forked from OSchip/llvm-project
285 lines
9.8 KiB
C++
285 lines
9.8 KiB
C++
//===-- Sink.cpp - Code Sinking -------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass moves instructions into successor blocks, when possible, so that
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// they aren't executed on paths where their results aren't needed.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/Sink.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Module.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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using namespace llvm;
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#define DEBUG_TYPE "sink"
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STATISTIC(NumSunk, "Number of instructions sunk");
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STATISTIC(NumSinkIter, "Number of sinking iterations");
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static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA,
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SmallPtrSetImpl<Instruction *> &Stores) {
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if (Inst->mayWriteToMemory()) {
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Stores.insert(Inst);
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return false;
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}
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if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
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MemoryLocation Loc = MemoryLocation::get(L);
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for (Instruction *S : Stores)
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if (isModSet(AA.getModRefInfo(S, Loc)))
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return false;
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}
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if (Inst->isTerminator() || isa<PHINode>(Inst) || Inst->isEHPad() ||
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Inst->mayThrow())
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return false;
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if (auto *Call = dyn_cast<CallBase>(Inst)) {
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// Convergent operations cannot be made control-dependent on additional
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// values.
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if (Call->isConvergent())
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return false;
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for (Instruction *S : Stores)
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if (isModSet(AA.getModRefInfo(S, Call)))
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return false;
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}
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return true;
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}
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/// IsAcceptableTarget - Return true if it is possible to sink the instruction
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/// in the specified basic block.
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static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo,
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DominatorTree &DT, LoopInfo &LI) {
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assert(Inst && "Instruction to be sunk is null");
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assert(SuccToSinkTo && "Candidate sink target is null");
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// It's never legal to sink an instruction into a block which terminates in an
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// EH-pad.
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if (SuccToSinkTo->getTerminator()->isExceptionalTerminator())
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return false;
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// If the block has multiple predecessors, this would introduce computation
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// on different code paths. We could split the critical edge, but for now we
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// just punt.
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// FIXME: Split critical edges if not backedges.
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if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
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// We cannot sink a load across a critical edge - there may be stores in
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// other code paths.
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if (Inst->mayReadFromMemory())
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return false;
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// We don't want to sink across a critical edge if we don't dominate the
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// successor. We could be introducing calculations to new code paths.
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if (!DT.dominates(Inst->getParent(), SuccToSinkTo))
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return false;
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// Don't sink instructions into a loop.
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Loop *succ = LI.getLoopFor(SuccToSinkTo);
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Loop *cur = LI.getLoopFor(Inst->getParent());
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if (succ != nullptr && succ != cur)
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return false;
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}
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return true;
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}
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/// SinkInstruction - Determine whether it is safe to sink the specified machine
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/// instruction out of its current block into a successor.
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static bool SinkInstruction(Instruction *Inst,
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SmallPtrSetImpl<Instruction *> &Stores,
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DominatorTree &DT, LoopInfo &LI, AAResults &AA) {
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// Don't sink static alloca instructions. CodeGen assumes allocas outside the
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// entry block are dynamically sized stack objects.
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if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
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if (AI->isStaticAlloca())
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return false;
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// Check if it's safe to move the instruction.
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if (!isSafeToMove(Inst, AA, Stores))
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return false;
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// FIXME: This should include support for sinking instructions within the
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// block they are currently in to shorten the live ranges. We often get
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// instructions sunk into the top of a large block, but it would be better to
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// also sink them down before their first use in the block. This xform has to
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// be careful not to *increase* register pressure though, e.g. sinking
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// "x = y + z" down if it kills y and z would increase the live ranges of y
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// and z and only shrink the live range of x.
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// SuccToSinkTo - This is the successor to sink this instruction to, once we
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// decide.
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BasicBlock *SuccToSinkTo = nullptr;
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// Find the nearest common dominator of all users as the candidate.
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BasicBlock *BB = Inst->getParent();
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for (Use &U : Inst->uses()) {
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Instruction *UseInst = cast<Instruction>(U.getUser());
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BasicBlock *UseBlock = UseInst->getParent();
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// Don't worry about dead users.
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if (!DT.isReachableFromEntry(UseBlock))
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continue;
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if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
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// PHI nodes use the operand in the predecessor block, not the block with
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// the PHI.
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unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo());
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UseBlock = PN->getIncomingBlock(Num);
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}
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if (SuccToSinkTo)
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SuccToSinkTo = DT.findNearestCommonDominator(SuccToSinkTo, UseBlock);
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else
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SuccToSinkTo = UseBlock;
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// The current basic block needs to dominate the candidate.
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if (!DT.dominates(BB, SuccToSinkTo))
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return false;
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}
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if (SuccToSinkTo) {
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// The nearest common dominator may be in a parent loop of BB, which may not
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// be beneficial. Find an ancestor.
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while (SuccToSinkTo != BB &&
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!IsAcceptableTarget(Inst, SuccToSinkTo, DT, LI))
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SuccToSinkTo = DT.getNode(SuccToSinkTo)->getIDom()->getBlock();
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if (SuccToSinkTo == BB)
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SuccToSinkTo = nullptr;
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}
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// If we couldn't find a block to sink to, ignore this instruction.
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if (!SuccToSinkTo)
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return false;
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LLVM_DEBUG(dbgs() << "Sink" << *Inst << " (";
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Inst->getParent()->printAsOperand(dbgs(), false); dbgs() << " -> ";
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SuccToSinkTo->printAsOperand(dbgs(), false); dbgs() << ")\n");
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// Move the instruction.
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Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt());
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return true;
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}
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static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI,
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AAResults &AA) {
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// Can't sink anything out of a block that has less than two successors.
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if (BB.getTerminator()->getNumSuccessors() <= 1) return false;
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// Don't bother sinking code out of unreachable blocks. In addition to being
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// unprofitable, it can also lead to infinite looping, because in an
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// unreachable loop there may be nowhere to stop.
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if (!DT.isReachableFromEntry(&BB)) return false;
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bool MadeChange = false;
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// Walk the basic block bottom-up. Remember if we saw a store.
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BasicBlock::iterator I = BB.end();
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--I;
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bool ProcessedBegin = false;
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SmallPtrSet<Instruction *, 8> Stores;
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do {
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Instruction *Inst = &*I; // The instruction to sink.
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// Predecrement I (if it's not begin) so that it isn't invalidated by
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// sinking.
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ProcessedBegin = I == BB.begin();
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if (!ProcessedBegin)
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--I;
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if (Inst->isDebugOrPseudoInst())
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continue;
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if (SinkInstruction(Inst, Stores, DT, LI, AA)) {
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++NumSunk;
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MadeChange = true;
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}
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// If we just processed the first instruction in the block, we're done.
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} while (!ProcessedBegin);
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return MadeChange;
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}
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static bool iterativelySinkInstructions(Function &F, DominatorTree &DT,
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LoopInfo &LI, AAResults &AA) {
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bool MadeChange, EverMadeChange = false;
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do {
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MadeChange = false;
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LLVM_DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
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// Process all basic blocks.
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for (BasicBlock &I : F)
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MadeChange |= ProcessBlock(I, DT, LI, AA);
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EverMadeChange |= MadeChange;
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NumSinkIter++;
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} while (MadeChange);
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return EverMadeChange;
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}
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PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) {
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auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
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auto &LI = AM.getResult<LoopAnalysis>(F);
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auto &AA = AM.getResult<AAManager>(F);
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if (!iterativelySinkInstructions(F, DT, LI, AA))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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return PA;
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}
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namespace {
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class SinkingLegacyPass : public FunctionPass {
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public:
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static char ID; // Pass identification
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SinkingLegacyPass() : FunctionPass(ID) {
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initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override {
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auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
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return iterativelySinkInstructions(F, DT, LI, AA);
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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FunctionPass::getAnalysisUsage(AU);
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AU.addRequired<AAResultsWrapperPass>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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}
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};
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} // end anonymous namespace
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char SinkingLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
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INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false)
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FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); }
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