1 //===-- LoopSink.cpp - Loop Sink Pass -------------------------------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass does the inverse transformation of what LICM does.
11 // It traverses all of the instructions in the loop's preheader and sinks
12 // them to the loop body where frequency is lower than the loop's preheader.
13 // This pass is a reverse-transformation of LICM. It differs from the Sink
14 // pass in the following ways:
15 //
16 // * It only handles sinking of instructions from the loop's preheader to the
17 // loop's body
18 // * It uses alias set tracker to get more accurate alias info
19 // * It uses block frequency info to find the optimal sinking locations
20 //
21 // Overall algorithm:
22 //
23 // For I in Preheader:
24 // InsertBBs = BBs that uses I
25 // For BB in sorted(LoopBBs):
26 // DomBBs = BBs in InsertBBs that are dominated by BB
27 // if freq(DomBBs) > freq(BB)
28 // InsertBBs = UseBBs - DomBBs + BB
29 // For BB in InsertBBs:
30 // Insert I at BB's beginning
31 //
32 //===----------------------------------------------------------------------===//
33
34 #include "llvm/Transforms/Scalar/LoopSink.h"
35 #include "llvm/ADT/Statistic.h"
36 #include "llvm/Analysis/AliasAnalysis.h"
37 #include "llvm/Analysis/AliasSetTracker.h"
38 #include "llvm/Analysis/BasicAliasAnalysis.h"
39 #include "llvm/Analysis/BlockFrequencyInfo.h"
40 #include "llvm/Analysis/Loads.h"
41 #include "llvm/Analysis/LoopInfo.h"
42 #include "llvm/Analysis/LoopPass.h"
43 #include "llvm/Analysis/ScalarEvolution.h"
44 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
45 #include "llvm/Transforms/Utils/Local.h"
46 #include "llvm/IR/Dominators.h"
47 #include "llvm/IR/Instructions.h"
48 #include "llvm/IR/LLVMContext.h"
49 #include "llvm/IR/Metadata.h"
50 #include "llvm/Support/CommandLine.h"
51 #include "llvm/Transforms/Scalar.h"
52 #include "llvm/Transforms/Scalar/LoopPassManager.h"
53 #include "llvm/Transforms/Utils/LoopUtils.h"
54 using namespace llvm;
55
56 #define DEBUG_TYPE "loopsink"
57
58 STATISTIC(NumLoopSunk, "Number of instructions sunk into loop");
59 STATISTIC(NumLoopSunkCloned, "Number of cloned instructions sunk into loop");
60
61 static cl::opt<unsigned> SinkFrequencyPercentThreshold(
62 "sink-freq-percent-threshold", cl::Hidden, cl::init(90),
63 cl::desc("Do not sink instructions that require cloning unless they "
64 "execute less than this percent of the time."));
65
66 static cl::opt<unsigned> MaxNumberOfUseBBsForSinking(
67 "max-uses-for-sinking", cl::Hidden, cl::init(30),
68 cl::desc("Do not sink instructions that have too many uses."));
69
70 /// Return adjusted total frequency of \p BBs.
71 ///
72 /// * If there is only one BB, sinking instruction will not introduce code
73 /// size increase. Thus there is no need to adjust the frequency.
74 /// * If there are more than one BB, sinking would lead to code size increase.
75 /// In this case, we add some "tax" to the total frequency to make it harder
76 /// to sink. E.g.
77 /// Freq(Preheader) = 100
78 /// Freq(BBs) = sum(50, 49) = 99
79 /// Even if Freq(BBs) < Freq(Preheader), we will not sink from Preheade to
80 /// BBs as the difference is too small to justify the code size increase.
81 /// To model this, The adjusted Freq(BBs) will be:
82 /// AdjustedFreq(BBs) = 99 / SinkFrequencyPercentThreshold%
adjustedSumFreq(SmallPtrSetImpl<BasicBlock * > & BBs,BlockFrequencyInfo & BFI)83 static BlockFrequency adjustedSumFreq(SmallPtrSetImpl<BasicBlock *> &BBs,
84 BlockFrequencyInfo &BFI) {
85 BlockFrequency T = 0;
86 for (BasicBlock *B : BBs)
87 T += BFI.getBlockFreq(B);
88 if (BBs.size() > 1)
89 T /= BranchProbability(SinkFrequencyPercentThreshold, 100);
90 return T;
91 }
92
93 /// Return a set of basic blocks to insert sinked instructions.
94 ///
95 /// The returned set of basic blocks (BBsToSinkInto) should satisfy:
96 ///
97 /// * Inside the loop \p L
98 /// * For each UseBB in \p UseBBs, there is at least one BB in BBsToSinkInto
99 /// that domintates the UseBB
100 /// * Has minimum total frequency that is no greater than preheader frequency
101 ///
102 /// The purpose of the function is to find the optimal sinking points to
103 /// minimize execution cost, which is defined as "sum of frequency of
104 /// BBsToSinkInto".
105 /// As a result, the returned BBsToSinkInto needs to have minimum total
106 /// frequency.
107 /// Additionally, if the total frequency of BBsToSinkInto exceeds preheader
108 /// frequency, the optimal solution is not sinking (return empty set).
109 ///
110 /// \p ColdLoopBBs is used to help find the optimal sinking locations.
111 /// It stores a list of BBs that is:
112 ///
113 /// * Inside the loop \p L
114 /// * Has a frequency no larger than the loop's preheader
115 /// * Sorted by BB frequency
116 ///
117 /// The complexity of the function is O(UseBBs.size() * ColdLoopBBs.size()).
118 /// To avoid expensive computation, we cap the maximum UseBBs.size() in its
119 /// caller.
120 static SmallPtrSet<BasicBlock *, 2>
findBBsToSinkInto(const Loop & L,const SmallPtrSetImpl<BasicBlock * > & UseBBs,const SmallVectorImpl<BasicBlock * > & ColdLoopBBs,DominatorTree & DT,BlockFrequencyInfo & BFI)121 findBBsToSinkInto(const Loop &L, const SmallPtrSetImpl<BasicBlock *> &UseBBs,
122 const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
123 DominatorTree &DT, BlockFrequencyInfo &BFI) {
124 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto;
125 if (UseBBs.size() == 0)
126 return BBsToSinkInto;
127
128 BBsToSinkInto.insert(UseBBs.begin(), UseBBs.end());
129 SmallPtrSet<BasicBlock *, 2> BBsDominatedByColdestBB;
130
131 // For every iteration:
132 // * Pick the ColdestBB from ColdLoopBBs
133 // * Find the set BBsDominatedByColdestBB that satisfy:
134 // - BBsDominatedByColdestBB is a subset of BBsToSinkInto
135 // - Every BB in BBsDominatedByColdestBB is dominated by ColdestBB
136 // * If Freq(ColdestBB) < Freq(BBsDominatedByColdestBB), remove
137 // BBsDominatedByColdestBB from BBsToSinkInto, add ColdestBB to
138 // BBsToSinkInto
139 for (BasicBlock *ColdestBB : ColdLoopBBs) {
140 BBsDominatedByColdestBB.clear();
141 for (BasicBlock *SinkedBB : BBsToSinkInto)
142 if (DT.dominates(ColdestBB, SinkedBB))
143 BBsDominatedByColdestBB.insert(SinkedBB);
144 if (BBsDominatedByColdestBB.size() == 0)
145 continue;
146 if (adjustedSumFreq(BBsDominatedByColdestBB, BFI) >
147 BFI.getBlockFreq(ColdestBB)) {
148 for (BasicBlock *DominatedBB : BBsDominatedByColdestBB) {
149 BBsToSinkInto.erase(DominatedBB);
150 }
151 BBsToSinkInto.insert(ColdestBB);
152 }
153 }
154
155 // Can't sink into blocks that have no valid insertion point.
156 for (BasicBlock *BB : BBsToSinkInto) {
157 if (BB->getFirstInsertionPt() == BB->end()) {
158 BBsToSinkInto.clear();
159 break;
160 }
161 }
162
163 // If the total frequency of BBsToSinkInto is larger than preheader frequency,
164 // do not sink.
165 if (adjustedSumFreq(BBsToSinkInto, BFI) >
166 BFI.getBlockFreq(L.getLoopPreheader()))
167 BBsToSinkInto.clear();
168 return BBsToSinkInto;
169 }
170
171 // Sinks \p I from the loop \p L's preheader to its uses. Returns true if
172 // sinking is successful.
173 // \p LoopBlockNumber is used to sort the insertion blocks to ensure
174 // determinism.
sinkInstruction(Loop & L,Instruction & I,const SmallVectorImpl<BasicBlock * > & ColdLoopBBs,const SmallDenseMap<BasicBlock *,int,16> & LoopBlockNumber,LoopInfo & LI,DominatorTree & DT,BlockFrequencyInfo & BFI)175 static bool sinkInstruction(Loop &L, Instruction &I,
176 const SmallVectorImpl<BasicBlock *> &ColdLoopBBs,
177 const SmallDenseMap<BasicBlock *, int, 16> &LoopBlockNumber,
178 LoopInfo &LI, DominatorTree &DT,
179 BlockFrequencyInfo &BFI) {
180 // Compute the set of blocks in loop L which contain a use of I.
181 SmallPtrSet<BasicBlock *, 2> BBs;
182 for (auto &U : I.uses()) {
183 Instruction *UI = cast<Instruction>(U.getUser());
184 // We cannot sink I to PHI-uses.
185 if (dyn_cast<PHINode>(UI))
186 return false;
187 // We cannot sink I if it has uses outside of the loop.
188 if (!L.contains(LI.getLoopFor(UI->getParent())))
189 return false;
190 BBs.insert(UI->getParent());
191 }
192
193 // findBBsToSinkInto is O(BBs.size() * ColdLoopBBs.size()). We cap the max
194 // BBs.size() to avoid expensive computation.
195 // FIXME: Handle code size growth for min_size and opt_size.
196 if (BBs.size() > MaxNumberOfUseBBsForSinking)
197 return false;
198
199 // Find the set of BBs that we should insert a copy of I.
200 SmallPtrSet<BasicBlock *, 2> BBsToSinkInto =
201 findBBsToSinkInto(L, BBs, ColdLoopBBs, DT, BFI);
202 if (BBsToSinkInto.empty())
203 return false;
204
205 // Copy the final BBs into a vector and sort them using the total ordering
206 // of the loop block numbers as iterating the set doesn't give a useful
207 // order. No need to stable sort as the block numbers are a total ordering.
208 SmallVector<BasicBlock *, 2> SortedBBsToSinkInto;
209 SortedBBsToSinkInto.insert(SortedBBsToSinkInto.begin(), BBsToSinkInto.begin(),
210 BBsToSinkInto.end());
211 llvm::sort(SortedBBsToSinkInto.begin(), SortedBBsToSinkInto.end(),
212 [&](BasicBlock *A, BasicBlock *B) {
213 return LoopBlockNumber.find(A)->second <
214 LoopBlockNumber.find(B)->second;
215 });
216
217 BasicBlock *MoveBB = *SortedBBsToSinkInto.begin();
218 // FIXME: Optimize the efficiency for cloned value replacement. The current
219 // implementation is O(SortedBBsToSinkInto.size() * I.num_uses()).
220 for (BasicBlock *N : makeArrayRef(SortedBBsToSinkInto).drop_front(1)) {
221 assert(LoopBlockNumber.find(N)->second >
222 LoopBlockNumber.find(MoveBB)->second &&
223 "BBs not sorted!");
224 // Clone I and replace its uses.
225 Instruction *IC = I.clone();
226 IC->setName(I.getName());
227 IC->insertBefore(&*N->getFirstInsertionPt());
228 // Replaces uses of I with IC in N
229 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
230 Use &U = *UI++;
231 auto *I = cast<Instruction>(U.getUser());
232 if (I->getParent() == N)
233 U.set(IC);
234 }
235 // Replaces uses of I with IC in blocks dominated by N
236 replaceDominatedUsesWith(&I, IC, DT, N);
237 LLVM_DEBUG(dbgs() << "Sinking a clone of " << I << " To: " << N->getName()
238 << '\n');
239 NumLoopSunkCloned++;
240 }
241 LLVM_DEBUG(dbgs() << "Sinking " << I << " To: " << MoveBB->getName() << '\n');
242 NumLoopSunk++;
243 I.moveBefore(&*MoveBB->getFirstInsertionPt());
244
245 return true;
246 }
247
248 /// Sinks instructions from loop's preheader to the loop body if the
249 /// sum frequency of inserted copy is smaller than preheader's frequency.
sinkLoopInvariantInstructions(Loop & L,AAResults & AA,LoopInfo & LI,DominatorTree & DT,BlockFrequencyInfo & BFI,ScalarEvolution * SE)250 static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
251 DominatorTree &DT,
252 BlockFrequencyInfo &BFI,
253 ScalarEvolution *SE) {
254 BasicBlock *Preheader = L.getLoopPreheader();
255 if (!Preheader)
256 return false;
257
258 // Enable LoopSink only when runtime profile is available.
259 // With static profile, the sinking decision may be sub-optimal.
260 if (!Preheader->getParent()->hasProfileData())
261 return false;
262
263 const BlockFrequency PreheaderFreq = BFI.getBlockFreq(Preheader);
264 // If there are no basic blocks with lower frequency than the preheader then
265 // we can avoid the detailed analysis as we will never find profitable sinking
266 // opportunities.
267 if (all_of(L.blocks(), [&](const BasicBlock *BB) {
268 return BFI.getBlockFreq(BB) > PreheaderFreq;
269 }))
270 return false;
271
272 bool Changed = false;
273 AliasSetTracker CurAST(AA);
274
275 // Compute alias set.
276 for (BasicBlock *BB : L.blocks())
277 CurAST.add(*BB);
278
279 // Sort loop's basic blocks by frequency
280 SmallVector<BasicBlock *, 10> ColdLoopBBs;
281 SmallDenseMap<BasicBlock *, int, 16> LoopBlockNumber;
282 int i = 0;
283 for (BasicBlock *B : L.blocks())
284 if (BFI.getBlockFreq(B) < BFI.getBlockFreq(L.getLoopPreheader())) {
285 ColdLoopBBs.push_back(B);
286 LoopBlockNumber[B] = ++i;
287 }
288 std::stable_sort(ColdLoopBBs.begin(), ColdLoopBBs.end(),
289 [&](BasicBlock *A, BasicBlock *B) {
290 return BFI.getBlockFreq(A) < BFI.getBlockFreq(B);
291 });
292
293 // Traverse preheader's instructions in reverse order becaue if A depends
294 // on B (A appears after B), A needs to be sinked first before B can be
295 // sinked.
296 for (auto II = Preheader->rbegin(), E = Preheader->rend(); II != E;) {
297 Instruction *I = &*II++;
298 // No need to check for instruction's operands are loop invariant.
299 assert(L.hasLoopInvariantOperands(I) &&
300 "Insts in a loop's preheader should have loop invariant operands!");
301 if (!canSinkOrHoistInst(*I, &AA, &DT, &L, &CurAST, nullptr))
302 continue;
303 if (sinkInstruction(L, *I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI))
304 Changed = true;
305 }
306
307 if (Changed && SE)
308 SE->forgetLoopDispositions(&L);
309 return Changed;
310 }
311
run(Function & F,FunctionAnalysisManager & FAM)312 PreservedAnalyses LoopSinkPass::run(Function &F, FunctionAnalysisManager &FAM) {
313 LoopInfo &LI = FAM.getResult<LoopAnalysis>(F);
314 // Nothing to do if there are no loops.
315 if (LI.empty())
316 return PreservedAnalyses::all();
317
318 AAResults &AA = FAM.getResult<AAManager>(F);
319 DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
320 BlockFrequencyInfo &BFI = FAM.getResult<BlockFrequencyAnalysis>(F);
321
322 // We want to do a postorder walk over the loops. Since loops are a tree this
323 // is equivalent to a reversed preorder walk and preorder is easy to compute
324 // without recursion. Since we reverse the preorder, we will visit siblings
325 // in reverse program order. This isn't expected to matter at all but is more
326 // consistent with sinking algorithms which generally work bottom-up.
327 SmallVector<Loop *, 4> PreorderLoops = LI.getLoopsInPreorder();
328
329 bool Changed = false;
330 do {
331 Loop &L = *PreorderLoops.pop_back_val();
332
333 // Note that we don't pass SCEV here because it is only used to invalidate
334 // loops in SCEV and we don't preserve (or request) SCEV at all making that
335 // unnecessary.
336 Changed |= sinkLoopInvariantInstructions(L, AA, LI, DT, BFI,
337 /*ScalarEvolution*/ nullptr);
338 } while (!PreorderLoops.empty());
339
340 if (!Changed)
341 return PreservedAnalyses::all();
342
343 PreservedAnalyses PA;
344 PA.preserveSet<CFGAnalyses>();
345 return PA;
346 }
347
348 namespace {
349 struct LegacyLoopSinkPass : public LoopPass {
350 static char ID;
LegacyLoopSinkPass__anoncebf8f450411::LegacyLoopSinkPass351 LegacyLoopSinkPass() : LoopPass(ID) {
352 initializeLegacyLoopSinkPassPass(*PassRegistry::getPassRegistry());
353 }
354
runOnLoop__anoncebf8f450411::LegacyLoopSinkPass355 bool runOnLoop(Loop *L, LPPassManager &LPM) override {
356 if (skipLoop(L))
357 return false;
358
359 auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
360 return sinkLoopInvariantInstructions(
361 *L, getAnalysis<AAResultsWrapperPass>().getAAResults(),
362 getAnalysis<LoopInfoWrapperPass>().getLoopInfo(),
363 getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
364 getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI(),
365 SE ? &SE->getSE() : nullptr);
366 }
367
getAnalysisUsage__anoncebf8f450411::LegacyLoopSinkPass368 void getAnalysisUsage(AnalysisUsage &AU) const override {
369 AU.setPreservesCFG();
370 AU.addRequired<BlockFrequencyInfoWrapperPass>();
371 getLoopAnalysisUsage(AU);
372 }
373 };
374 }
375
376 char LegacyLoopSinkPass::ID = 0;
377 INITIALIZE_PASS_BEGIN(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false,
378 false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)379 INITIALIZE_PASS_DEPENDENCY(LoopPass)
380 INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
381 INITIALIZE_PASS_END(LegacyLoopSinkPass, "loop-sink", "Loop Sink", false, false)
382
383 Pass *llvm::createLoopSinkPass() { return new LegacyLoopSinkPass(); }
384