lib/Transforms/Scalar/Sink.cpp

   1 //===-- Sink.cpp - Code Sinking -------------------------------------------===//
   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 moves instructions into successor blocks, when possible, so that
  11 // they aren't executed on paths where their results aren't needed.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  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/Assembly/Writer.h"
  22 #include "llvm/ADT/Statistic.h"
  23 #include "llvm/Support/CFG.h"
  24 #include "llvm/Support/Debug.h"
  25 #include "llvm/Support/raw_ostream.h"
  26 using namespace llvm;
  27
  28 STATISTIC(NumSunk, "Number of instructions sunk");
  29
  30 namespace {
  31   class Sinking : public FunctionPass {
  32     DominatorTree *DT;
  33     LoopInfo *LI;
  34     AliasAnalysis *AA;
  35
  36   public:
  37     static char ID; // Pass identification
  38     Sinking() : FunctionPass(ID) {
  39       initializeSinkingPass(*PassRegistry::getPassRegistry());
  40     }
  41
  42     virtual bool runOnFunction(Function &F);
  43
  44     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  45       AU.setPreservesCFG();
  46       FunctionPass::getAnalysisUsage(AU);
  47       AU.addRequired<AliasAnalysis>();
  48       AU.addRequired<DominatorTree>();
  49       AU.addRequired<LoopInfo>();
  50       AU.addPreserved<DominatorTree>();
  51       AU.addPreserved<LoopInfo>();
  52     }
  53   private:
  54     bool ProcessBlock(BasicBlock &BB);
  55     bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
  56     bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
  57   };
  58 } // end anonymous namespace
  59
  60 char Sinking::ID = 0;
  61 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
  62 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  63 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  64 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
  65 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
  66
  67 FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
  68
  69 /// AllUsesDominatedByBlock - Return true if all uses of the specified value
  70 /// occur in blocks dominated by the specified block.
  71 bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
  72                                       BasicBlock *BB) const {
  73   // Ignoring debug uses is necessary so debug info doesn't affect the code.
  74   // This may leave a referencing dbg_value in the original block, before
  75   // the definition of the vreg.  Dwarf generator handles this although the
  76   // user might not get the right info at runtime.
  77   for (Value::use_iterator I = Inst->use_begin(),
  78        E = Inst->use_end(); I != E; ++I) {
  79     // Determine the block of the use.
  80     Instruction *UseInst = cast<Instruction>(*I);
  81     BasicBlock *UseBlock = UseInst->getParent();
  82     if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
  83       // PHI nodes use the operand in the predecessor block, not the block with
  84       // the PHI.
  85       unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
  86       UseBlock = PN->getIncomingBlock(Num);
  87     }
  88     // Check that it dominates.
  89     if (!DT->dominates(BB, UseBlock))
  90       return false;
  91   }
  92   return true;
  93 }
  94
  95 bool Sinking::runOnFunction(Function &F) {
  96   DT = &getAnalysis<DominatorTree>();
  97   LI = &getAnalysis<LoopInfo>();
  98   AA = &getAnalysis<AliasAnalysis>();
  99
 100   bool EverMadeChange = false;
 101
 102   while (1) {
 103     bool MadeChange = false;
 104
 105     // Process all basic blocks.
 106     for (Function::iterator I = F.begin(), E = F.end();
 107          I != E; ++I)
 108       MadeChange |= ProcessBlock(*I);
 109
 110     // If this iteration over the code changed anything, keep iterating.
 111     if (!MadeChange) break;
 112     EverMadeChange = true;
 113   }
 114   return EverMadeChange;
 115 }
 116
 117 bool Sinking::ProcessBlock(BasicBlock &BB) {
 118   // Can't sink anything out of a block that has less than two successors.
 119   if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
 120
 121   // Don't bother sinking code out of unreachable blocks. In addition to being
 122   // unprofitable, it can also lead to infinite looping, because in an unreachable
 123   // loop there may be nowhere to stop.
 124   if (!DT->isReachableFromEntry(&BB)) return false;
 125
 126   bool MadeChange = false;
 127
 128   // Walk the basic block bottom-up.  Remember if we saw a store.
 129   BasicBlock::iterator I = BB.end();
 130   --I;
 131   bool ProcessedBegin = false;
 132   SmallPtrSet<Instruction *, 8> Stores;
 133   do {
 134     Instruction *Inst = I;  // The instruction to sink.
 135
 136     // Predecrement I (if it's not begin) so that it isn't invalidated by
 137     // sinking.
 138     ProcessedBegin = I == BB.begin();
 139     if (!ProcessedBegin)
 140       --I;
 141
 142     if (isa<DbgInfoIntrinsic>(Inst))
 143       continue;
 144
 145     if (SinkInstruction(Inst, Stores))
 146       ++NumSunk, MadeChange = true;
 147
 148     // If we just processed the first instruction in the block, we're done.
 149   } while (!ProcessedBegin);
 150
 151   return MadeChange;
 152 }
 153
 154 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
 155                          SmallPtrSet<Instruction *, 8> &Stores) {
 156   if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
 157     if (L->isVolatile()) return false;
 158
 159     AliasAnalysis::Location Loc = AA->getLocation(L);
 160     for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(),
 161          E = Stores.end(); I != E; ++I)
 162       if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod)
 163         return false;
 164   }
 165
 166   if (Inst->mayWriteToMemory()) {
 167     Stores.insert(Inst);
 168     return false;
 169   }
 170
 171   if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
 172     return false;
 173
 174   return true;
 175 }
 176
 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))
 183     return false;
 184
 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.
 192
 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();
 196
 197   // SuccToSinkTo - This is the successor to sink this instruction to, once we
 198   // decide.
 199   BasicBlock *SuccToSinkTo = 0;
 200
 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
 204   // example:
 205   //   x = computation
 206   //   if () {} else {}
 207   //   use x
 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
 210   // after that.
 211
 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)) {
 219       SuccToSinkTo = *SI;
 220       break;
 221     }
 222   }
 223
 224   // If we couldn't find a block to sink to, ignore this instruction.
 225   if (SuccToSinkTo == 0)
 226     return false;
 227
 228   // It is not possible to sink an instruction into its own block.  This can
 229   // happen with loops.
 230   if (Inst->getParent() == SuccToSinkTo)
 231     return false;
 232
 233   DEBUG(dbgs() << "Sink instr " << *Inst);
 234   DEBUG(dbgs() << "to block ";
 235         WriteAsOperand(dbgs(), SuccToSinkTo, false));
 236
 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
 243     // other code paths.
 244     if (!Inst->isSafeToSpeculativelyExecute()) {
 245       DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
 246       return false;
 247     }
 248
 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");
 253       return false;
 254     }
 255
 256     // Don't sink instructions into a loop.
 257     if (LI->isLoopHeader(SuccToSinkTo)) {
 258       DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
 259       return false;
 260     }
 261
 262     // Otherwise we are OK with sinking along a critical edge.
 263     DEBUG(dbgs() << "Sinking along critical edge.\n");
 264   }
 265
 266   // Determine where to insert into.  Skip phi nodes.
 267   BasicBlock::iterator InsertPos = SuccToSinkTo->begin();
 268   while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos))
 269     ++InsertPos;
 270
 271   // Move the instruction.
 272   Inst->moveBefore(InsertPos);
 273   return true;
 274 }