llvm-6502/lib/Transforms/Scalar/LoopUnroll.cpp
2008-02-20 11:08:44 +00:00

500 lines
18 KiB
C++

//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop unroller. It works best when loops have
// been canonicalized by the -indvars pass, allowing it to determine the trip
// counts of loops easily.
//
// This pass will multi-block loops only if they contain no non-unrolled
// subloops. The process of unrolling can produce extraneous basic blocks
// linked with unconditional branches. This will be corrected in the future.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-unroll"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IntrinsicInst.h"
#include <algorithm>
#include <climits>
#include <cstdio>
using namespace llvm;
STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
namespace {
cl::opt<unsigned>
UnrollThreshold
("unroll-threshold", cl::init(100), cl::Hidden,
cl::desc("The cut-off point for automatic loop unrolling"));
cl::opt<unsigned>
UnrollCount
("unroll-count", cl::init(0), cl::Hidden,
cl::desc("Use this unroll count for all loops, for testing purposes"));
class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
LoopInfo *LI; // The current loop information
public:
static char ID; // Pass ID, replacement for typeid
LoopUnroll() : LoopPass((intptr_t)&ID) {}
/// A magic value for use with the Threshold parameter to indicate
/// that the loop unroll should be performed regardless of how much
/// code expansion would result.
static const unsigned NoThreshold = UINT_MAX;
bool runOnLoop(Loop *L, LPPassManager &LPM);
bool unrollLoop(Loop *L, unsigned Count, unsigned Threshold);
BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addRequired<LoopInfo>();
AU.addPreservedID(LCSSAID);
AU.addPreserved<LoopInfo>();
}
};
char LoopUnroll::ID = 0;
RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
}
LoopPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L) {
unsigned Size = 0;
for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) {
BasicBlock *BB = L->getBlocks()[i];
Instruction *Term = BB->getTerminator();
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
if (isa<PHINode>(I) && BB == L->getHeader()) {
// Ignore PHI nodes in the header.
} else if (I->hasOneUse() && I->use_back() == Term) {
// Ignore instructions only used by the loop terminator.
} else if (isa<DbgInfoIntrinsic>(I)) {
// Ignore debug instructions
} else {
++Size;
}
// TODO: Ignore expressions derived from PHI and constants if inval of phi
// is a constant, or if operation is associative. This will get induction
// variables.
}
}
return Size;
}
// RemapInstruction - Convert the instruction operands from referencing the
// current values into those specified by ValueMap.
//
static inline void RemapInstruction(Instruction *I,
DenseMap<const Value *, Value*> &ValueMap) {
for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
Value *Op = I->getOperand(op);
DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
if (It != ValueMap.end()) Op = It->second;
I->setOperand(op, Op);
}
}
// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
// only has one predecessor, and that predecessor only has one successor.
// Returns the new combined block.
BasicBlock *LoopUnroll::FoldBlockIntoPredecessor(BasicBlock *BB) {
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
//
BasicBlock *OnlyPred = BB->getSinglePredecessor();
if (!OnlyPred) return 0;
if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
return 0;
DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
// Resolve any PHI nodes at the start of the block. They are all
// guaranteed to have exactly one entry if they exist, unless there are
// multiple duplicate (but guaranteed to be equal) entries for the
// incoming edges. This occurs when there are multiple edges from
// OnlyPred to OnlySucc.
//
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
PN->replaceAllUsesWith(PN->getIncomingValue(0));
BB->getInstList().pop_front(); // Delete the phi node...
}
// Delete the unconditional branch from the predecessor...
OnlyPred->getInstList().pop_back();
// Move all definitions in the successor to the predecessor...
OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
// Make all PHI nodes that referred to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(OnlyPred);
std::string OldName = BB->getName();
// Erase basic block from the function...
LI->removeBlock(BB);
BB->eraseFromParent();
// Inherit predecessor's name if it exists...
if (!OldName.empty() && !OnlyPred->hasName())
OnlyPred->setName(OldName);
return OnlyPred;
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
LI = &getAnalysis<LoopInfo>();
// Unroll the loop.
if (!unrollLoop(L, UnrollCount, UnrollThreshold))
return false;
// Update the loop information for this loop.
// If we completely unrolled the loop, remove it from the parent.
if (L->getNumBackEdges() == 0)
LPM.deleteLoopFromQueue(L);
return true;
}
/// Unroll the given loop by UnrollCount, or by a heuristically-determined
/// value if Count is zero. If Threshold is not NoThreshold, it is a value
/// to limit code size expansion. If the loop size would expand beyond the
/// threshold value, unrolling is suppressed. The return value is true if
/// any transformations are performed.
///
bool LoopUnroll::unrollLoop(Loop *L, unsigned Count, unsigned Threshold) {
assert(L->isLCSSAForm());
BasicBlock *Header = L->getHeader();
BasicBlock *LatchBlock = L->getLoopLatch();
BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << "\n";
if (!BI || BI->isUnconditional()) {
// The loop-rotate pass can be helpful to avoid this in many cases.
DOUT << " Can't unroll; loop not terminated by a conditional branch.\n";
return false;
}
// Determine the trip count and/or trip multiple. A TripCount value of zero
// is used to mean an unknown trip count. The TripMultiple value is the
// greatest known integer multiple of the trip count.
unsigned TripCount = 0;
unsigned TripMultiple = 1;
if (Value *TripCountValue = L->getTripCount()) {
if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCountValue)) {
// Guard against huge trip counts. This also guards against assertions in
// APInt from the use of getZExtValue, below.
if (TripCountC->getValue().getActiveBits() <= 32) {
TripCount = (unsigned)TripCountC->getZExtValue();
}
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCountValue)) {
switch (BO->getOpcode()) {
case BinaryOperator::Mul:
if (ConstantInt *MultipleC = dyn_cast<ConstantInt>(BO->getOperand(1))) {
if (MultipleC->getValue().getActiveBits() <= 32) {
TripMultiple = (unsigned)MultipleC->getZExtValue();
}
}
break;
default: break;
}
}
}
if (TripCount != 0)
DOUT << " Trip Count = " << TripCount << "\n";
if (TripMultiple != 1)
DOUT << " Trip Multiple = " << TripMultiple << "\n";
// Automatically select an unroll count.
if (Count == 0) {
// Conservative heuristic: if we know the trip count, see if we can
// completely unroll (subject to the threshold, checked below); otherwise
// don't unroll.
if (TripCount != 0) {
Count = TripCount;
} else {
return false;
}
}
// Effectively "DCE" unrolled iterations that are beyond the tripcount
// and will never be executed.
if (TripCount != 0 && Count > TripCount)
Count = TripCount;
assert(Count > 0);
assert(TripMultiple > 0);
assert(TripCount == 0 || TripCount % TripMultiple == 0);
// Enforce the threshold.
if (Threshold != NoThreshold) {
unsigned LoopSize = ApproximateLoopSize(L);
DOUT << " Loop Size = " << LoopSize << "\n";
uint64_t Size = (uint64_t)LoopSize*Count;
if (TripCount != 1 && Size > Threshold) {
DOUT << " TOO LARGE TO UNROLL: "
<< Size << ">" << Threshold << "\n";
return false;
}
}
// Are we eliminating the loop control altogether?
bool CompletelyUnroll = Count == TripCount;
// If we know the trip count, we know the multiple...
unsigned BreakoutTrip = 0;
if (TripCount != 0) {
BreakoutTrip = TripCount % Count;
TripMultiple = 0;
} else {
// Figure out what multiple to use.
BreakoutTrip = TripMultiple =
(unsigned)GreatestCommonDivisor64(Count, TripMultiple);
}
if (CompletelyUnroll) {
DOUT << "COMPLETELY UNROLLING loop %" << Header->getName()
<< " with trip count " << TripCount << "!\n";
} else {
DOUT << "UNROLLING loop %" << Header->getName()
<< " by " << Count;
if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
DOUT << " with a breakout at trip " << BreakoutTrip;
} else if (TripMultiple != 1) {
DOUT << " with " << TripMultiple << " trips per branch";
}
DOUT << "!\n";
}
std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
// For the first iteration of the loop, we should use the precloned values for
// PHI nodes. Insert associations now.
typedef DenseMap<const Value*, Value*> ValueMapTy;
ValueMapTy LastValueMap;
std::vector<PHINode*> OrigPHINode;
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
OrigPHINode.push_back(PN);
if (Instruction *I =
dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock)))
if (L->contains(I->getParent()))
LastValueMap[I] = I;
}
std::vector<BasicBlock*> Headers;
std::vector<BasicBlock*> Latches;
Headers.push_back(Header);
Latches.push_back(LatchBlock);
for (unsigned It = 1; It != Count; ++It) {
char SuffixBuffer[100];
sprintf(SuffixBuffer, ".%d", It);
std::vector<BasicBlock*> NewBlocks;
for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(),
E = LoopBlocks.end(); BB != E; ++BB) {
ValueMapTy ValueMap;
BasicBlock *New = CloneBasicBlock(*BB, ValueMap, SuffixBuffer);
Header->getParent()->getBasicBlockList().push_back(New);
// Loop over all of the PHI nodes in the block, changing them to use the
// incoming values from the previous block.
if (*BB == Header)
for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]);
Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
if (Instruction *InValI = dyn_cast<Instruction>(InVal))
if (It > 1 && L->contains(InValI->getParent()))
InVal = LastValueMap[InValI];
ValueMap[OrigPHINode[i]] = InVal;
New->getInstList().erase(NewPHI);
}
// Update our running map of newest clones
LastValueMap[*BB] = New;
for (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end();
VI != VE; ++VI)
LastValueMap[VI->first] = VI->second;
L->addBasicBlockToLoop(New, LI->getBase());
// Add phi entries for newly created values to all exit blocks except
// the successor of the latch block. The successor of the exit block will
// be updated specially after unrolling all the way.
if (*BB != LatchBlock)
for (Value::use_iterator UI = (*BB)->use_begin(), UE = (*BB)->use_end();
UI != UE;) {
Instruction *UseInst = cast<Instruction>(*UI);
++UI;
if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) {
PHINode *phi = cast<PHINode>(UseInst);
Value *Incoming = phi->getIncomingValueForBlock(*BB);
phi->addIncoming(Incoming, New);
}
}
// Keep track of new headers and latches as we create them, so that
// we can insert the proper branches later.
if (*BB == Header)
Headers.push_back(New);
if (*BB == LatchBlock) {
Latches.push_back(New);
// Also, clear out the new latch's back edge so that it doesn't look
// like a new loop, so that it's amenable to being merged with adjacent
// blocks later on.
TerminatorInst *Term = New->getTerminator();
assert(L->contains(Term->getSuccessor(!ContinueOnTrue)));
assert(Term->getSuccessor(ContinueOnTrue) == LoopExit);
Term->setSuccessor(!ContinueOnTrue, NULL);
}
NewBlocks.push_back(New);
}
// Remap all instructions in the most recent iteration
for (unsigned i = 0; i < NewBlocks.size(); ++i)
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
E = NewBlocks[i]->end(); I != E; ++I)
RemapInstruction(I, LastValueMap);
}
// The latch block exits the loop. If there are any PHI nodes in the
// successor blocks, update them to use the appropriate values computed as the
// last iteration of the loop.
if (Count != 1) {
SmallPtrSet<PHINode*, 8> Users;
for (Value::use_iterator UI = LatchBlock->use_begin(),
UE = LatchBlock->use_end(); UI != UE; ++UI)
if (PHINode *phi = dyn_cast<PHINode>(*UI))
Users.insert(phi);
BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]);
for (SmallPtrSet<PHINode*,8>::iterator SI = Users.begin(), SE = Users.end();
SI != SE; ++SI) {
PHINode *PN = *SI;
Value *InVal = PN->removeIncomingValue(LatchBlock, false);
// If this value was defined in the loop, take the value defined by the
// last iteration of the loop.
if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
if (L->contains(InValI->getParent()))
InVal = LastValueMap[InVal];
}
PN->addIncoming(InVal, LastIterationBB);
}
}
// Now, if we're doing complete unrolling, loop over the PHI nodes in the
// original block, setting them to their incoming values.
if (CompletelyUnroll) {
BasicBlock *Preheader = L->getLoopPreheader();
for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
PHINode *PN = OrigPHINode[i];
PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
Header->getInstList().erase(PN);
}
}
// Now that all the basic blocks for the unrolled iterations are in place,
// set up the branches to connect them.
for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
// The original branch was replicated in each unrolled iteration.
BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
// The branch destination.
unsigned j = (i + 1) % e;
BasicBlock *Dest = Headers[j];
bool NeedConditional = true;
// For a complete unroll, make the last iteration end with a branch
// to the exit block.
if (CompletelyUnroll && j == 0) {
Dest = LoopExit;
NeedConditional = false;
}
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
NeedConditional = false;
}
if (NeedConditional) {
// Update the conditional branch's successor for the following
// iteration.
Term->setSuccessor(!ContinueOnTrue, Dest);
} else {
Term->setUnconditionalDest(Dest);
// Merge adjacent basic blocks, if possible.
if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest)) {
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
std::replace(Headers.begin(), Headers.end(), Dest, Fold);
}
}
}
// At this point, the code is well formed. We now do a quick sweep over the
// inserted code, doing constant propagation and dead code elimination as we
// go.
const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
Instruction *Inst = I++;
if (isInstructionTriviallyDead(Inst))
(*BB)->getInstList().erase(Inst);
else if (Constant *C = ConstantFoldInstruction(Inst)) {
Inst->replaceAllUsesWith(C);
(*BB)->getInstList().erase(Inst);
}
}
NumCompletelyUnrolled += CompletelyUnroll;
++NumUnrolled;
return true;
}