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llvm-project/llvm/lib/Transforms/Scalar/LoopFuse.cpp

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//===- LoopFuse.cpp - Loop Fusion Pass ------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements the loop fusion pass.
/// The implementation is largely based on the following document:
///
/// Code Transformations to Augment the Scope of Loop Fusion in a
/// Production Compiler
/// Christopher Mark Barton
/// MSc Thesis
/// https://webdocs.cs.ualberta.ca/~amaral/thesis/ChristopherBartonMSc.pdf
///
/// The general approach taken is to collect sets of control flow equivalent
/// loops and test whether they can be fused. The necessary conditions for
/// fusion are:
/// 1. The loops must be adjacent (there cannot be any statements between
/// the two loops).
/// 2. The loops must be conforming (they must execute the same number of
/// iterations).
/// 3. The loops must be control flow equivalent (if one loop executes, the
/// other is guaranteed to execute).
/// 4. There cannot be any negative distance dependencies between the loops.
/// If all of these conditions are satisfied, it is safe to fuse the loops.
///
/// This implementation creates FusionCandidates that represent the loop and the
/// necessary information needed by fusion. It then operates on the fusion
/// candidates, first confirming that the candidate is eligible for fusion. The
/// candidates are then collected into control flow equivalent sets, sorted in
/// dominance order. Each set of control flow equivalent candidates is then
/// traversed, attempting to fuse pairs of candidates in the set. If all
/// requirements for fusion are met, the two candidates are fused, creating a
/// new (fused) candidate which is then added back into the set to consider for
/// additional fusion.
///
/// This implementation currently does not make any modifications to remove
/// conditions for fusion. Code transformations to make loops conform to each of
/// the conditions for fusion are discussed in more detail in the document
/// above. These can be added to the current implementation in the future.
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LoopFuse.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Verifier.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/CodeMoverUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-fusion"
STATISTIC(FuseCounter, "Loops fused");
STATISTIC(NumFusionCandidates, "Number of candidates for loop fusion");
STATISTIC(InvalidPreheader, "Loop has invalid preheader");
STATISTIC(InvalidHeader, "Loop has invalid header");
STATISTIC(InvalidExitingBlock, "Loop has invalid exiting blocks");
STATISTIC(InvalidExitBlock, "Loop has invalid exit block");
STATISTIC(InvalidLatch, "Loop has invalid latch");
STATISTIC(InvalidLoop, "Loop is invalid");
STATISTIC(AddressTakenBB, "Basic block has address taken");
STATISTIC(MayThrowException, "Loop may throw an exception");
STATISTIC(ContainsVolatileAccess, "Loop contains a volatile access");
STATISTIC(NotSimplifiedForm, "Loop is not in simplified form");
STATISTIC(InvalidDependencies, "Dependencies prevent fusion");
STATISTIC(UnknownTripCount, "Loop has unknown trip count");
STATISTIC(UncomputableTripCount, "SCEV cannot compute trip count of loop");
STATISTIC(NonEqualTripCount, "Loop trip counts are not the same");
STATISTIC(NonAdjacent, "Loops are not adjacent");
STATISTIC(
NonEmptyPreheader,
"Loop has a non-empty preheader with instructions that cannot be moved");
STATISTIC(FusionNotBeneficial, "Fusion is not beneficial");
STATISTIC(NonIdenticalGuards, "Candidates have different guards");
STATISTIC(NonEmptyExitBlock, "Candidate has a non-empty exit block with "
"instructions that cannot be moved");
STATISTIC(NonEmptyGuardBlock, "Candidate has a non-empty guard block with "
"instructions that cannot be moved");
STATISTIC(NotRotated, "Candidate is not rotated");
enum FusionDependenceAnalysisChoice {
FUSION_DEPENDENCE_ANALYSIS_SCEV,
FUSION_DEPENDENCE_ANALYSIS_DA,
FUSION_DEPENDENCE_ANALYSIS_ALL,
};
static cl::opt<FusionDependenceAnalysisChoice> FusionDependenceAnalysis(
"loop-fusion-dependence-analysis",
cl::desc("Which dependence analysis should loop fusion use?"),
cl::values(clEnumValN(FUSION_DEPENDENCE_ANALYSIS_SCEV, "scev",
"Use the scalar evolution interface"),
clEnumValN(FUSION_DEPENDENCE_ANALYSIS_DA, "da",
"Use the dependence analysis interface"),
clEnumValN(FUSION_DEPENDENCE_ANALYSIS_ALL, "all",
"Use all available analyses")),
cl::Hidden, cl::init(FUSION_DEPENDENCE_ANALYSIS_ALL), cl::ZeroOrMore);
#ifndef NDEBUG
static cl::opt<bool>
VerboseFusionDebugging("loop-fusion-verbose-debug",
cl::desc("Enable verbose debugging for Loop Fusion"),
cl::Hidden, cl::init(false), cl::ZeroOrMore);
#endif
namespace {
/// This class is used to represent a candidate for loop fusion. When it is
/// constructed, it checks the conditions for loop fusion to ensure that it
/// represents a valid candidate. It caches several parts of a loop that are
/// used throughout loop fusion (e.g., loop preheader, loop header, etc) instead
/// of continually querying the underlying Loop to retrieve these values. It is
/// assumed these will not change throughout loop fusion.
///
/// The invalidate method should be used to indicate that the FusionCandidate is
/// no longer a valid candidate for fusion. Similarly, the isValid() method can
/// be used to ensure that the FusionCandidate is still valid for fusion.
struct FusionCandidate {
/// Cache of parts of the loop used throughout loop fusion. These should not
/// need to change throughout the analysis and transformation.
/// These parts are cached to avoid repeatedly looking up in the Loop class.
/// Preheader of the loop this candidate represents
BasicBlock *Preheader;
/// Header of the loop this candidate represents
BasicBlock *Header;
/// Blocks in the loop that exit the loop
BasicBlock *ExitingBlock;
/// The successor block of this loop (where the exiting blocks go to)
BasicBlock *ExitBlock;
/// Latch of the loop
BasicBlock *Latch;
/// The loop that this fusion candidate represents
Loop *L;
/// Vector of instructions in this loop that read from memory
SmallVector<Instruction *, 16> MemReads;
/// Vector of instructions in this loop that write to memory
SmallVector<Instruction *, 16> MemWrites;
/// Are all of the members of this fusion candidate still valid
bool Valid;
/// Guard branch of the loop, if it exists
BranchInst *GuardBranch;
/// Dominator and PostDominator trees are needed for the
/// FusionCandidateCompare function, required by FusionCandidateSet to
/// determine where the FusionCandidate should be inserted into the set. These
/// are used to establish ordering of the FusionCandidates based on dominance.
const DominatorTree *DT;
const PostDominatorTree *PDT;
OptimizationRemarkEmitter &ORE;
FusionCandidate(Loop *L, const DominatorTree *DT,
const PostDominatorTree *PDT, OptimizationRemarkEmitter &ORE)
: Preheader(L->getLoopPreheader()), Header(L->getHeader()),
ExitingBlock(L->getExitingBlock()), ExitBlock(L->getExitBlock()),
Latch(L->getLoopLatch()), L(L), Valid(true),
GuardBranch(L->getLoopGuardBranch()), DT(DT), PDT(PDT), ORE(ORE) {
// Walk over all blocks in the loop and check for conditions that may
// prevent fusion. For each block, walk over all instructions and collect
// the memory reads and writes If any instructions that prevent fusion are
// found, invalidate this object and return.
for (BasicBlock *BB : L->blocks()) {
if (BB->hasAddressTaken()) {
invalidate();
reportInvalidCandidate(AddressTakenBB);
return;
}
for (Instruction &I : *BB) {
if (I.mayThrow()) {
invalidate();
reportInvalidCandidate(MayThrowException);
return;
}
if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
if (SI->isVolatile()) {
invalidate();
reportInvalidCandidate(ContainsVolatileAccess);
return;
}
}
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
if (LI->isVolatile()) {
invalidate();
reportInvalidCandidate(ContainsVolatileAccess);
return;
}
}
if (I.mayWriteToMemory())
MemWrites.push_back(&I);
if (I.mayReadFromMemory())
MemReads.push_back(&I);
}
}
}
/// Check if all members of the class are valid.
bool isValid() const {
return Preheader && Header && ExitingBlock && ExitBlock && Latch && L &&
!L->isInvalid() && Valid;
}
/// Verify that all members are in sync with the Loop object.
void verify() const {
assert(isValid() && "Candidate is not valid!!");
assert(!L->isInvalid() && "Loop is invalid!");
assert(Preheader == L->getLoopPreheader() && "Preheader is out of sync");
assert(Header == L->getHeader() && "Header is out of sync");
assert(ExitingBlock == L->getExitingBlock() &&
"Exiting Blocks is out of sync");
assert(ExitBlock == L->getExitBlock() && "Exit block is out of sync");
assert(Latch == L->getLoopLatch() && "Latch is out of sync");
}
/// Get the entry block for this fusion candidate.
///
/// If this fusion candidate represents a guarded loop, the entry block is the
/// loop guard block. If it represents an unguarded loop, the entry block is
/// the preheader of the loop.
BasicBlock *getEntryBlock() const {
if (GuardBranch)
return GuardBranch->getParent();
else
return Preheader;
}
/// Given a guarded loop, get the successor of the guard that is not in the
/// loop.
///
/// This method returns the successor of the loop guard that is not located
/// within the loop (i.e., the successor of the guard that is not the
/// preheader).
/// This method is only valid for guarded loops.
BasicBlock *getNonLoopBlock() const {
assert(GuardBranch && "Only valid on guarded loops.");
assert(GuardBranch->isConditional() &&
"Expecting guard to be a conditional branch.");
return (GuardBranch->getSuccessor(0) == Preheader)
? GuardBranch->getSuccessor(1)
: GuardBranch->getSuccessor(0);
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void dump() const {
dbgs() << "\tGuardBranch: ";
if (GuardBranch)
dbgs() << *GuardBranch;
else
dbgs() << "nullptr";
dbgs() << "\n"
<< (GuardBranch ? GuardBranch->getName() : "nullptr") << "\n"
<< "\tPreheader: " << (Preheader ? Preheader->getName() : "nullptr")
<< "\n"
<< "\tHeader: " << (Header ? Header->getName() : "nullptr") << "\n"
<< "\tExitingBB: "
<< (ExitingBlock ? ExitingBlock->getName() : "nullptr") << "\n"
<< "\tExitBB: " << (ExitBlock ? ExitBlock->getName() : "nullptr")
<< "\n"
<< "\tLatch: " << (Latch ? Latch->getName() : "nullptr") << "\n"
<< "\tEntryBlock: "
<< (getEntryBlock() ? getEntryBlock()->getName() : "nullptr")
<< "\n";
}
#endif
/// Determine if a fusion candidate (representing a loop) is eligible for
/// fusion. Note that this only checks whether a single loop can be fused - it
/// does not check whether it is *legal* to fuse two loops together.
bool isEligibleForFusion(ScalarEvolution &SE) const {
if (!isValid()) {
LLVM_DEBUG(dbgs() << "FC has invalid CFG requirements!\n");
if (!Preheader)
++InvalidPreheader;
if (!Header)
++InvalidHeader;
if (!ExitingBlock)
++InvalidExitingBlock;
if (!ExitBlock)
++InvalidExitBlock;
if (!Latch)
++InvalidLatch;
if (L->isInvalid())
++InvalidLoop;
return false;
}
// Require ScalarEvolution to be able to determine a trip count.
if (!SE.hasLoopInvariantBackedgeTakenCount(L)) {
LLVM_DEBUG(dbgs() << "Loop " << L->getName()
<< " trip count not computable!\n");
return reportInvalidCandidate(UnknownTripCount);
}
if (!L->isLoopSimplifyForm()) {
LLVM_DEBUG(dbgs() << "Loop " << L->getName()
<< " is not in simplified form!\n");
return reportInvalidCandidate(NotSimplifiedForm);
}
if (!L->isRotatedForm()) {
LLVM_DEBUG(dbgs() << "Loop " << L->getName() << " is not rotated!\n");
return reportInvalidCandidate(NotRotated);
}
return true;
}
private:
// This is only used internally for now, to clear the MemWrites and MemReads
// list and setting Valid to false. I can't envision other uses of this right
// now, since once FusionCandidates are put into the FusionCandidateSet they
// are immutable. Thus, any time we need to change/update a FusionCandidate,
// we must create a new one and insert it into the FusionCandidateSet to
// ensure the FusionCandidateSet remains ordered correctly.
void invalidate() {
MemWrites.clear();
MemReads.clear();
Valid = false;
}
bool reportInvalidCandidate(llvm::Statistic &Stat) const {
using namespace ore;
assert(L && Preheader && "Fusion candidate not initialized properly!");
++Stat;
ORE.emit(OptimizationRemarkAnalysis(DEBUG_TYPE, Stat.getName(),
L->getStartLoc(), Preheader)
<< "[" << Preheader->getParent()->getName() << "]: "
<< "Loop is not a candidate for fusion: " << Stat.getDesc());
return false;
}
};
struct FusionCandidateCompare {
/// Comparison functor to sort two Control Flow Equivalent fusion candidates
/// into dominance order.
/// If LHS dominates RHS and RHS post-dominates LHS, return true;
/// IF RHS dominates LHS and LHS post-dominates RHS, return false;
bool operator()(const FusionCandidate &LHS,
const FusionCandidate &RHS) const {
const DominatorTree *DT = LHS.DT;
BasicBlock *LHSEntryBlock = LHS.getEntryBlock();
BasicBlock *RHSEntryBlock = RHS.getEntryBlock();
// Do not save PDT to local variable as it is only used in asserts and thus
// will trigger an unused variable warning if building without asserts.
assert(DT && LHS.PDT && "Expecting valid dominator tree");
// Do this compare first so if LHS == RHS, function returns false.
if (DT->dominates(RHSEntryBlock, LHSEntryBlock)) {
// RHS dominates LHS
// Verify LHS post-dominates RHS
assert(LHS.PDT->dominates(LHSEntryBlock, RHSEntryBlock));
return false;
}
if (DT->dominates(LHSEntryBlock, RHSEntryBlock)) {
// Verify RHS Postdominates LHS
assert(LHS.PDT->dominates(RHSEntryBlock, LHSEntryBlock));
return true;
}
// If LHS does not dominate RHS and RHS does not dominate LHS then there is
// no dominance relationship between the two FusionCandidates. Thus, they
// should not be in the same set together.
llvm_unreachable(
"No dominance relationship between these fusion candidates!");
}
};
using LoopVector = SmallVector<Loop *, 4>;
// Set of Control Flow Equivalent (CFE) Fusion Candidates, sorted in dominance
// order. Thus, if FC0 comes *before* FC1 in a FusionCandidateSet, then FC0
// dominates FC1 and FC1 post-dominates FC0.
// std::set was chosen because we want a sorted data structure with stable
// iterators. A subsequent patch to loop fusion will enable fusing non-ajdacent
// loops by moving intervening code around. When this intervening code contains
// loops, those loops will be moved also. The corresponding FusionCandidates
// will also need to be moved accordingly. As this is done, having stable
// iterators will simplify the logic. Similarly, having an efficient insert that
// keeps the FusionCandidateSet sorted will also simplify the implementation.
using FusionCandidateSet = std::set<FusionCandidate, FusionCandidateCompare>;
using FusionCandidateCollection = SmallVector<FusionCandidateSet, 4>;
#if !defined(NDEBUG)
static llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
const FusionCandidate &FC) {
if (FC.isValid())
OS << FC.Preheader->getName();
else
OS << "<Invalid>";
return OS;
}
static llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
const FusionCandidateSet &CandSet) {
for (const FusionCandidate &FC : CandSet)
OS << FC << '\n';
return OS;
}
static void
printFusionCandidates(const FusionCandidateCollection &FusionCandidates) {
dbgs() << "Fusion Candidates: \n";
for (const auto &CandidateSet : FusionCandidates) {
dbgs() << "*** Fusion Candidate Set ***\n";
dbgs() << CandidateSet;
dbgs() << "****************************\n";
}
}
#endif
/// Collect all loops in function at the same nest level, starting at the
/// outermost level.
///
/// This data structure collects all loops at the same nest level for a
/// given function (specified by the LoopInfo object). It starts at the
/// outermost level.
struct LoopDepthTree {
using LoopsOnLevelTy = SmallVector<LoopVector, 4>;
using iterator = LoopsOnLevelTy::iterator;
using const_iterator = LoopsOnLevelTy::const_iterator;
LoopDepthTree(LoopInfo &LI) : Depth(1) {
if (!LI.empty())
LoopsOnLevel.emplace_back(LoopVector(LI.rbegin(), LI.rend()));
}
/// Test whether a given loop has been removed from the function, and thus is
/// no longer valid.
bool isRemovedLoop(const Loop *L) const { return RemovedLoops.count(L); }
/// Record that a given loop has been removed from the function and is no
/// longer valid.
void removeLoop(const Loop *L) { RemovedLoops.insert(L); }
/// Descend the tree to the next (inner) nesting level
void descend() {
LoopsOnLevelTy LoopsOnNextLevel;
for (const LoopVector &LV : *this)
for (Loop *L : LV)
if (!isRemovedLoop(L) && L->begin() != L->end())
LoopsOnNextLevel.emplace_back(LoopVector(L->begin(), L->end()));
LoopsOnLevel = LoopsOnNextLevel;
RemovedLoops.clear();
Depth++;
}
bool empty() const { return size() == 0; }
size_t size() const { return LoopsOnLevel.size() - RemovedLoops.size(); }
unsigned getDepth() const { return Depth; }
iterator begin() { return LoopsOnLevel.begin(); }
iterator end() { return LoopsOnLevel.end(); }
const_iterator begin() const { return LoopsOnLevel.begin(); }
const_iterator end() const { return LoopsOnLevel.end(); }
private:
/// Set of loops that have been removed from the function and are no longer
/// valid.
SmallPtrSet<const Loop *, 8> RemovedLoops;
/// Depth of the current level, starting at 1 (outermost loops).
unsigned Depth;
/// Vector of loops at the current depth level that have the same parent loop
LoopsOnLevelTy LoopsOnLevel;
};
#ifndef NDEBUG
static void printLoopVector(const LoopVector &LV) {
dbgs() << "****************************\n";
for (auto L : LV)
printLoop(*L, dbgs());
dbgs() << "****************************\n";
}
#endif
struct LoopFuser {
private:
// Sets of control flow equivalent fusion candidates for a given nest level.
FusionCandidateCollection FusionCandidates;
LoopDepthTree LDT;
DomTreeUpdater DTU;
LoopInfo &LI;
DominatorTree &DT;
DependenceInfo &DI;
ScalarEvolution &SE;
PostDominatorTree &PDT;
OptimizationRemarkEmitter &ORE;
public:
LoopFuser(LoopInfo &LI, DominatorTree &DT, DependenceInfo &DI,
ScalarEvolution &SE, PostDominatorTree &PDT,
OptimizationRemarkEmitter &ORE, const DataLayout &DL)
: LDT(LI), DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy), LI(LI),
DT(DT), DI(DI), SE(SE), PDT(PDT), ORE(ORE) {}
/// This is the main entry point for loop fusion. It will traverse the
/// specified function and collect candidate loops to fuse, starting at the
/// outermost nesting level and working inwards.
bool fuseLoops(Function &F) {
#ifndef NDEBUG
if (VerboseFusionDebugging) {
LI.print(dbgs());
}
#endif
LLVM_DEBUG(dbgs() << "Performing Loop Fusion on function " << F.getName()
<< "\n");
bool Changed = false;
while (!LDT.empty()) {
LLVM_DEBUG(dbgs() << "Got " << LDT.size() << " loop sets for depth "
<< LDT.getDepth() << "\n";);
for (const LoopVector &LV : LDT) {
assert(LV.size() > 0 && "Empty loop set was build!");
// Skip singleton loop sets as they do not offer fusion opportunities on
// this level.
if (LV.size() == 1)
continue;
#ifndef NDEBUG
if (VerboseFusionDebugging) {
LLVM_DEBUG({
dbgs() << " Visit loop set (#" << LV.size() << "):\n";
printLoopVector(LV);
});
}
#endif
collectFusionCandidates(LV);
Changed |= fuseCandidates();
}
// Finished analyzing candidates at this level.
// Descend to the next level and clear all of the candidates currently
// collected. Note that it will not be possible to fuse any of the
// existing candidates with new candidates because the new candidates will
// be at a different nest level and thus not be control flow equivalent
// with all of the candidates collected so far.
LLVM_DEBUG(dbgs() << "Descend one level!\n");
LDT.descend();
FusionCandidates.clear();
}
if (Changed)
LLVM_DEBUG(dbgs() << "Function after Loop Fusion: \n"; F.dump(););
#ifndef NDEBUG
assert(DT.verify());
assert(PDT.verify());
LI.verify(DT);
SE.verify();
#endif
LLVM_DEBUG(dbgs() << "Loop Fusion complete\n");
return Changed;
}
private:
/// Determine if two fusion candidates are control flow equivalent.
///
/// Two fusion candidates are control flow equivalent if when one executes,
/// the other is guaranteed to execute. This is determined using dominators
/// and post-dominators: if A dominates B and B post-dominates A then A and B
/// are control-flow equivalent.
bool isControlFlowEquivalent(const FusionCandidate &FC0,
const FusionCandidate &FC1) const {
assert(FC0.Preheader && FC1.Preheader && "Expecting valid preheaders");
return ::isControlFlowEquivalent(*FC0.getEntryBlock(), *FC1.getEntryBlock(),
DT, PDT);
}
/// Iterate over all loops in the given loop set and identify the loops that
/// are eligible for fusion. Place all eligible fusion candidates into Control
/// Flow Equivalent sets, sorted by dominance.
void collectFusionCandidates(const LoopVector &LV) {
for (Loop *L : LV) {
FusionCandidate CurrCand(L, &DT, &PDT, ORE);
if (!CurrCand.isEligibleForFusion(SE))
continue;
// Go through each list in FusionCandidates and determine if L is control
// flow equivalent with the first loop in that list. If it is, append LV.
// If not, go to the next list.
// If no suitable list is found, start another list and add it to
// FusionCandidates.
bool FoundSet = false;
for (auto &CurrCandSet : FusionCandidates) {
if (isControlFlowEquivalent(*CurrCandSet.begin(), CurrCand)) {
CurrCandSet.insert(CurrCand);
FoundSet = true;
#ifndef NDEBUG
if (VerboseFusionDebugging)
LLVM_DEBUG(dbgs() << "Adding " << CurrCand
<< " to existing candidate set\n");
#endif
break;
}
}
if (!FoundSet) {
// No set was found. Create a new set and add to FusionCandidates
#ifndef NDEBUG
if (VerboseFusionDebugging)
LLVM_DEBUG(dbgs() << "Adding " << CurrCand << " to new set\n");
#endif
FusionCandidateSet NewCandSet;
NewCandSet.insert(CurrCand);
FusionCandidates.push_back(NewCandSet);
}
NumFusionCandidates++;
}
}
/// Determine if it is beneficial to fuse two loops.
///
/// For now, this method simply returns true because we want to fuse as much
/// as possible (primarily to test the pass). This method will evolve, over
/// time, to add heuristics for profitability of fusion.
bool isBeneficialFusion(const FusionCandidate &FC0,
const FusionCandidate &FC1) {
return true;
}
/// Determine if two fusion candidates have the same trip count (i.e., they
/// execute the same number of iterations).
///
/// Note that for now this method simply returns a boolean value because there
/// are no mechanisms in loop fusion to handle different trip counts. In the
/// future, this behaviour can be extended to adjust one of the loops to make
/// the trip counts equal (e.g., loop peeling). When this is added, this
/// interface may need to change to return more information than just a
/// boolean value.
bool identicalTripCounts(const FusionCandidate &FC0,
const FusionCandidate &FC1) const {
const SCEV *TripCount0 = SE.getBackedgeTakenCount(FC0.L);
if (isa<SCEVCouldNotCompute>(TripCount0)) {
UncomputableTripCount++;
LLVM_DEBUG(dbgs() << "Trip count of first loop could not be computed!");
return false;
}
const SCEV *TripCount1 = SE.getBackedgeTakenCount(FC1.L);
if (isa<SCEVCouldNotCompute>(TripCount1)) {
UncomputableTripCount++;
LLVM_DEBUG(dbgs() << "Trip count of second loop could not be computed!");
return false;
}
LLVM_DEBUG(dbgs() << "\tTrip counts: " << *TripCount0 << " & "
<< *TripCount1 << " are "
<< (TripCount0 == TripCount1 ? "identical" : "different")
<< "\n");
return (TripCount0 == TripCount1);
}
/// Walk each set of control flow equivalent fusion candidates and attempt to
/// fuse them. This does a single linear traversal of all candidates in the
/// set. The conditions for legal fusion are checked at this point. If a pair
/// of fusion candidates passes all legality checks, they are fused together
/// and a new fusion candidate is created and added to the FusionCandidateSet.
/// The original fusion candidates are then removed, as they are no longer
/// valid.
bool fuseCandidates() {
bool Fused = false;
LLVM_DEBUG(printFusionCandidates(FusionCandidates));
for (auto &CandidateSet : FusionCandidates) {
if (CandidateSet.size() < 2)
continue;
LLVM_DEBUG(dbgs() << "Attempting fusion on Candidate Set:\n"
<< CandidateSet << "\n");
for (auto FC0 = CandidateSet.begin(); FC0 != CandidateSet.end(); ++FC0) {
assert(!LDT.isRemovedLoop(FC0->L) &&
"Should not have removed loops in CandidateSet!");
auto FC1 = FC0;
for (++FC1; FC1 != CandidateSet.end(); ++FC1) {
assert(!LDT.isRemovedLoop(FC1->L) &&
"Should not have removed loops in CandidateSet!");
LLVM_DEBUG(dbgs() << "Attempting to fuse candidate \n"; FC0->dump();
dbgs() << " with\n"; FC1->dump(); dbgs() << "\n");
FC0->verify();
FC1->verify();
if (!identicalTripCounts(*FC0, *FC1)) {
LLVM_DEBUG(dbgs() << "Fusion candidates do not have identical trip "
"counts. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonEqualTripCount);
continue;
}
if (!isAdjacent(*FC0, *FC1)) {
LLVM_DEBUG(dbgs()
<< "Fusion candidates are not adjacent. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1, NonAdjacent);
continue;
}
// Ensure that FC0 and FC1 have identical guards.
// If one (or both) are not guarded, this check is not necessary.
if (FC0->GuardBranch && FC1->GuardBranch &&
!haveIdenticalGuards(*FC0, *FC1)) {
LLVM_DEBUG(dbgs() << "Fusion candidates do not have identical "
"guards. Not Fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonIdenticalGuards);
continue;
}
if (!isSafeToMoveBefore(*FC1->Preheader,
*FC0->Preheader->getTerminator(), DT, PDT,
DI)) {
LLVM_DEBUG(dbgs() << "Fusion candidate contains unsafe "
"instructions in preheader. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonEmptyPreheader);
continue;
}
if (FC0->GuardBranch) {
assert(FC1->GuardBranch && "Expecting valid FC1 guard branch");
if (!isSafeToMoveBefore(*FC0->ExitBlock,
*FC1->ExitBlock->getFirstNonPHIOrDbg(), DT,
PDT, DI)) {
LLVM_DEBUG(dbgs() << "Fusion candidate contains unsafe "
"instructions in exit block. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonEmptyExitBlock);
continue;
}
if (!isSafeToMoveBefore(
*FC1->GuardBranch->getParent(),
*FC0->GuardBranch->getParent()->getTerminator(), DT, PDT,
DI)) {
LLVM_DEBUG(dbgs()
<< "Fusion candidate contains unsafe "
"instructions in guard block. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonEmptyGuardBlock);
continue;
}
}
// Check the dependencies across the loops and do not fuse if it would
// violate them.
if (!dependencesAllowFusion(*FC0, *FC1)) {
LLVM_DEBUG(dbgs() << "Memory dependencies do not allow fusion!\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
InvalidDependencies);
continue;
}
bool BeneficialToFuse = isBeneficialFusion(*FC0, *FC1);
LLVM_DEBUG(dbgs()
<< "\tFusion appears to be "
<< (BeneficialToFuse ? "" : "un") << "profitable!\n");
if (!BeneficialToFuse) {
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
FusionNotBeneficial);
continue;
}
// All analysis has completed and has determined that fusion is legal
// and profitable. At this point, start transforming the code and
// perform fusion.
LLVM_DEBUG(dbgs() << "\tFusion is performed: " << *FC0 << " and "
<< *FC1 << "\n");
// Report fusion to the Optimization Remarks.
// Note this needs to be done *before* performFusion because
// performFusion will change the original loops, making it not
// possible to identify them after fusion is complete.
reportLoopFusion<OptimizationRemark>(*FC0, *FC1, FuseCounter);
FusionCandidate FusedCand(performFusion(*FC0, *FC1), &DT, &PDT, ORE);
FusedCand.verify();
assert(FusedCand.isEligibleForFusion(SE) &&
"Fused candidate should be eligible for fusion!");
// Notify the loop-depth-tree that these loops are not valid objects
LDT.removeLoop(FC1->L);
CandidateSet.erase(FC0);
CandidateSet.erase(FC1);
auto InsertPos = CandidateSet.insert(FusedCand);
assert(InsertPos.second &&
"Unable to insert TargetCandidate in CandidateSet!");
// Reset FC0 and FC1 the new (fused) candidate. Subsequent iterations
// of the FC1 loop will attempt to fuse the new (fused) loop with the
// remaining candidates in the current candidate set.
FC0 = FC1 = InsertPos.first;
LLVM_DEBUG(dbgs() << "Candidate Set (after fusion): " << CandidateSet
<< "\n");
Fused = true;
}
}
}
return Fused;
}
/// Rewrite all additive recurrences in a SCEV to use a new loop.
class AddRecLoopReplacer : public SCEVRewriteVisitor<AddRecLoopReplacer> {
public:
AddRecLoopReplacer(ScalarEvolution &SE, const Loop &OldL, const Loop &NewL,
bool UseMax = true)
: SCEVRewriteVisitor(SE), Valid(true), UseMax(UseMax), OldL(OldL),
NewL(NewL) {}
const SCEV *visitAddRecExpr(const SCEVAddRecExpr *Expr) {
const Loop *ExprL = Expr->getLoop();
SmallVector<const SCEV *, 2> Operands;
if (ExprL == &OldL) {
Operands.append(Expr->op_begin(), Expr->op_end());
return SE.getAddRecExpr(Operands, &NewL, Expr->getNoWrapFlags());
}
if (OldL.contains(ExprL)) {
bool Pos = SE.isKnownPositive(Expr->getStepRecurrence(SE));
if (!UseMax || !Pos || !Expr->isAffine()) {
Valid = false;
return Expr;
}
return visit(Expr->getStart());
}
for (const SCEV *Op : Expr->operands())
Operands.push_back(visit(Op));
return SE.getAddRecExpr(Operands, ExprL, Expr->getNoWrapFlags());
}
bool wasValidSCEV() const { return Valid; }
private:
bool Valid, UseMax;
const Loop &OldL, &NewL;
};
/// Return false if the access functions of \p I0 and \p I1 could cause
/// a negative dependence.
bool accessDiffIsPositive(const Loop &L0, const Loop &L1, Instruction &I0,
Instruction &I1, bool EqualIsInvalid) {
Value *Ptr0 = getLoadStorePointerOperand(&I0);
Value *Ptr1 = getLoadStorePointerOperand(&I1);
if (!Ptr0 || !Ptr1)
return false;
const SCEV *SCEVPtr0 = SE.getSCEVAtScope(Ptr0, &L0);
const SCEV *SCEVPtr1 = SE.getSCEVAtScope(Ptr1, &L1);
#ifndef NDEBUG
if (VerboseFusionDebugging)
LLVM_DEBUG(dbgs() << " Access function check: " << *SCEVPtr0 << " vs "
<< *SCEVPtr1 << "\n");
#endif
AddRecLoopReplacer Rewriter(SE, L0, L1);
SCEVPtr0 = Rewriter.visit(SCEVPtr0);
#ifndef NDEBUG
if (VerboseFusionDebugging)
LLVM_DEBUG(dbgs() << " Access function after rewrite: " << *SCEVPtr0
<< " [Valid: " << Rewriter.wasValidSCEV() << "]\n");
#endif
if (!Rewriter.wasValidSCEV())
return false;
// TODO: isKnownPredicate doesnt work well when one SCEV is loop carried (by
// L0) and the other is not. We could check if it is monotone and test
// the beginning and end value instead.
BasicBlock *L0Header = L0.getHeader();
auto HasNonLinearDominanceRelation = [&](const SCEV *S) {
const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(S);
if (!AddRec)
return false;
return !DT.dominates(L0Header, AddRec->getLoop()->getHeader()) &&
!DT.dominates(AddRec->getLoop()->getHeader(), L0Header);
};
if (SCEVExprContains(SCEVPtr1, HasNonLinearDominanceRelation))
return false;
ICmpInst::Predicate Pred =
EqualIsInvalid ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_SGE;
bool IsAlwaysGE = SE.isKnownPredicate(Pred, SCEVPtr0, SCEVPtr1);
#ifndef NDEBUG
if (VerboseFusionDebugging)
LLVM_DEBUG(dbgs() << " Relation: " << *SCEVPtr0
<< (IsAlwaysGE ? " >= " : " may < ") << *SCEVPtr1
<< "\n");
#endif
return IsAlwaysGE;
}
/// Return true if the dependences between @p I0 (in @p L0) and @p I1 (in
/// @p L1) allow loop fusion of @p L0 and @p L1. The dependence analyses
/// specified by @p DepChoice are used to determine this.
bool dependencesAllowFusion(const FusionCandidate &FC0,
const FusionCandidate &FC1, Instruction &I0,
Instruction &I1, bool AnyDep,
FusionDependenceAnalysisChoice DepChoice) {
#ifndef NDEBUG
if (VerboseFusionDebugging) {
LLVM_DEBUG(dbgs() << "Check dep: " << I0 << " vs " << I1 << " : "
<< DepChoice << "\n");
}
#endif
switch (DepChoice) {
case FUSION_DEPENDENCE_ANALYSIS_SCEV:
return accessDiffIsPositive(*FC0.L, *FC1.L, I0, I1, AnyDep);
case FUSION_DEPENDENCE_ANALYSIS_DA: {
auto DepResult = DI.depends(&I0, &I1, true);
if (!DepResult)
return true;
#ifndef NDEBUG
if (VerboseFusionDebugging) {
LLVM_DEBUG(dbgs() << "DA res: "; DepResult->dump(dbgs());
dbgs() << " [#l: " << DepResult->getLevels() << "][Ordered: "
<< (DepResult->isOrdered() ? "true" : "false")
<< "]\n");
LLVM_DEBUG(dbgs() << "DepResult Levels: " << DepResult->getLevels()
<< "\n");
}
#endif
if (DepResult->getNextPredecessor() || DepResult->getNextSuccessor())
LLVM_DEBUG(
dbgs() << "TODO: Implement pred/succ dependence handling!\n");
// TODO: Can we actually use the dependence info analysis here?
return false;
}
case FUSION_DEPENDENCE_ANALYSIS_ALL:
return dependencesAllowFusion(FC0, FC1, I0, I1, AnyDep,
FUSION_DEPENDENCE_ANALYSIS_SCEV) ||
dependencesAllowFusion(FC0, FC1, I0, I1, AnyDep,
FUSION_DEPENDENCE_ANALYSIS_DA);
}
llvm_unreachable("Unknown fusion dependence analysis choice!");
}
/// Perform a dependence check and return if @p FC0 and @p FC1 can be fused.
bool dependencesAllowFusion(const FusionCandidate &FC0,
const FusionCandidate &FC1) {
LLVM_DEBUG(dbgs() << "Check if " << FC0 << " can be fused with " << FC1
<< "\n");
assert(FC0.L->getLoopDepth() == FC1.L->getLoopDepth());
assert(DT.dominates(FC0.getEntryBlock(), FC1.getEntryBlock()));
for (Instruction *WriteL0 : FC0.MemWrites) {
for (Instruction *WriteL1 : FC1.MemWrites)
if (!dependencesAllowFusion(FC0, FC1, *WriteL0, *WriteL1,
/* AnyDep */ false,
FusionDependenceAnalysis)) {
InvalidDependencies++;
return false;
}
for (Instruction *ReadL1 : FC1.MemReads)
if (!dependencesAllowFusion(FC0, FC1, *WriteL0, *ReadL1,
/* AnyDep */ false,
FusionDependenceAnalysis)) {
InvalidDependencies++;
return false;
}
}
for (Instruction *WriteL1 : FC1.MemWrites) {
for (Instruction *WriteL0 : FC0.MemWrites)
if (!dependencesAllowFusion(FC0, FC1, *WriteL0, *WriteL1,
/* AnyDep */ false,
FusionDependenceAnalysis)) {
InvalidDependencies++;
return false;
}
for (Instruction *ReadL0 : FC0.MemReads)
if (!dependencesAllowFusion(FC0, FC1, *ReadL0, *WriteL1,
/* AnyDep */ false,
FusionDependenceAnalysis)) {
InvalidDependencies++;
return false;
}
}
// Walk through all uses in FC1. For each use, find the reaching def. If the
// def is located in FC0 then it is is not safe to fuse.
for (BasicBlock *BB : FC1.L->blocks())
for (Instruction &I : *BB)
for (auto &Op : I.operands())
if (Instruction *Def = dyn_cast<Instruction>(Op))
if (FC0.L->contains(Def->getParent())) {
InvalidDependencies++;
return false;
}
return true;
}
/// Determine if two fusion candidates are adjacent in the CFG.
///
/// This method will determine if there are additional basic blocks in the CFG
/// between the exit of \p FC0 and the entry of \p FC1.
/// If the two candidates are guarded loops, then it checks whether the
/// non-loop successor of the \p FC0 guard branch is the entry block of \p
/// FC1. If not, then the loops are not adjacent. If the two candidates are
/// not guarded loops, then it checks whether the exit block of \p FC0 is the
/// preheader of \p FC1.
bool isAdjacent(const FusionCandidate &FC0,
const FusionCandidate &FC1) const {
// If the successor of the guard branch is FC1, then the loops are adjacent
if (FC0.GuardBranch)
return FC0.getNonLoopBlock() == FC1.getEntryBlock();
else
return FC0.ExitBlock == FC1.getEntryBlock();
}
/// Determine if two fusion candidates have identical guards
///
/// This method will determine if two fusion candidates have the same guards.
/// The guards are considered the same if:
/// 1. The instructions to compute the condition used in the compare are
/// identical.
/// 2. The successors of the guard have the same flow into/around the loop.
/// If the compare instructions are identical, then the first successor of the
/// guard must go to the same place (either the preheader of the loop or the
/// NonLoopBlock). In other words, the the first successor of both loops must
/// both go into the loop (i.e., the preheader) or go around the loop (i.e.,
/// the NonLoopBlock). The same must be true for the second successor.
bool haveIdenticalGuards(const FusionCandidate &FC0,
const FusionCandidate &FC1) const {
assert(FC0.GuardBranch && FC1.GuardBranch &&
"Expecting FC0 and FC1 to be guarded loops.");
if (auto FC0CmpInst =
dyn_cast<Instruction>(FC0.GuardBranch->getCondition()))
if (auto FC1CmpInst =
dyn_cast<Instruction>(FC1.GuardBranch->getCondition()))
if (!FC0CmpInst->isIdenticalTo(FC1CmpInst))
return false;
// The compare instructions are identical.
// Now make sure the successor of the guards have the same flow into/around
// the loop
if (FC0.GuardBranch->getSuccessor(0) == FC0.Preheader)
return (FC1.GuardBranch->getSuccessor(0) == FC1.Preheader);
else
return (FC1.GuardBranch->getSuccessor(1) == FC1.Preheader);
}
/// Simplify the condition of the latch branch of \p FC to true, when both of
/// its successors are the same.
void simplifyLatchBranch(const FusionCandidate &FC) const {
BranchInst *FCLatchBranch = dyn_cast<BranchInst>(FC.Latch->getTerminator());
if (FCLatchBranch) {
assert(FCLatchBranch->isConditional() &&
FCLatchBranch->getSuccessor(0) == FCLatchBranch->getSuccessor(1) &&
"Expecting the two successors of FCLatchBranch to be the same");
FCLatchBranch->setCondition(
llvm::ConstantInt::getTrue(FCLatchBranch->getCondition()->getType()));
}
}
/// Move instructions from FC0.Latch to FC1.Latch. If FC0.Latch has an unique
/// successor, then merge FC0.Latch with its unique successor.
void mergeLatch(const FusionCandidate &FC0, const FusionCandidate &FC1) {
moveInstructionsToTheBeginning(*FC0.Latch, *FC1.Latch, DT, PDT, DI);
if (BasicBlock *Succ = FC0.Latch->getUniqueSuccessor()) {
MergeBlockIntoPredecessor(Succ, &DTU, &LI);
DTU.flush();
}
}
/// Fuse two fusion candidates, creating a new fused loop.
///
/// This method contains the mechanics of fusing two loops, represented by \p
/// FC0 and \p FC1. It is assumed that \p FC0 dominates \p FC1 and \p FC1
/// postdominates \p FC0 (making them control flow equivalent). It also
/// assumes that the other conditions for fusion have been met: adjacent,
/// identical trip counts, and no negative distance dependencies exist that
/// would prevent fusion. Thus, there is no checking for these conditions in
/// this method.
///
/// Fusion is performed by rewiring the CFG to update successor blocks of the
/// components of tho loop. Specifically, the following changes are done:
///
/// 1. The preheader of \p FC1 is removed as it is no longer necessary
/// (because it is currently only a single statement block).
/// 2. The latch of \p FC0 is modified to jump to the header of \p FC1.
/// 3. The latch of \p FC1 i modified to jump to the header of \p FC0.
/// 4. All blocks from \p FC1 are removed from FC1 and added to FC0.
///
/// All of these modifications are done with dominator tree updates, thus
/// keeping the dominator (and post dominator) information up-to-date.
///
/// This can be improved in the future by actually merging blocks during
/// fusion. For example, the preheader of \p FC1 can be merged with the
/// preheader of \p FC0. This would allow loops with more than a single
/// statement in the preheader to be fused. Similarly, the latch blocks of the
/// two loops could also be fused into a single block. This will require
/// analysis to prove it is safe to move the contents of the block past
/// existing code, which currently has not been implemented.
Loop *performFusion(const FusionCandidate &FC0, const FusionCandidate &FC1) {
assert(FC0.isValid() && FC1.isValid() &&
"Expecting valid fusion candidates");
LLVM_DEBUG(dbgs() << "Fusion Candidate 0: \n"; FC0.dump();
dbgs() << "Fusion Candidate 1: \n"; FC1.dump(););
// Move instructions from the preheader of FC1 to the end of the preheader
// of FC0.
moveInstructionsToTheEnd(*FC1.Preheader, *FC0.Preheader, DT, PDT, DI);
// Fusing guarded loops is handled slightly differently than non-guarded
// loops and has been broken out into a separate method instead of trying to
// intersperse the logic within a single method.
if (FC0.GuardBranch)
return fuseGuardedLoops(FC0, FC1);
assert(FC1.Preheader == FC0.ExitBlock);
assert(FC1.Preheader->size() == 1 &&
FC1.Preheader->getSingleSuccessor() == FC1.Header);
// Remember the phi nodes originally in the header of FC0 in order to rewire
// them later. However, this is only necessary if the new loop carried
// values might not dominate the exiting branch. While we do not generally
// test if this is the case but simply insert intermediate phi nodes, we
// need to make sure these intermediate phi nodes have different
// predecessors. To this end, we filter the special case where the exiting
// block is the latch block of the first loop. Nothing needs to be done
// anyway as all loop carried values dominate the latch and thereby also the
// exiting branch.
SmallVector<PHINode *, 8> OriginalFC0PHIs;
if (FC0.ExitingBlock != FC0.Latch)
for (PHINode &PHI : FC0.Header->phis())
OriginalFC0PHIs.push_back(&PHI);
// Replace incoming blocks for header PHIs first.
FC1.Preheader->replaceSuccessorsPhiUsesWith(FC0.Preheader);
FC0.Latch->replaceSuccessorsPhiUsesWith(FC1.Latch);
// Then modify the control flow and update DT and PDT.
SmallVector<DominatorTree::UpdateType, 8> TreeUpdates;
// The old exiting block of the first loop (FC0) has to jump to the header
// of the second as we need to execute the code in the second header block
// regardless of the trip count. That is, if the trip count is 0, so the
// back edge is never taken, we still have to execute both loop headers,
// especially (but not only!) if the second is a do-while style loop.
// However, doing so might invalidate the phi nodes of the first loop as
// the new values do only need to dominate their latch and not the exiting
// predicate. To remedy this potential problem we always introduce phi
// nodes in the header of the second loop later that select the loop carried
// value, if the second header was reached through an old latch of the
// first, or undef otherwise. This is sound as exiting the first implies the
// second will exit too, __without__ taking the back-edge. [Their
// trip-counts are equal after all.
// KB: Would this sequence be simpler to just just make FC0.ExitingBlock go
// to FC1.Header? I think this is basically what the three sequences are
// trying to accomplish; however, doing this directly in the CFG may mean
// the DT/PDT becomes invalid
FC0.ExitingBlock->getTerminator()->replaceUsesOfWith(FC1.Preheader,
FC1.Header);
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC0.ExitingBlock, FC1.Preheader));
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Insert, FC0.ExitingBlock, FC1.Header));
// The pre-header of L1 is not necessary anymore.
assert(pred_begin(FC1.Preheader) == pred_end(FC1.Preheader));
FC1.Preheader->getTerminator()->eraseFromParent();
new UnreachableInst(FC1.Preheader->getContext(), FC1.Preheader);
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC1.Preheader, FC1.Header));
// Moves the phi nodes from the second to the first loops header block.
while (PHINode *PHI = dyn_cast<PHINode>(&FC1.Header->front())) {
if (SE.isSCEVable(PHI->getType()))
SE.forgetValue(PHI);
if (PHI->hasNUsesOrMore(1))
PHI->moveBefore(&*FC0.Header->getFirstInsertionPt());
else
PHI->eraseFromParent();
}
// Introduce new phi nodes in the second loop header to ensure
// exiting the first and jumping to the header of the second does not break
// the SSA property of the phis originally in the first loop. See also the
// comment above.
Instruction *L1HeaderIP = &FC1.Header->front();
for (PHINode *LCPHI : OriginalFC0PHIs) {
int L1LatchBBIdx = LCPHI->getBasicBlockIndex(FC1.Latch);
assert(L1LatchBBIdx >= 0 &&
"Expected loop carried value to be rewired at this point!");
Value *LCV = LCPHI->getIncomingValue(L1LatchBBIdx);
PHINode *L1HeaderPHI = PHINode::Create(
LCV->getType(), 2, LCPHI->getName() + ".afterFC0", L1HeaderIP);
L1HeaderPHI->addIncoming(LCV, FC0.Latch);
L1HeaderPHI->addIncoming(UndefValue::get(LCV->getType()),
FC0.ExitingBlock);
LCPHI->setIncomingValue(L1LatchBBIdx, L1HeaderPHI);
}
// Replace latch terminator destinations.
FC0.Latch->getTerminator()->replaceUsesOfWith(FC0.Header, FC1.Header);
FC1.Latch->getTerminator()->replaceUsesOfWith(FC1.Header, FC0.Header);
// Change the condition of FC0 latch branch to true, as both successors of
// the branch are the same.
simplifyLatchBranch(FC0);
// If FC0.Latch and FC0.ExitingBlock are the same then we have already
// performed the updates above.
if (FC0.Latch != FC0.ExitingBlock)
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Insert, FC0.Latch, FC1.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Delete,
FC0.Latch, FC0.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Insert,
FC1.Latch, FC0.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Delete,
FC1.Latch, FC1.Header));
// Update DT/PDT
DTU.applyUpdates(TreeUpdates);
LI.removeBlock(FC1.Preheader);
DTU.deleteBB(FC1.Preheader);
DTU.flush();
// Is there a way to keep SE up-to-date so we don't need to forget the loops
// and rebuild the information in subsequent passes of fusion?
// Note: Need to forget the loops before merging the loop latches, as
// mergeLatch may remove the only block in FC1.
SE.forgetLoop(FC1.L);
SE.forgetLoop(FC0.L);
// Move instructions from FC0.Latch to FC1.Latch.
// Note: mergeLatch requires an updated DT.
mergeLatch(FC0, FC1);
// Merge the loops.
SmallVector<BasicBlock *, 8> Blocks(FC1.L->block_begin(),
FC1.L->block_end());
for (BasicBlock *BB : Blocks) {
FC0.L->addBlockEntry(BB);
FC1.L->removeBlockFromLoop(BB);
if (LI.getLoopFor(BB) != FC1.L)
continue;
LI.changeLoopFor(BB, FC0.L);
}
while (!FC1.L->empty()) {
const auto &ChildLoopIt = FC1.L->begin();
Loop *ChildLoop = *ChildLoopIt;
FC1.L->removeChildLoop(ChildLoopIt);
FC0.L->addChildLoop(ChildLoop);
}
// Delete the now empty loop L1.
LI.erase(FC1.L);
#ifndef NDEBUG
assert(!verifyFunction(*FC0.Header->getParent(), &errs()));
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
assert(PDT.verify());
LI.verify(DT);
SE.verify();
#endif
LLVM_DEBUG(dbgs() << "Fusion done:\n");
return FC0.L;
}
/// Report details on loop fusion opportunities.
///
/// This template function can be used to report both successful and missed
/// loop fusion opportunities, based on the RemarkKind. The RemarkKind should
/// be one of:
/// - OptimizationRemarkMissed to report when loop fusion is unsuccessful
/// given two valid fusion candidates.
/// - OptimizationRemark to report successful fusion of two fusion
/// candidates.
/// The remarks will be printed using the form:
/// <path/filename>:<line number>:<column number>: [<function name>]:
/// <Cand1 Preheader> and <Cand2 Preheader>: <Stat Description>
template <typename RemarkKind>
void reportLoopFusion(const FusionCandidate &FC0, const FusionCandidate &FC1,
llvm::Statistic &Stat) {
assert(FC0.Preheader && FC1.Preheader &&
"Expecting valid fusion candidates");
using namespace ore;
++Stat;
ORE.emit(RemarkKind(DEBUG_TYPE, Stat.getName(), FC0.L->getStartLoc(),
FC0.Preheader)
<< "[" << FC0.Preheader->getParent()->getName()
<< "]: " << NV("Cand1", StringRef(FC0.Preheader->getName()))
<< " and " << NV("Cand2", StringRef(FC1.Preheader->getName()))
<< ": " << Stat.getDesc());
}
/// Fuse two guarded fusion candidates, creating a new fused loop.
///
/// Fusing guarded loops is handled much the same way as fusing non-guarded
/// loops. The rewiring of the CFG is slightly different though, because of
/// the presence of the guards around the loops and the exit blocks after the
/// loop body. As such, the new loop is rewired as follows:
/// 1. Keep the guard branch from FC0 and use the non-loop block target
/// from the FC1 guard branch.
/// 2. Remove the exit block from FC0 (this exit block should be empty
/// right now).
/// 3. Remove the guard branch for FC1
/// 4. Remove the preheader for FC1.
/// The exit block successor for the latch of FC0 is updated to be the header
/// of FC1 and the non-exit block successor of the latch of FC1 is updated to
/// be the header of FC0, thus creating the fused loop.
Loop *fuseGuardedLoops(const FusionCandidate &FC0,
const FusionCandidate &FC1) {
assert(FC0.GuardBranch && FC1.GuardBranch && "Expecting guarded loops");
BasicBlock *FC0GuardBlock = FC0.GuardBranch->getParent();
BasicBlock *FC1GuardBlock = FC1.GuardBranch->getParent();
BasicBlock *FC0NonLoopBlock = FC0.getNonLoopBlock();
BasicBlock *FC1NonLoopBlock = FC1.getNonLoopBlock();
// Move instructions from the exit block of FC0 to the beginning of the exit
// block of FC1.
moveInstructionsToTheBeginning(*FC0.ExitBlock, *FC1.ExitBlock, DT, PDT, DI);
// Move instructions from the guard block of FC1 to the end of the guard
// block of FC0.
moveInstructionsToTheEnd(*FC1GuardBlock, *FC0GuardBlock, DT, PDT, DI);
assert(FC0NonLoopBlock == FC1GuardBlock && "Loops are not adjacent");
SmallVector<DominatorTree::UpdateType, 8> TreeUpdates;
////////////////////////////////////////////////////////////////////////////
// Update the Loop Guard
////////////////////////////////////////////////////////////////////////////
// The guard for FC0 is updated to guard both FC0 and FC1. This is done by
// changing the NonLoopGuardBlock for FC0 to the NonLoopGuardBlock for FC1.
// Thus, one path from the guard goes to the preheader for FC0 (and thus
// executes the new fused loop) and the other path goes to the NonLoopBlock
// for FC1 (where FC1 guard would have gone if FC1 was not executed).
FC0.GuardBranch->replaceUsesOfWith(FC0NonLoopBlock, FC1NonLoopBlock);
FC0.ExitBlock->getTerminator()->replaceUsesOfWith(FC1GuardBlock,
FC1.Header);
// The guard of FC1 is not necessary anymore.
FC1.GuardBranch->eraseFromParent();
new UnreachableInst(FC1GuardBlock->getContext(), FC1GuardBlock);
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC1GuardBlock, FC1.Preheader));
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC1GuardBlock, FC1NonLoopBlock));
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC0GuardBlock, FC1GuardBlock));
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Insert, FC0GuardBlock, FC1NonLoopBlock));
assert(pred_begin(FC1GuardBlock) == pred_end(FC1GuardBlock) &&
"Expecting guard block to have no predecessors");
assert(succ_begin(FC1GuardBlock) == succ_end(FC1GuardBlock) &&
"Expecting guard block to have no successors");
// Remember the phi nodes originally in the header of FC0 in order to rewire
// them later. However, this is only necessary if the new loop carried
// values might not dominate the exiting branch. While we do not generally
// test if this is the case but simply insert intermediate phi nodes, we
// need to make sure these intermediate phi nodes have different
// predecessors. To this end, we filter the special case where the exiting
// block is the latch block of the first loop. Nothing needs to be done
// anyway as all loop carried values dominate the latch and thereby also the
// exiting branch.
// KB: This is no longer necessary because FC0.ExitingBlock == FC0.Latch
// (because the loops are rotated. Thus, nothing will ever be added to
// OriginalFC0PHIs.
SmallVector<PHINode *, 8> OriginalFC0PHIs;
if (FC0.ExitingBlock != FC0.Latch)
for (PHINode &PHI : FC0.Header->phis())
OriginalFC0PHIs.push_back(&PHI);
assert(OriginalFC0PHIs.empty() && "Expecting OriginalFC0PHIs to be empty!");
// Replace incoming blocks for header PHIs first.
FC1.Preheader->replaceSuccessorsPhiUsesWith(FC0.Preheader);
FC0.Latch->replaceSuccessorsPhiUsesWith(FC1.Latch);
// The old exiting block of the first loop (FC0) has to jump to the header
// of the second as we need to execute the code in the second header block
// regardless of the trip count. That is, if the trip count is 0, so the
// back edge is never taken, we still have to execute both loop headers,
// especially (but not only!) if the second is a do-while style loop.
// However, doing so might invalidate the phi nodes of the first loop as
// the new values do only need to dominate their latch and not the exiting
// predicate. To remedy this potential problem we always introduce phi
// nodes in the header of the second loop later that select the loop carried
// value, if the second header was reached through an old latch of the
// first, or undef otherwise. This is sound as exiting the first implies the
// second will exit too, __without__ taking the back-edge (their
// trip-counts are equal after all).
FC0.ExitingBlock->getTerminator()->replaceUsesOfWith(FC0.ExitBlock,
FC1.Header);
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC0.ExitingBlock, FC0.ExitBlock));
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Insert, FC0.ExitingBlock, FC1.Header));
// Remove FC0 Exit Block
// The exit block for FC0 is no longer needed since control will flow
// directly to the header of FC1. Since it is an empty block, it can be
// removed at this point.
// TODO: In the future, we can handle non-empty exit blocks my merging any
// instructions from FC0 exit block into FC1 exit block prior to removing
// the block.
assert(pred_begin(FC0.ExitBlock) == pred_end(FC0.ExitBlock) &&
"Expecting exit block to be empty");
FC0.ExitBlock->getTerminator()->eraseFromParent();
new UnreachableInst(FC0.ExitBlock->getContext(), FC0.ExitBlock);
// Remove FC1 Preheader
// The pre-header of L1 is not necessary anymore.
assert(pred_begin(FC1.Preheader) == pred_end(FC1.Preheader));
FC1.Preheader->getTerminator()->eraseFromParent();
new UnreachableInst(FC1.Preheader->getContext(), FC1.Preheader);
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Delete, FC1.Preheader, FC1.Header));
// Moves the phi nodes from the second to the first loops header block.
while (PHINode *PHI = dyn_cast<PHINode>(&FC1.Header->front())) {
if (SE.isSCEVable(PHI->getType()))
SE.forgetValue(PHI);
if (PHI->hasNUsesOrMore(1))
PHI->moveBefore(&*FC0.Header->getFirstInsertionPt());
else
PHI->eraseFromParent();
}
// Introduce new phi nodes in the second loop header to ensure
// exiting the first and jumping to the header of the second does not break
// the SSA property of the phis originally in the first loop. See also the
// comment above.
Instruction *L1HeaderIP = &FC1.Header->front();
for (PHINode *LCPHI : OriginalFC0PHIs) {
int L1LatchBBIdx = LCPHI->getBasicBlockIndex(FC1.Latch);
assert(L1LatchBBIdx >= 0 &&
"Expected loop carried value to be rewired at this point!");
Value *LCV = LCPHI->getIncomingValue(L1LatchBBIdx);
PHINode *L1HeaderPHI = PHINode::Create(
LCV->getType(), 2, LCPHI->getName() + ".afterFC0", L1HeaderIP);
L1HeaderPHI->addIncoming(LCV, FC0.Latch);
L1HeaderPHI->addIncoming(UndefValue::get(LCV->getType()),
FC0.ExitingBlock);
LCPHI->setIncomingValue(L1LatchBBIdx, L1HeaderPHI);
}
// Update the latches
// Replace latch terminator destinations.
FC0.Latch->getTerminator()->replaceUsesOfWith(FC0.Header, FC1.Header);
FC1.Latch->getTerminator()->replaceUsesOfWith(FC1.Header, FC0.Header);
// Change the condition of FC0 latch branch to true, as both successors of
// the branch are the same.
simplifyLatchBranch(FC0);
// If FC0.Latch and FC0.ExitingBlock are the same then we have already
// performed the updates above.
if (FC0.Latch != FC0.ExitingBlock)
TreeUpdates.emplace_back(DominatorTree::UpdateType(
DominatorTree::Insert, FC0.Latch, FC1.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Delete,
FC0.Latch, FC0.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Insert,
FC1.Latch, FC0.Header));
TreeUpdates.emplace_back(DominatorTree::UpdateType(DominatorTree::Delete,
FC1.Latch, FC1.Header));
// All done
// Apply the updates to the Dominator Tree and cleanup.
assert(succ_begin(FC1GuardBlock) == succ_end(FC1GuardBlock) &&
"FC1GuardBlock has successors!!");
assert(pred_begin(FC1GuardBlock) == pred_end(FC1GuardBlock) &&
"FC1GuardBlock has predecessors!!");
// Update DT/PDT
DTU.applyUpdates(TreeUpdates);
LI.removeBlock(FC1.Preheader);
DTU.deleteBB(FC1.Preheader);
DTU.deleteBB(FC0.ExitBlock);
DTU.flush();
// Is there a way to keep SE up-to-date so we don't need to forget the loops
// and rebuild the information in subsequent passes of fusion?
// Note: Need to forget the loops before merging the loop latches, as
// mergeLatch may remove the only block in FC1.
SE.forgetLoop(FC1.L);
SE.forgetLoop(FC0.L);
// Move instructions from FC0.Latch to FC1.Latch.
// Note: mergeLatch requires an updated DT.
mergeLatch(FC0, FC1);
// Merge the loops.
SmallVector<BasicBlock *, 8> Blocks(FC1.L->block_begin(),
FC1.L->block_end());
for (BasicBlock *BB : Blocks) {
FC0.L->addBlockEntry(BB);
FC1.L->removeBlockFromLoop(BB);
if (LI.getLoopFor(BB) != FC1.L)
continue;
LI.changeLoopFor(BB, FC0.L);
}
while (!FC1.L->empty()) {
const auto &ChildLoopIt = FC1.L->begin();
Loop *ChildLoop = *ChildLoopIt;
FC1.L->removeChildLoop(ChildLoopIt);
FC0.L->addChildLoop(ChildLoop);
}
// Delete the now empty loop L1.
LI.erase(FC1.L);
#ifndef NDEBUG
assert(!verifyFunction(*FC0.Header->getParent(), &errs()));
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
assert(PDT.verify());
LI.verify(DT);
SE.verify();
#endif
LLVM_DEBUG(dbgs() << "Fusion done:\n");
return FC0.L;
}
};
struct LoopFuseLegacy : public FunctionPass {
static char ID;
LoopFuseLegacy() : FunctionPass(ID) {
initializeLoopFuseLegacyPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequiredID(LoopSimplifyID);
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<PostDominatorTreeWrapperPass>();
AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
AU.addRequired<DependenceAnalysisWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<PostDominatorTreeWrapperPass>();
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &DI = getAnalysis<DependenceAnalysisWrapperPass>().getDI();
auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
auto &ORE = getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
const DataLayout &DL = F.getParent()->getDataLayout();
LoopFuser LF(LI, DT, DI, SE, PDT, ORE, DL);
return LF.fuseLoops(F);
}
};
} // namespace
PreservedAnalyses LoopFusePass::run(Function &F, FunctionAnalysisManager &AM) {
auto &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &DI = AM.getResult<DependenceAnalysis>(F);
auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
auto &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
const DataLayout &DL = F.getParent()->getDataLayout();
LoopFuser LF(LI, DT, DI, SE, PDT, ORE, DL);
bool Changed = LF.fuseLoops(F);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<PostDominatorTreeAnalysis>();
PA.preserve<ScalarEvolutionAnalysis>();
PA.preserve<LoopAnalysis>();
return PA;
}
char LoopFuseLegacy::ID = 0;
INITIALIZE_PASS_BEGIN(LoopFuseLegacy, "loop-fusion", "Loop Fusion", false,
false)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DependenceAnalysisWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
INITIALIZE_PASS_END(LoopFuseLegacy, "loop-fusion", "Loop Fusion", false, false)
FunctionPass *llvm::createLoopFusePass() { return new LoopFuseLegacy(); }
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