1//===-- Sink.cpp - Code Sinking -------------------------------------------===//
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 moves instructions into successor blocks, when possible, so that
10// they aren't executed on paths where their results aren't needed.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Transforms/Scalar/Sink.h"
15#include "llvm/ADT/Statistic.h"
16#include "llvm/Analysis/AliasAnalysis.h"
17#include "llvm/Analysis/LoopInfo.h"
18#include "llvm/IR/Dominators.h"
19#include "llvm/InitializePasses.h"
20#include "llvm/Support/Debug.h"
21#include "llvm/Support/raw_ostream.h"
22#include "llvm/Transforms/Scalar.h"
23using namespace llvm;
24
25#define DEBUG_TYPE "sink"
26
27STATISTIC(NumSunk, "Number of instructions sunk");
28STATISTIC(NumSinkIter, "Number of sinking iterations");
29
30static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA,
31 SmallPtrSetImpl<Instruction *> &Stores) {
32
33 if (Inst->mayWriteToMemory()) {
34 Stores.insert(Ptr: Inst);
35 return false;
36 }
37
38 // Don't sink static alloca instructions. CodeGen assumes allocas outside the
39 // entry block are dynamically sized stack objects.
40 if (AllocaInst *AI = dyn_cast<AllocaInst>(Val: Inst))
41 if (AI->isStaticAlloca())
42 return false;
43
44 if (LoadInst *L = dyn_cast<LoadInst>(Val: Inst)) {
45 MemoryLocation Loc = MemoryLocation::get(LI: L);
46 for (Instruction *S : Stores)
47 if (isModSet(MRI: AA.getModRefInfo(I: S, OptLoc: Loc)))
48 return false;
49 }
50
51 if (Inst->isTerminator() || isa<PHINode>(Val: Inst) || Inst->isEHPad() ||
52 Inst->mayThrow() || !Inst->willReturn())
53 return false;
54
55 if (auto *Call = dyn_cast<CallBase>(Val: Inst)) {
56 // Convergent operations cannot be made control-dependent on additional
57 // values.
58 if (Call->isConvergent())
59 return false;
60
61 for (Instruction *S : Stores)
62 if (isModSet(MRI: AA.getModRefInfo(I: S, Call)))
63 return false;
64 }
65
66 return true;
67}
68
69/// IsAcceptableTarget - Return true if it is possible to sink the instruction
70/// in the specified basic block.
71static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo,
72 DominatorTree &DT, LoopInfo &LI) {
73 assert(Inst && "Instruction to be sunk is null");
74 assert(SuccToSinkTo && "Candidate sink target is null");
75
76 // It's never legal to sink an instruction into an EH-pad block.
77 if (SuccToSinkTo->isEHPad())
78 return false;
79
80 // If the block has multiple predecessors, this would introduce computation
81 // on different code paths. We could split the critical edge, but for now we
82 // just punt.
83 // FIXME: Split critical edges if not backedges.
84 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
85 // We cannot sink a load across a critical edge - there may be stores in
86 // other code paths.
87 if (Inst->mayReadFromMemory() &&
88 !Inst->hasMetadata(KindID: LLVMContext::MD_invariant_load))
89 return false;
90
91 // Don't sink instructions into a loop.
92 Loop *succ = LI.getLoopFor(BB: SuccToSinkTo);
93 Loop *cur = LI.getLoopFor(BB: Inst->getParent());
94 if (succ != nullptr && succ != cur)
95 return false;
96 }
97
98 return true;
99}
100
101/// SinkInstruction - Determine whether it is safe to sink the specified machine
102/// instruction out of its current block into a successor.
103static bool SinkInstruction(Instruction *Inst,
104 SmallPtrSetImpl<Instruction *> &Stores,
105 DominatorTree &DT, LoopInfo &LI, AAResults &AA) {
106
107 // Check if it's safe to move the instruction.
108 if (!isSafeToMove(Inst, AA, Stores))
109 return false;
110
111 // FIXME: This should include support for sinking instructions within the
112 // block they are currently in to shorten the live ranges. We often get
113 // instructions sunk into the top of a large block, but it would be better to
114 // also sink them down before their first use in the block. This xform has to
115 // be careful not to *increase* register pressure though, e.g. sinking
116 // "x = y + z" down if it kills y and z would increase the live ranges of y
117 // and z and only shrink the live range of x.
118
119 // SuccToSinkTo - This is the successor to sink this instruction to, once we
120 // decide.
121 BasicBlock *SuccToSinkTo = nullptr;
122
123 // Find the nearest common dominator of all users as the candidate.
124 BasicBlock *BB = Inst->getParent();
125 for (Use &U : Inst->uses()) {
126 Instruction *UseInst = cast<Instruction>(Val: U.getUser());
127 BasicBlock *UseBlock = UseInst->getParent();
128 if (PHINode *PN = dyn_cast<PHINode>(Val: UseInst)) {
129 // PHI nodes use the operand in the predecessor block, not the block with
130 // the PHI.
131 unsigned Num = PHINode::getIncomingValueNumForOperand(i: U.getOperandNo());
132 UseBlock = PN->getIncomingBlock(i: Num);
133 }
134 // Don't worry about dead users.
135 if (!DT.isReachableFromEntry(A: UseBlock))
136 continue;
137
138 if (SuccToSinkTo)
139 SuccToSinkTo = DT.findNearestCommonDominator(A: SuccToSinkTo, B: UseBlock);
140 else
141 SuccToSinkTo = UseBlock;
142 }
143
144 if (SuccToSinkTo) {
145 // The nearest common dominator may be in a parent loop of BB, which may not
146 // be beneficial. Find an ancestor.
147 while (SuccToSinkTo != BB &&
148 !IsAcceptableTarget(Inst, SuccToSinkTo, DT, LI))
149 SuccToSinkTo = DT.getNode(BB: SuccToSinkTo)->getIDom()->getBlock();
150 if (SuccToSinkTo == BB)
151 SuccToSinkTo = nullptr;
152 }
153
154 // If we couldn't find a block to sink to, ignore this instruction.
155 if (!SuccToSinkTo)
156 return false;
157
158 LLVM_DEBUG(dbgs() << "Sink" << *Inst << " (";
159 Inst->getParent()->printAsOperand(dbgs(), false); dbgs() << " -> ";
160 SuccToSinkTo->printAsOperand(dbgs(), false); dbgs() << ")\n");
161
162 // The current location of Inst dominates all uses, thus it must dominate
163 // SuccToSinkTo, which is on the IDom chain between the nearest common
164 // dominator to all uses and the current location.
165 assert(DT.dominates(BB, SuccToSinkTo) &&
166 "SuccToSinkTo must be dominated by current Inst location!");
167
168 // Move the instruction.
169 Inst->moveBefore(InsertPos: SuccToSinkTo->getFirstInsertionPt());
170 return true;
171}
172
173static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI,
174 AAResults &AA) {
175 // Don't bother sinking code out of unreachable blocks. In addition to being
176 // unprofitable, it can also lead to infinite looping, because in an
177 // unreachable loop there may be nowhere to stop.
178 if (!DT.isReachableFromEntry(A: &BB)) return false;
179
180 bool MadeChange = false;
181
182 // Walk the basic block bottom-up. Remember if we saw a store.
183 BasicBlock::iterator I = BB.end();
184 --I;
185 bool ProcessedBegin = false;
186 SmallPtrSet<Instruction *, 8> Stores;
187 do {
188 Instruction *Inst = &*I; // The instruction to sink.
189
190 // Predecrement I (if it's not begin) so that it isn't invalidated by
191 // sinking.
192 ProcessedBegin = I == BB.begin();
193 if (!ProcessedBegin)
194 --I;
195
196 if (Inst->isDebugOrPseudoInst())
197 continue;
198
199 if (SinkInstruction(Inst, Stores, DT, LI, AA)) {
200 ++NumSunk;
201 MadeChange = true;
202 }
203
204 // If we just processed the first instruction in the block, we're done.
205 } while (!ProcessedBegin);
206
207 return MadeChange;
208}
209
210static bool iterativelySinkInstructions(Function &F, DominatorTree &DT,
211 LoopInfo &LI, AAResults &AA) {
212 bool MadeChange, EverMadeChange = false;
213
214 do {
215 MadeChange = false;
216 LLVM_DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
217 // Process all basic blocks.
218 for (BasicBlock &I : F)
219 MadeChange |= ProcessBlock(BB&: I, DT, LI, AA);
220 EverMadeChange |= MadeChange;
221 NumSinkIter++;
222 } while (MadeChange);
223
224 return EverMadeChange;
225}
226
227PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) {
228 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
229 auto &LI = AM.getResult<LoopAnalysis>(IR&: F);
230 auto &AA = AM.getResult<AAManager>(IR&: F);
231
232 if (!iterativelySinkInstructions(F, DT, LI, AA))
233 return PreservedAnalyses::all();
234
235 PreservedAnalyses PA;
236 PA.preserveSet<CFGAnalyses>();
237 return PA;
238}
239
240namespace {
241 class SinkingLegacyPass : public FunctionPass {
242 public:
243 static char ID; // Pass identification
244 SinkingLegacyPass() : FunctionPass(ID) {
245 initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry());
246 }
247
248 bool runOnFunction(Function &F) override {
249 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
250 auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
251 auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
252
253 return iterativelySinkInstructions(F, DT, LI, AA);
254 }
255
256 void getAnalysisUsage(AnalysisUsage &AU) const override {
257 AU.setPreservesCFG();
258 FunctionPass::getAnalysisUsage(AU);
259 AU.addRequired<AAResultsWrapperPass>();
260 AU.addRequired<DominatorTreeWrapperPass>();
261 AU.addRequired<LoopInfoWrapperPass>();
262 AU.addPreserved<DominatorTreeWrapperPass>();
263 AU.addPreserved<LoopInfoWrapperPass>();
264 }
265 };
266} // end anonymous namespace
267
268char SinkingLegacyPass::ID = 0;
269INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false)
270INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
271INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
272INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
273INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false)
274
275FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); }
276