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

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//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Loop Rotation Pass.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-rotate"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallVector.h"
using namespace llvm;
#define MAX_HEADER_SIZE 16
STATISTIC(NumRotated, "Number of loops rotated");
namespace {
class RenameData {
public:
RenameData(Instruction *O, Value *P, Instruction *H)
: Original(O), PreHeader(P), Header(H) { }
public:
Instruction *Original; // Original instruction
Value *PreHeader; // Original pre-header replacement
Instruction *Header; // New header replacement
};
class LoopRotate : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopRotate() : LoopPass(&ID) {}
// Rotate Loop L as many times as possible. Return true if
// loop is rotated at least once.
bool runOnLoop(Loop *L, LPPassManager &LPM);
// LCSSA form makes instruction renaming easier.
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addPreserved<ScalarEvolution>();
AU.addPreserved<LoopInfo>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<DominanceFrontier>();
}
// Helper functions
/// Do actual work
bool rotateLoop(Loop *L, LPPassManager &LPM);
/// Initialize local data
void initialize();
/// Make sure all Exit block PHINodes have required incoming values.
/// If incoming value is constant or defined outside the loop then
/// PHINode may not have an entry for original pre-header.
void updateExitBlock();
/// Return true if this instruction is used outside original header.
bool usedOutsideOriginalHeader(Instruction *In);
/// Find Replacement information for instruction. Return NULL if it is
/// not available.
const RenameData *findReplacementData(Instruction *I);
/// After loop rotation, loop pre-header has multiple sucessors.
/// Insert one forwarding basic block to ensure that loop pre-header
/// has only one successor.
void preserveCanonicalLoopForm(LPPassManager &LPM);
private:
Loop *L;
BasicBlock *OrigHeader;
BasicBlock *OrigPreHeader;
BasicBlock *OrigLatch;
BasicBlock *NewHeader;
BasicBlock *Exit;
LPPassManager *LPM_Ptr;
SmallVector<RenameData, MAX_HEADER_SIZE> LoopHeaderInfo;
};
}
char LoopRotate::ID = 0;
static RegisterPass<LoopRotate> X("loop-rotate", "Rotate Loops");
Pass *llvm::createLoopRotatePass() { return new LoopRotate(); }
/// Rotate Loop L as many times as possible. Return true if
/// the loop is rotated at least once.
bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
bool RotatedOneLoop = false;
initialize();
LPM_Ptr = &LPM;
// One loop can be rotated multiple times.
while (rotateLoop(Lp,LPM)) {
RotatedOneLoop = true;
initialize();
}
return RotatedOneLoop;
}
/// Rotate loop LP. Return true if the loop is rotated.
bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
L = Lp;
OrigHeader = L->getHeader();
OrigPreHeader = L->getLoopPreheader();
OrigLatch = L->getLoopLatch();
// If the loop has only one block then there is not much to rotate.
if (L->getBlocks().size() == 1)
return false;
assert(OrigHeader && OrigLatch && OrigPreHeader &&
"Loop is not in canonical form");
// If the loop header is not one of the loop exiting blocks then
// either this loop is already rotated or it is not
// suitable for loop rotation transformations.
if (!L->isLoopExit(OrigHeader))
return false;
BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
if (!BI)
return false;
assert(BI->isConditional() && "Branch Instruction is not conditional");
// Updating PHInodes in loops with multiple exits adds complexity.
// Keep it simple, and restrict loop rotation to loops with one exit only.
// In future, lift this restriction and support for multiple exits if
// required.
SmallVector<BasicBlock*, 8> ExitBlocks;
L->getExitBlocks(ExitBlocks);
if (ExitBlocks.size() > 1)
return false;
// Check size of original header and reject
// loop if it is very big.
unsigned Size = 0;
// FIXME: Use common api to estimate size.
for (BasicBlock::const_iterator OI = OrigHeader->begin(),
OE = OrigHeader->end(); OI != OE; ++OI) {
if (isa<PHINode>(OI))
continue; // PHI nodes don't count.
if (isa<DbgInfoIntrinsic>(OI))
continue; // Debug intrinsics don't count as size.
Size++;
}
if (Size > MAX_HEADER_SIZE)
return false;
// Now, this loop is suitable for rotation.
// Find new Loop header. NewHeader is a Header's one and only successor
// that is inside loop. Header's other successor is outside the
// loop. Otherwise loop is not suitable for rotation.
Exit = BI->getSuccessor(0);
NewHeader = BI->getSuccessor(1);
if (L->contains(Exit))
std::swap(Exit, NewHeader);
assert(NewHeader && "Unable to determine new loop header");
assert(L->contains(NewHeader) && !L->contains(Exit) &&
"Unable to determine loop header and exit blocks");
// This code assumes that the new header has exactly one predecessor.
// Remove any single-entry PHI nodes in it.
assert(NewHeader->getSinglePredecessor() &&
"New header doesn't have one pred!");
FoldSingleEntryPHINodes(NewHeader);
// Copy PHI nodes and other instructions from the original header
// into the original pre-header. Unlike the original header, the original
// pre-header is not a member of the loop.
//
// The new loop header is the one and only successor of original header that
// is inside the loop. All other original header successors are outside
// the loop. Copy PHI Nodes from the original header into the new loop header.
// Add second incoming value, from original loop pre-header into these phi
// nodes. If a value defined in original header is used outside original
// header then new loop header will need new phi nodes with two incoming
// values, one definition from original header and second definition is
// from original loop pre-header.
// Remove terminator from Original pre-header. Original pre-header will
// receive a clone of original header terminator as a new terminator.
OrigPreHeader->getInstList().pop_back();
BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
PHINode *PN = 0;
for (; (PN = dyn_cast<PHINode>(I)); ++I) {
// PHI nodes are not copied into original pre-header. Instead their values
// are directly propagated.
Value *NPV = PN->getIncomingValueForBlock(OrigPreHeader);
// Create a new PHI node with two incoming values for NewHeader.
// One incoming value is from OrigLatch (through OrigHeader) and the
// second incoming value is from original pre-header.
PHINode *NH = PHINode::Create(PN->getType(), PN->getName(),
NewHeader->begin());
NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
NH->addIncoming(NPV, OrigPreHeader);
// "In" can be replaced by NH at various places.
LoopHeaderInfo.push_back(RenameData(PN, NPV, NH));
}
// Now, handle non-phi instructions.
for (; I != E; ++I) {
Instruction *In = I;
assert(!isa<PHINode>(In) && "PHINode is not expected here");
// This is not a PHI instruction. Insert its clone into original pre-header.
// If this instruction is using a value from same basic block then
// update it to use value from cloned instruction.
Instruction *C = In->clone(In->getContext());
C->setName(In->getName());
OrigPreHeader->getInstList().push_back(C);
for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) {
Instruction *OpInsn = dyn_cast<Instruction>(In->getOperand(opi));
if (!OpInsn) continue; // Ignore non-instruction values.
if (const RenameData *D = findReplacementData(OpInsn))
C->setOperand(opi, D->PreHeader);
}
// If this instruction is used outside this basic block then
// create new PHINode for this instruction.
Instruction *NewHeaderReplacement = NULL;
if (usedOutsideOriginalHeader(In)) {
PHINode *PN = PHINode::Create(In->getType(), In->getName(),
NewHeader->begin());
PN->addIncoming(In, OrigHeader);
PN->addIncoming(C, OrigPreHeader);
NewHeaderReplacement = PN;
}
LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement));
}
// Rename uses of original header instructions to reflect their new
// definitions (either from original pre-header node or from newly created
// new header PHINodes.
//
// Original header instructions are used in
// 1) Original header:
//
// If instruction is used in non-phi instructions then it is using
// defintion from original heder iteself. Do not replace this use
// with definition from new header or original pre-header.
//
// If instruction is used in phi node then it is an incoming
// value. Rename its use to reflect new definition from new-preheader
// or new header.
//
// 2) Inside loop but not in original header
//
// Replace this use to reflect definition from new header.
for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
if (!ILoopHeaderInfo.Header)
continue;
Instruction *OldPhi = ILoopHeaderInfo.Original;
Instruction *NewPhi = ILoopHeaderInfo.Header;
// Before replacing uses, collect them first, so that iterator is
// not invalidated.
SmallVector<Instruction *, 16> AllUses;
for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
UI != UE; ++UI)
AllUses.push_back(cast<Instruction>(UI));
for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(),
UE = AllUses.end(); UI != UE; ++UI) {
Instruction *U = *UI;
BasicBlock *Parent = U->getParent();
// Used inside original header
if (Parent == OrigHeader) {
// Do not rename uses inside original header non-phi instructions.
PHINode *PU = dyn_cast<PHINode>(U);
if (!PU)
continue;
// Do not rename uses inside original header phi nodes, if the
// incoming value is for new header.
if (PU->getBasicBlockIndex(NewHeader) != -1
&& PU->getIncomingValueForBlock(NewHeader) == U)
continue;
U->replaceUsesOfWith(OldPhi, NewPhi);
continue;
}
// Used inside loop, but not in original header.
if (L->contains(U->getParent())) {
if (U != NewPhi)
U->replaceUsesOfWith(OldPhi, NewPhi);
continue;
}
// Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
if (U->getParent() == Exit) {
assert(isa<PHINode>(U) && "Use in Exit Block that is not PHINode");
PHINode *UPhi = cast<PHINode>(U);
// UPhi already has one incoming argument from original header.
// Add second incoming argument from new Pre header.
UPhi->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
} else {
// Used outside Exit block. Create a new PHI node in the exit block
// to receive the value from the new header and pre-header.
PHINode *PN = PHINode::Create(U->getType(), U->getName(),
Exit->begin());
PN->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
PN->addIncoming(OldPhi, OrigHeader);
U->replaceUsesOfWith(OldPhi, PN);
}
}
}
/// Make sure all Exit block PHINodes have required incoming values.
updateExitBlock();
// Update CFG
// Removing incoming branch from loop preheader to original header.
// Now original header is inside the loop.
for (BasicBlock::iterator I = OrigHeader->begin();
(PN = dyn_cast<PHINode>(I)); ++I)
PN->removeIncomingValue(OrigPreHeader);
// Make NewHeader as the new header for the loop.
L->moveToHeader(NewHeader);
preserveCanonicalLoopForm(LPM);
NumRotated++;
return true;
}
/// Make sure all Exit block PHINodes have required incoming values.
/// If an incoming value is constant or defined outside the loop then
/// PHINode may not have an entry for the original pre-header.
void LoopRotate::updateExitBlock() {
PHINode *PN;
for (BasicBlock::iterator I = Exit->begin();
(PN = dyn_cast<PHINode>(I)); ++I) {
// There is already one incoming value from original pre-header block.
if (PN->getBasicBlockIndex(OrigPreHeader) != -1)
continue;
const RenameData *ILoopHeaderInfo;
Value *V = PN->getIncomingValueForBlock(OrigHeader);
if (isa<Instruction>(V) &&
(ILoopHeaderInfo = findReplacementData(cast<Instruction>(V)))) {
assert(ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction");
PN->addIncoming(ILoopHeaderInfo->PreHeader, OrigPreHeader);
} else {
PN->addIncoming(V, OrigPreHeader);
}
}
}
/// Initialize local data
void LoopRotate::initialize() {
L = NULL;
OrigHeader = NULL;
OrigPreHeader = NULL;
NewHeader = NULL;
Exit = NULL;
LoopHeaderInfo.clear();
}
/// Return true if this instruction is used by any instructions in the loop that
/// aren't in original header.
bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
UI != UE; ++UI) {
BasicBlock *UserBB = cast<Instruction>(UI)->getParent();
if (UserBB != OrigHeader && L->contains(UserBB))
return true;
}
return false;
}
/// Find Replacement information for instruction. Return NULL if it is
/// not available.
const RenameData *LoopRotate::findReplacementData(Instruction *In) {
// Since LoopHeaderInfo is small, linear walk is OK.
for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
if (ILoopHeaderInfo.Original == In)
return &ILoopHeaderInfo;
}
return NULL;
}
/// After loop rotation, loop pre-header has multiple sucessors.
/// Insert one forwarding basic block to ensure that loop pre-header
/// has only one successor.
void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) {
// Right now original pre-header has two successors, new header and
// exit block. Insert new block between original pre-header and
// new header such that loop's new pre-header has only one successor.
BasicBlock *NewPreHeader = BasicBlock::Create(OrigHeader->getContext(),
"bb.nph",
OrigHeader->getParent(),
NewHeader);
LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
if (Loop *PL = LI.getLoopFor(OrigPreHeader))
PL->addBasicBlockToLoop(NewPreHeader, LI.getBase());
BranchInst::Create(NewHeader, NewPreHeader);
BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
if (OrigPH_BI->getSuccessor(0) == NewHeader)
OrigPH_BI->setSuccessor(0, NewPreHeader);
else {
assert(OrigPH_BI->getSuccessor(1) == NewHeader &&
"Unexpected original pre-header terminator");
OrigPH_BI->setSuccessor(1, NewPreHeader);
}
PHINode *PN;
for (BasicBlock::iterator I = NewHeader->begin();
(PN = dyn_cast<PHINode>(I)); ++I) {
int index = PN->getBasicBlockIndex(OrigPreHeader);
assert(index != -1 && "Expected incoming value from Original PreHeader");
PN->setIncomingBlock(index, NewPreHeader);
assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 &&
"Expected only one incoming value from Original PreHeader");
}
if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
DT->addNewBlock(NewPreHeader, OrigPreHeader);
DT->changeImmediateDominator(L->getHeader(), NewPreHeader);
DT->changeImmediateDominator(Exit, OrigPreHeader);
for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
BI != BE; ++BI) {
BasicBlock *B = *BI;
if (L->getHeader() != B) {
DomTreeNode *Node = DT->getNode(B);
if (Node && Node->getBlock() == OrigHeader)
DT->changeImmediateDominator(*BI, L->getHeader());
}
}
DT->changeImmediateDominator(OrigHeader, OrigLatch);
}
if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) {
// New Preheader's dominance frontier is Exit block.
DominanceFrontier::DomSetType NewPHSet;
NewPHSet.insert(Exit);
DF->addBasicBlock(NewPreHeader, NewPHSet);
// New Header's dominance frontier now includes itself and Exit block
DominanceFrontier::iterator HeadI = DF->find(L->getHeader());
if (HeadI != DF->end()) {
DominanceFrontier::DomSetType & HeaderSet = HeadI->second;
HeaderSet.clear();
HeaderSet.insert(L->getHeader());
HeaderSet.insert(Exit);
} else {
DominanceFrontier::DomSetType HeaderSet;
HeaderSet.insert(L->getHeader());
HeaderSet.insert(Exit);
DF->addBasicBlock(L->getHeader(), HeaderSet);
}
// Original header (new Loop Latch)'s dominance frontier is Exit.
DominanceFrontier::iterator LatchI = DF->find(L->getLoopLatch());
if (LatchI != DF->end()) {
DominanceFrontier::DomSetType &LatchSet = LatchI->second;
LatchSet = LatchI->second;
LatchSet.clear();
LatchSet.insert(Exit);
} else {
DominanceFrontier::DomSetType LatchSet;
LatchSet.insert(Exit);
DF->addBasicBlock(L->getHeader(), LatchSet);
}
// If a loop block dominates new loop latch then add to its frontiers
// new header and Exit and remove new latch (which is equal to original
// header).
BasicBlock *NewLatch = L->getLoopLatch();
assert(NewLatch == OrigHeader && "NewLatch is inequal to OrigHeader");
if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
BI != BE; ++BI) {
BasicBlock *B = *BI;
if (DT->dominates(B, NewLatch)) {
DominanceFrontier::iterator BDFI = DF->find(B);
if (BDFI != DF->end()) {
DominanceFrontier::DomSetType &BSet = BDFI->second;
BSet.erase(NewLatch);
BSet.insert(L->getHeader());
BSet.insert(Exit);
} else {
DominanceFrontier::DomSetType BSet;
BSet.insert(L->getHeader());
BSet.insert(Exit);
DF->addBasicBlock(B, BSet);
}
}
}
}
}
// Preserve canonical loop form, which means Exit block should
// have only one predecessor.
SplitEdge(L->getLoopLatch(), Exit, this);
assert(NewHeader && L->getHeader() == NewHeader &&
"Invalid loop header after loop rotation");
assert(NewPreHeader && L->getLoopPreheader() == NewPreHeader &&
"Invalid loop preheader after loop rotation");
assert(L->getLoopLatch() &&
"Invalid loop latch after loop rotation");
}
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