop *, 4> PreorderLoops = LI.getLoopsInPreorder();
363
364 bool Changed = false;
365 do {
366 Loop &L = *PreorderLoops.pop_back_val();
367
368 BasicBlock *Preheader = L.getLoopPreheader();
369 if (!Preheader)
370 continue;
371
372 // Note that we don't pass SCEV here because it is only used to invalidate
373 // loops in SCEV and we don't preserve (or request) SCEV at all making that
374 // unnecessary.
375 Changed |= sinkLoopInvariantInstructions(L, AA, LI, DT, BFI, MSSA,
376 /*ScalarEvolution*/ SE: nullptr);
377 } while (!PreorderLoops.empty());
378
379 if (!Changed)
380 return PreservedAnalyses::all();
381
382 PreservedAnalyses PA;
383 PA.preserveSet<CFGAnalyses>();
384 PA.preserve<MemorySSAAnalysis>();
385
386 if (VerifyMemorySSA)
387 MSSA.verifyMemorySSA();
388
389 return PA;
390}
391

source code of llvm/lib/Transforms/Scalar/LoopSink.cpp