1 //===-- Sink.cpp - Code Sinking -------------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass moves instructions into successor blocks, when possible, so that
11 // they aren't executed on paths where their results aren't needed.
13 //===----------------------------------------------------------------------===//
15 #define DEBUG_TYPE "sink"
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/IntrinsicInst.h"
18 #include "llvm/Analysis/Dominators.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/Analysis/ValueTracking.h"
22 #include "llvm/Assembly/Writer.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Support/CFG.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Support/raw_ostream.h"
29 STATISTIC(NumSunk, "Number of instructions sunk");
32 class Sinking : public FunctionPass {
38 static char ID; // Pass identification
39 Sinking() : FunctionPass(ID) {
40 initializeSinkingPass(*PassRegistry::getPassRegistry());
43 virtual bool runOnFunction(Function &F);
45 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 FunctionPass::getAnalysisUsage(AU);
48 AU.addRequired<AliasAnalysis>();
49 AU.addRequired<DominatorTree>();
50 AU.addRequired<LoopInfo>();
51 AU.addPreserved<DominatorTree>();
52 AU.addPreserved<LoopInfo>();
55 bool ProcessBlock(BasicBlock &BB);
56 bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
57 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
59 } // end anonymous namespace
62 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
63 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
64 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
65 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
66 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
68 FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
70 /// AllUsesDominatedByBlock - Return true if all uses of the specified value
71 /// occur in blocks dominated by the specified block.
72 bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
73 BasicBlock *BB) const {
74 // Ignoring debug uses is necessary so debug info doesn't affect the code.
75 // This may leave a referencing dbg_value in the original block, before
76 // the definition of the vreg. Dwarf generator handles this although the
77 // user might not get the right info at runtime.
78 for (Value::use_iterator I = Inst->use_begin(),
79 E = Inst->use_end(); I != E; ++I) {
80 // Determine the block of the use.
81 Instruction *UseInst = cast<Instruction>(*I);
82 BasicBlock *UseBlock = UseInst->getParent();
83 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
84 // PHI nodes use the operand in the predecessor block, not the block with
86 unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
87 UseBlock = PN->getIncomingBlock(Num);
89 // Check that it dominates.
90 if (!DT->dominates(BB, UseBlock))
96 bool Sinking::runOnFunction(Function &F) {
97 DT = &getAnalysis<DominatorTree>();
98 LI = &getAnalysis<LoopInfo>();
99 AA = &getAnalysis<AliasAnalysis>();
101 bool EverMadeChange = false;
104 bool MadeChange = false;
106 // Process all basic blocks.
107 for (Function::iterator I = F.begin(), E = F.end();
109 MadeChange |= ProcessBlock(*I);
111 // If this iteration over the code changed anything, keep iterating.
112 if (!MadeChange) break;
113 EverMadeChange = true;
115 return EverMadeChange;
118 bool Sinking::ProcessBlock(BasicBlock &BB) {
119 // Can't sink anything out of a block that has less than two successors.
120 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
122 // Don't bother sinking code out of unreachable blocks. In addition to being
123 // unprofitable, it can also lead to infinite looping, because in an unreachable
124 // loop there may be nowhere to stop.
125 if (!DT->isReachableFromEntry(&BB)) return false;
127 bool MadeChange = false;
129 // Walk the basic block bottom-up. Remember if we saw a store.
130 BasicBlock::iterator I = BB.end();
132 bool ProcessedBegin = false;
133 SmallPtrSet<Instruction *, 8> Stores;
135 Instruction *Inst = I; // The instruction to sink.
137 // Predecrement I (if it's not begin) so that it isn't invalidated by
139 ProcessedBegin = I == BB.begin();
143 if (isa<DbgInfoIntrinsic>(Inst))
146 if (SinkInstruction(Inst, Stores))
147 ++NumSunk, MadeChange = true;
149 // If we just processed the first instruction in the block, we're done.
150 } while (!ProcessedBegin);
155 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
156 SmallPtrSet<Instruction *, 8> &Stores) {
158 if (Inst->mayWriteToMemory()) {
163 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
164 AliasAnalysis::Location Loc = AA->getLocation(L);
165 for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(),
166 E = Stores.end(); I != E; ++I)
167 if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod)
171 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
177 /// SinkInstruction - Determine whether it is safe to sink the specified machine
178 /// instruction out of its current block into a successor.
179 bool Sinking::SinkInstruction(Instruction *Inst,
180 SmallPtrSet<Instruction *, 8> &Stores) {
181 // Check if it's safe to move the instruction.
182 if (!isSafeToMove(Inst, AA, Stores))
185 // FIXME: This should include support for sinking instructions within the
186 // block they are currently in to shorten the live ranges. We often get
187 // instructions sunk into the top of a large block, but it would be better to
188 // also sink them down before their first use in the block. This xform has to
189 // be careful not to *increase* register pressure though, e.g. sinking
190 // "x = y + z" down if it kills y and z would increase the live ranges of y
191 // and z and only shrink the live range of x.
193 // Loop over all the operands of the specified instruction. If there is
194 // anything we can't handle, bail out.
195 BasicBlock *ParentBlock = Inst->getParent();
197 // SuccToSinkTo - This is the successor to sink this instruction to, once we
199 BasicBlock *SuccToSinkTo = 0;
201 // FIXME: This picks a successor to sink into based on having one
202 // successor that dominates all the uses. However, there are cases where
203 // sinking can happen but where the sink point isn't a successor. For
208 // the instruction could be sunk over the whole diamond for the
209 // if/then/else (or loop, etc), allowing it to be sunk into other blocks
212 // Instructions can only be sunk if all their uses are in blocks
213 // dominated by one of the successors.
214 // Look at all the successors and decide which one
215 // we should sink to.
216 for (succ_iterator SI = succ_begin(ParentBlock),
217 E = succ_end(ParentBlock); SI != E; ++SI) {
218 if (AllUsesDominatedByBlock(Inst, *SI)) {
224 // If we couldn't find a block to sink to, ignore this instruction.
225 if (SuccToSinkTo == 0)
228 // It is not possible to sink an instruction into its own block. This can
229 // happen with loops.
230 if (Inst->getParent() == SuccToSinkTo)
233 DEBUG(dbgs() << "Sink instr " << *Inst);
234 DEBUG(dbgs() << "to block ";
235 WriteAsOperand(dbgs(), SuccToSinkTo, false));
237 // If the block has multiple predecessors, this would introduce computation on
238 // a path that it doesn't already exist. We could split the critical edge,
239 // but for now we just punt.
240 // FIXME: Split critical edges if not backedges.
241 if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) {
242 // We cannot sink a load across a critical edge - there may be stores in
244 if (!isSafeToSpeculativelyExecute(Inst)) {
245 DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
249 // We don't want to sink across a critical edge if we don't dominate the
250 // successor. We could be introducing calculations to new code paths.
251 if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
252 DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
256 // Don't sink instructions into a loop.
257 if (LI->isLoopHeader(SuccToSinkTo)) {
258 DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
262 // Otherwise we are OK with sinking along a critical edge.
263 DEBUG(dbgs() << "Sinking along critical edge.\n");
266 // Determine where to insert into. Skip phi nodes.
267 BasicBlock::iterator InsertPos = SuccToSinkTo->begin();
268 while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos))
271 // Move the instruction.
272 Inst->moveBefore(InsertPos);