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

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//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and 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/LoopInfo.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/IntrinsicInst.h"
#include <cstdio>
#include <set>
#include <algorithm>
#include <iostream>
using namespace llvm;
namespace {
Statistic<> NumUnrolled("loop-unroll", "Number of loops completely unrolled");
cl::opt<unsigned>
UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
cl::desc("The cut-off point for loop unrolling"));
class LoopUnroll : public FunctionPass {
LoopInfo *LI; // The current loop information
public:
virtual bool runOnFunction(Function &F);
bool visitLoop(Loop *L);
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>();
}
};
RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
}
FunctionPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
bool LoopUnroll::runOnFunction(Function &F) {
bool Changed = false;
LI = &getAnalysis<LoopInfo>();
// Transform all the top-level loops. Copy the loop list so that the child
// can update the loop tree if it needs to delete the loop.
std::vector<Loop*> SubLoops(LI->begin(), LI->end());
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= visitLoop(SubLoops[i]);
return Changed;
}
/// ApproximateLoopSize - Approximate the size of the loop after it has been
/// unrolled.
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 (DbgInfoIntrinsic *DbgI = dyn_cast<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,
std::map<const Value *, Value*> &ValueMap) {
for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
Value *Op = I->getOperand(op);
std::map<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;
DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
TerminatorInst *Term = OnlyPred->getTerminator();
// 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 predecessors name if it exists...
if (!OldName.empty() && !OnlyPred->hasName())
OnlyPred->setName(OldName);
return OnlyPred;
}
bool LoopUnroll::visitLoop(Loop *L) {
bool Changed = false;
// Recurse through all subloops before we process this loop. Copy the loop
// list so that the child can update the loop tree if it needs to delete the
// loop.
std::vector<Loop*> SubLoops(L->begin(), L->end());
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= visitLoop(SubLoops[i]);
BasicBlock* Header = L->getHeader();
BasicBlock* LatchBlock = L->getLoopLatch();
BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
if (BI == 0) return Changed; // Must end in a conditional branch
ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount());
if (!TripCountC) return Changed; // Must have constant trip count!
uint64_t TripCountFull = TripCountC->getRawValue();
if (TripCountFull != TripCountC->getRawValue() || TripCountFull == 0)
return Changed; // More than 2^32 iterations???
unsigned LoopSize = ApproximateLoopSize(L);
DEBUG(std::cerr << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << " Loop Size = "
<< LoopSize << " Trip Count = " << TripCountFull << " - ");
uint64_t Size = (uint64_t)LoopSize*TripCountFull;
if (Size > UnrollThreshold) {
DEBUG(std::cerr << "TOO LARGE: " << Size << ">" << UnrollThreshold << "\n");
return Changed;
}
DEBUG(std::cerr << "UNROLLING!\n");
std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
unsigned TripCount = (unsigned)TripCountFull;
BasicBlock *LoopExit = BI->getSuccessor(L->contains(BI->getSuccessor(0)));
// For the first iteration of the loop, we should use the precloned values for
// PHI nodes. Insert associations now.
std::map<const Value*, Value*> 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;
}
// Remove the exit branch from the loop
LatchBlock->getInstList().erase(BI);
std::vector<BasicBlock*> Headers;
std::vector<BasicBlock*> Latches;
Headers.push_back(Header);
Latches.push_back(LatchBlock);
assert(TripCount != 0 && "Trip count of 0 is impossible!");
for (unsigned It = 1; It != TripCount; ++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) {
std::map<const Value*, Value*> 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 (std::map<const Value*, Value*>::iterator VI = ValueMap.begin(),
VE = ValueMap.end(); VI != VE; ++VI)
LastValueMap[VI->first] = VI->second;
L->addBasicBlockToLoop(New, *LI);
// 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; ++UI) {
Instruction* UseInst = cast<Instruction>(*UI);
if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) {
PHINode* phi = cast<PHINode>(UseInst);
Value* Incoming = phi->getIncomingValueForBlock(*BB);
if (isa<Instruction>(Incoming))
Incoming = LastValueMap[Incoming];
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);
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);
}
// Update PHI nodes that reference the final latch block
if (TripCount > 1) {
std::set<PHINode*> 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);
for (std::set<PHINode*>::iterator SI = Users.begin(), SE = Users.end();
SI != SE; ++SI) {
Value* InVal = (*SI)->getIncomingValueForBlock(LatchBlock);
if (isa<Instruction>(InVal))
InVal = LastValueMap[InVal];
(*SI)->removeIncomingValue(LatchBlock, false);
if (InVal)
(*SI)->addIncoming(InVal, cast<BasicBlock>(LastValueMap[LatchBlock]));
}
}
// Now loop over the PHI nodes in the original block, setting them to their
// incoming values.
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);
}
// Insert the branches that link the different iterations together
for (unsigned i = 0; i < Latches.size()-1; ++i) {
new BranchInst(Headers[i+1], Latches[i]);
if(BasicBlock* Fold = FoldBlockIntoPredecessor(Headers[i+1])) {
std::replace(Latches.begin(), Latches.end(), Headers[i+1], Fold);
std::replace(Headers.begin(), Headers.end(), Headers[i+1], Fold);
}
}
// Finally, add an unconditional branch to the block to continue into the exit
// block.
new BranchInst(LoopExit, Latches[Latches.size()-1]);
FoldBlockIntoPredecessor(LoopExit);
// 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);
}
}
// Update the loop information for this loop.
Loop *Parent = L->getParentLoop();
// Move all of the basic blocks in the loop into the parent loop.
for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
E = NewLoopBlocks.end(); BB != E; ++BB)
LI->changeLoopFor(*BB, Parent);
// Remove the loop from the parent.
if (Parent)
delete Parent->removeChildLoop(std::find(Parent->begin(), Parent->end(),L));
else
delete LI->removeLoop(std::find(LI->begin(), LI->end(), L));
++NumUnrolled;
return true;
}
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