llvm-6502/lib/Transforms/Scalar/Sink.cpp

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//===-- Sink.cpp - Code Sinking -------------------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass moves instructions into successor blocks, when possible, so that
// they aren't executed on paths where their results aren't needed.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "sink"
#include "llvm/Transforms/Scalar.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
STATISTIC(NumSunk, "Number of instructions sunk");
namespace {
class Sinking : public FunctionPass {
DominatorTree *DT;
LoopInfo *LI;
AliasAnalysis *AA;
public:
static char ID; // Pass identification
Sinking() : FunctionPass(ID) {
initializeSinkingPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
FunctionPass::getAnalysisUsage(AU);
AU.addRequired<AliasAnalysis>();
AU.addRequired<DominatorTree>();
AU.addRequired<LoopInfo>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<LoopInfo>();
}
private:
bool ProcessBlock(BasicBlock &BB);
bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
};
} // end anonymous namespace
char Sinking::ID = 0;
INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
/// AllUsesDominatedByBlock - Return true if all uses of the specified value
/// occur in blocks dominated by the specified block.
bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
BasicBlock *BB) const {
// Ignoring debug uses is necessary so debug info doesn't affect the code.
// This may leave a referencing dbg_value in the original block, before
// the definition of the vreg. Dwarf generator handles this although the
// user might not get the right info at runtime.
for (Value::use_iterator I = Inst->use_begin(),
E = Inst->use_end(); I != E; ++I) {
// Determine the block of the use.
Instruction *UseInst = cast<Instruction>(*I);
BasicBlock *UseBlock = UseInst->getParent();
if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
// PHI nodes use the operand in the predecessor block, not the block with
// the PHI.
unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
UseBlock = PN->getIncomingBlock(Num);
}
// Check that it dominates.
if (!DT->dominates(BB, UseBlock))
return false;
}
return true;
}
bool Sinking::runOnFunction(Function &F) {
DT = &getAnalysis<DominatorTree>();
LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
bool EverMadeChange = false;
while (1) {
bool MadeChange = false;
// Process all basic blocks.
for (Function::iterator I = F.begin(), E = F.end();
I != E; ++I)
MadeChange |= ProcessBlock(*I);
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
}
return EverMadeChange;
}
bool Sinking::ProcessBlock(BasicBlock &BB) {
// Can't sink anything out of a block that has less than two successors.
if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
// Don't bother sinking code out of unreachable blocks. In addition to being
// unprofitable, it can also lead to infinite looping, because in an unreachable
// loop there may be nowhere to stop.
if (!DT->isReachableFromEntry(&BB)) return false;
bool MadeChange = false;
// Walk the basic block bottom-up. Remember if we saw a store.
BasicBlock::iterator I = BB.end();
--I;
bool ProcessedBegin = false;
SmallPtrSet<Instruction *, 8> Stores;
do {
Instruction *Inst = I; // The instruction to sink.
// Predecrement I (if it's not begin) so that it isn't invalidated by
// sinking.
ProcessedBegin = I == BB.begin();
if (!ProcessedBegin)
--I;
if (isa<DbgInfoIntrinsic>(Inst))
continue;
if (SinkInstruction(Inst, Stores))
++NumSunk, MadeChange = true;
// If we just processed the first instruction in the block, we're done.
} while (!ProcessedBegin);
return MadeChange;
}
static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
SmallPtrSet<Instruction *, 8> &Stores) {
if (Inst->mayWriteToMemory()) {
Stores.insert(Inst);
return false;
}
if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
AliasAnalysis::Location Loc = AA->getLocation(L);
for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(),
E = Stores.end(); I != E; ++I)
if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod)
return false;
}
if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
return false;
return true;
}
/// SinkInstruction - Determine whether it is safe to sink the specified machine
/// instruction out of its current block into a successor.
bool Sinking::SinkInstruction(Instruction *Inst,
SmallPtrSet<Instruction *, 8> &Stores) {
// Check if it's safe to move the instruction.
if (!isSafeToMove(Inst, AA, Stores))
return false;
// FIXME: This should include support for sinking instructions within the
// block they are currently in to shorten the live ranges. We often get
// instructions sunk into the top of a large block, but it would be better to
// also sink them down before their first use in the block. This xform has to
// be careful not to *increase* register pressure though, e.g. sinking
// "x = y + z" down if it kills y and z would increase the live ranges of y
// and z and only shrink the live range of x.
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
BasicBlock *ParentBlock = Inst->getParent();
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
BasicBlock *SuccToSinkTo = 0;
// FIXME: This picks a successor to sink into based on having one
// successor that dominates all the uses. However, there are cases where
// sinking can happen but where the sink point isn't a successor. For
// example:
// x = computation
// if () {} else {}
// use x
// the instruction could be sunk over the whole diamond for the
// if/then/else (or loop, etc), allowing it to be sunk into other blocks
// after that.
// Instructions can only be sunk if all their uses are in blocks
// dominated by one of the successors.
// Look at all the successors and decide which one
// we should sink to.
for (succ_iterator SI = succ_begin(ParentBlock),
E = succ_end(ParentBlock); SI != E; ++SI) {
if (AllUsesDominatedByBlock(Inst, *SI)) {
SuccToSinkTo = *SI;
break;
}
}
// If we couldn't find a block to sink to, ignore this instruction.
if (SuccToSinkTo == 0)
return false;
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (Inst->getParent() == SuccToSinkTo)
return false;
DEBUG(dbgs() << "Sink instr " << *Inst);
DEBUG(dbgs() << "to block ";
WriteAsOperand(dbgs(), SuccToSinkTo, false));
// If the block has multiple predecessors, this would introduce computation on
// a path that it doesn't already exist. We could split the critical edge,
// but for now we just punt.
// FIXME: Split critical edges if not backedges.
if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) {
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
if (!Inst->isSafeToSpeculativelyExecute()) {
DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
return false;
}
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
return false;
}
// Don't sink instructions into a loop.
if (LI->isLoopHeader(SuccToSinkTo)) {
DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
return false;
}
// Otherwise we are OK with sinking along a critical edge.
DEBUG(dbgs() << "Sinking along critical edge.\n");
}
// Determine where to insert into. Skip phi nodes.
BasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos))
++InsertPos;
// Move the instruction.
Inst->moveBefore(InsertPos);
return true;
}