forked from OSchip/llvm-project
306 lines
11 KiB
C++
306 lines
11 KiB
C++
//===-- 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.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#include "llvm/Transforms/Scalar/Sink.h"
|
|
#include "llvm/ADT/Statistic.h"
|
|
#include "llvm/Analysis/AliasAnalysis.h"
|
|
#include "llvm/Analysis/LoopInfo.h"
|
|
#include "llvm/Analysis/ValueTracking.h"
|
|
#include "llvm/IR/CFG.h"
|
|
#include "llvm/IR/DataLayout.h"
|
|
#include "llvm/IR/Dominators.h"
|
|
#include "llvm/IR/IntrinsicInst.h"
|
|
#include "llvm/IR/Module.h"
|
|
#include "llvm/Support/Debug.h"
|
|
#include "llvm/Support/raw_ostream.h"
|
|
#include "llvm/Transforms/Scalar.h"
|
|
using namespace llvm;
|
|
|
|
#define DEBUG_TYPE "sink"
|
|
|
|
STATISTIC(NumSunk, "Number of instructions sunk");
|
|
STATISTIC(NumSinkIter, "Number of sinking iterations");
|
|
|
|
/// AllUsesDominatedByBlock - Return true if all uses of the specified value
|
|
/// occur in blocks dominated by the specified block.
|
|
static bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB,
|
|
DominatorTree &DT) {
|
|
// 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 (Use &U : Inst->uses()) {
|
|
// Determine the block of the use.
|
|
Instruction *UseInst = cast<Instruction>(U.getUser());
|
|
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(U.getOperandNo());
|
|
UseBlock = PN->getIncomingBlock(Num);
|
|
}
|
|
// Check that it dominates.
|
|
if (!DT.dominates(BB, UseBlock))
|
|
return false;
|
|
}
|
|
return true;
|
|
}
|
|
|
|
static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA,
|
|
SmallPtrSetImpl<Instruction *> &Stores) {
|
|
|
|
if (Inst->mayWriteToMemory()) {
|
|
Stores.insert(Inst);
|
|
return false;
|
|
}
|
|
|
|
if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
|
|
MemoryLocation Loc = MemoryLocation::get(L);
|
|
for (Instruction *S : Stores)
|
|
if (AA.getModRefInfo(S, Loc) & MRI_Mod)
|
|
return false;
|
|
}
|
|
|
|
if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst) || Inst->isEHPad() ||
|
|
Inst->mayThrow())
|
|
return false;
|
|
|
|
if (auto CS = CallSite(Inst)) {
|
|
// Convergent operations cannot be made control-dependent on additional
|
|
// values.
|
|
if (CS.hasFnAttr(Attribute::Convergent))
|
|
return false;
|
|
|
|
for (Instruction *S : Stores)
|
|
if (AA.getModRefInfo(S, CS) & MRI_Mod)
|
|
return false;
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
/// IsAcceptableTarget - Return true if it is possible to sink the instruction
|
|
/// in the specified basic block.
|
|
static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo,
|
|
DominatorTree &DT, LoopInfo &LI) {
|
|
assert(Inst && "Instruction to be sunk is null");
|
|
assert(SuccToSinkTo && "Candidate sink target is null");
|
|
|
|
// It is not possible to sink an instruction into its own block. This can
|
|
// happen with loops.
|
|
if (Inst->getParent() == SuccToSinkTo)
|
|
return false;
|
|
|
|
// It's never legal to sink an instruction into a block which terminates in an
|
|
// EH-pad.
|
|
if (SuccToSinkTo->getTerminator()->isExceptional())
|
|
return false;
|
|
|
|
// If the block has multiple predecessors, this would introduce computation
|
|
// on different code paths. We could split the critical edge, but for now we
|
|
// just punt.
|
|
// FIXME: Split critical edges if not backedges.
|
|
if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
|
|
// We cannot sink a load across a critical edge - there may be stores in
|
|
// other code paths.
|
|
if (!isSafeToSpeculativelyExecute(Inst))
|
|
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(Inst->getParent(), SuccToSinkTo))
|
|
return false;
|
|
|
|
// Don't sink instructions into a loop.
|
|
Loop *succ = LI.getLoopFor(SuccToSinkTo);
|
|
Loop *cur = LI.getLoopFor(Inst->getParent());
|
|
if (succ != nullptr && succ != cur)
|
|
return false;
|
|
}
|
|
|
|
// Finally, check that all the uses of the instruction are actually
|
|
// dominated by the candidate
|
|
return AllUsesDominatedByBlock(Inst, SuccToSinkTo, DT);
|
|
}
|
|
|
|
/// SinkInstruction - Determine whether it is safe to sink the specified machine
|
|
/// instruction out of its current block into a successor.
|
|
static bool SinkInstruction(Instruction *Inst,
|
|
SmallPtrSetImpl<Instruction *> &Stores,
|
|
DominatorTree &DT, LoopInfo &LI, AAResults &AA) {
|
|
|
|
// Don't sink static alloca instructions. CodeGen assumes allocas outside the
|
|
// entry block are dynamically sized stack objects.
|
|
if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
|
|
if (AI->isStaticAlloca())
|
|
return false;
|
|
|
|
// 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.
|
|
|
|
// SuccToSinkTo - This is the successor to sink this instruction to, once we
|
|
// decide.
|
|
BasicBlock *SuccToSinkTo = nullptr;
|
|
|
|
// Instructions can only be sunk if all their uses are in blocks
|
|
// dominated by one of the successors.
|
|
// Look at all the postdominators and see if we can sink it in one.
|
|
DomTreeNode *DTN = DT.getNode(Inst->getParent());
|
|
for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
|
|
I != E && SuccToSinkTo == nullptr; ++I) {
|
|
BasicBlock *Candidate = (*I)->getBlock();
|
|
if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
|
|
IsAcceptableTarget(Inst, Candidate, DT, LI))
|
|
SuccToSinkTo = Candidate;
|
|
}
|
|
|
|
// If no suitable postdominator was found, look at all the successors and
|
|
// decide which one we should sink to, if any.
|
|
for (succ_iterator I = succ_begin(Inst->getParent()),
|
|
E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) {
|
|
if (IsAcceptableTarget(Inst, *I, DT, LI))
|
|
SuccToSinkTo = *I;
|
|
}
|
|
|
|
// If we couldn't find a block to sink to, ignore this instruction.
|
|
if (!SuccToSinkTo)
|
|
return false;
|
|
|
|
DEBUG(dbgs() << "Sink" << *Inst << " (";
|
|
Inst->getParent()->printAsOperand(dbgs(), false);
|
|
dbgs() << " -> ";
|
|
SuccToSinkTo->printAsOperand(dbgs(), false);
|
|
dbgs() << ")\n");
|
|
|
|
// Move the instruction.
|
|
Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt());
|
|
return true;
|
|
}
|
|
|
|
static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI,
|
|
AAResults &AA) {
|
|
// Can't sink anything out of a block that has less than two successors.
|
|
if (BB.getTerminator()->getNumSuccessors() <= 1) 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, DT, LI, AA)) {
|
|
++NumSunk;
|
|
MadeChange = true;
|
|
}
|
|
|
|
// If we just processed the first instruction in the block, we're done.
|
|
} while (!ProcessedBegin);
|
|
|
|
return MadeChange;
|
|
}
|
|
|
|
static bool iterativelySinkInstructions(Function &F, DominatorTree &DT,
|
|
LoopInfo &LI, AAResults &AA) {
|
|
bool MadeChange, EverMadeChange = false;
|
|
|
|
do {
|
|
MadeChange = false;
|
|
DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
|
|
// Process all basic blocks.
|
|
for (BasicBlock &I : F)
|
|
MadeChange |= ProcessBlock(I, DT, LI, AA);
|
|
EverMadeChange |= MadeChange;
|
|
NumSinkIter++;
|
|
} while (MadeChange);
|
|
|
|
return EverMadeChange;
|
|
}
|
|
|
|
PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) {
|
|
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
|
|
auto &LI = AM.getResult<LoopAnalysis>(F);
|
|
auto &AA = AM.getResult<AAManager>(F);
|
|
|
|
if (!iterativelySinkInstructions(F, DT, LI, AA))
|
|
return PreservedAnalyses::all();
|
|
|
|
PreservedAnalyses PA;
|
|
PA.preserveSet<CFGAnalyses>();
|
|
return PA;
|
|
}
|
|
|
|
namespace {
|
|
class SinkingLegacyPass : public FunctionPass {
|
|
public:
|
|
static char ID; // Pass identification
|
|
SinkingLegacyPass() : FunctionPass(ID) {
|
|
initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnFunction(Function &F) override {
|
|
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
|
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
|
|
auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
|
|
|
|
return iterativelySinkInstructions(F, DT, LI, AA);
|
|
}
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.setPreservesCFG();
|
|
FunctionPass::getAnalysisUsage(AU);
|
|
AU.addRequired<AAResultsWrapperPass>();
|
|
AU.addRequired<DominatorTreeWrapperPass>();
|
|
AU.addRequired<LoopInfoWrapperPass>();
|
|
AU.addPreserved<DominatorTreeWrapperPass>();
|
|
AU.addPreserved<LoopInfoWrapperPass>();
|
|
}
|
|
};
|
|
} // end anonymous namespace
|
|
|
|
char SinkingLegacyPass::ID = 0;
|
|
INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
|
|
INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false)
|
|
|
|
FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); }
|