blob: c1f97889e61c54d1b876c145ccd44734711759fd [file] [log] [blame]
//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
// 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 file defines the LoopInfo class that is used to identify natural loops
// and determine the loop depth of various nodes of the CFG. Note that the
// loops identified may actually be several natural loops that share the same
// header node... not just a single natural loop.
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/CFG.h"
#include "llvm/Assembly/Writer.h"
#include "Support/DepthFirstIterator.h"
#include <algorithm>
static RegisterAnalysis<LoopInfo>
X("loops", "Natural Loop Construction", true);
// Loop implementation
bool Loop::contains(const BasicBlock *BB) const {
return find(Blocks.begin(), Blocks.end(), BB) != Blocks.end();
bool Loop::isLoopExit(const BasicBlock *BB) const {
for (succ_const_iterator SI = succ_begin(BB), SE = succ_end(BB);
SI != SE; ++SI) {
if (!contains(*SI))
return true;
return false;
/// getNumBackEdges - Calculate the number of back edges to the loop header.
unsigned Loop::getNumBackEdges() const {
unsigned NumBackEdges = 0;
BasicBlock *H = getHeader();
for (pred_iterator I = pred_begin(H), E = pred_end(H); I != E; ++I)
if (contains(*I))
return NumBackEdges;
void Loop::print(std::ostream &OS, unsigned Depth) const {
OS << std::string(Depth*2, ' ') << "Loop Containing: ";
for (unsigned i = 0; i < getBlocks().size(); ++i) {
if (i) OS << ",";
WriteAsOperand(OS, getBlocks()[i], false);
if (!ExitBlocks.empty()) {
OS << "\tExitBlocks: ";
for (unsigned i = 0; i < getExitBlocks().size(); ++i) {
if (i) OS << ",";
WriteAsOperand(OS, getExitBlocks()[i], false);
OS << "\n";
for (unsigned i = 0, e = getSubLoops().size(); i != e; ++i)
getSubLoops()[i]->print(OS, Depth+2);
void Loop::dump() const {
// LoopInfo implementation
void LoopInfo::stub() {}
bool LoopInfo::runOnFunction(Function &) {
Calculate(getAnalysis<DominatorSet>()); // Update
return false;
void LoopInfo::releaseMemory() {
for (std::vector<Loop*>::iterator I = TopLevelLoops.begin(),
E = TopLevelLoops.end(); I != E; ++I)
delete *I; // Delete all of the loops...
BBMap.clear(); // Reset internal state of analysis
void LoopInfo::Calculate(const DominatorSet &DS) {
BasicBlock *RootNode = DS.getRoot();
for (df_iterator<BasicBlock*> NI = df_begin(RootNode),
NE = df_end(RootNode); NI != NE; ++NI)
if (Loop *L = ConsiderForLoop(*NI, DS))
for (unsigned i = 0; i < TopLevelLoops.size(); ++i)
void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
void LoopInfo::print(std::ostream &OS) const {
for (unsigned i = 0; i < TopLevelLoops.size(); ++i)
#if 0
for (std::map<BasicBlock*, Loop*>::const_iterator I = BBMap.begin(),
E = BBMap.end(); I != E; ++I)
OS << "BB '" << I->first->getName() << "' level = "
<< I->second->LoopDepth << "\n";
static bool isNotAlreadyContainedIn(Loop *SubLoop, Loop *ParentLoop) {
if (SubLoop == 0) return true;
if (SubLoop == ParentLoop) return false;
return isNotAlreadyContainedIn(SubLoop->getParentLoop(), ParentLoop);
Loop *LoopInfo::ConsiderForLoop(BasicBlock *BB, const DominatorSet &DS) {
if (BBMap.find(BB) != BBMap.end()) return 0; // Haven't processed this node?
std::vector<BasicBlock *> TodoStack;
// Scan the predecessors of BB, checking to see if BB dominates any of
// them. This identifies backedges which target this node...
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I)
if (DS.dominates(BB, *I)) // If BB dominates it's predecessor...
if (TodoStack.empty()) return 0; // No backedges to this block...
// Create a new loop to represent this basic block...
Loop *L = new Loop(BB);
BBMap[BB] = L;
BasicBlock *EntryBlock = &BB->getParent()->getEntryBlock();
while (!TodoStack.empty()) { // Process all the nodes in the loop
BasicBlock *X = TodoStack.back();
if (!L->contains(X) && // As of yet unprocessed??
DS.dominates(EntryBlock, X)) { // X is reachable from entry block?
// Check to see if this block already belongs to a loop. If this occurs
// then we have a case where a loop that is supposed to be a child of the
// current loop was processed before the current loop. When this occurs,
// this child loop gets added to a part of the current loop, making it a
// sibling to the current loop. We have to reparent this loop.
if (Loop *SubLoop = const_cast<Loop*>(getLoopFor(X)))
if (SubLoop->getHeader() == X && isNotAlreadyContainedIn(SubLoop, L)) {
// Remove the subloop from it's current parent...
assert(SubLoop->ParentLoop && SubLoop->ParentLoop != L);
Loop *SLP = SubLoop->ParentLoop; // SubLoopParent
std::vector<Loop*>::iterator I =
std::find(SLP->SubLoops.begin(), SLP->SubLoops.end(), SubLoop);
assert(I != SLP->SubLoops.end() && "SubLoop not a child of parent?");
SLP->SubLoops.erase(I); // Remove from parent...
// Add the subloop to THIS loop...
SubLoop->ParentLoop = L;
// Normal case, add the block to our loop...
// Add all of the predecessors of X to the end of the work stack...
TodoStack.insert(TodoStack.end(), pred_begin(X), pred_end(X));
// If there are any loops nested within this loop, create them now!
for (std::vector<BasicBlock*>::iterator I = L->Blocks.begin(),
E = L->Blocks.end(); I != E; ++I)
if (Loop *NewLoop = ConsiderForLoop(*I, DS)) {
NewLoop->ParentLoop = L;
// Add the basic blocks that comprise this loop to the BBMap so that this
// loop can be found for them.
for (std::vector<BasicBlock*>::iterator I = L->Blocks.begin(),
E = L->Blocks.end(); I != E; ++I) {
std::map<BasicBlock*, Loop*>::iterator BBMI = BBMap.lower_bound(*I);
if (BBMI == BBMap.end() || BBMI->first != *I) // Not in map yet...
BBMap.insert(BBMI, std::make_pair(*I, L)); // Must be at this level
// Now that we have a list of all of the child loops of this loop, check to
// see if any of them should actually be nested inside of each other. We can
// accidentally pull loops our of their parents, so we must make sure to
// organize the loop nests correctly now.
std::map<BasicBlock*, Loop*> ContainingLoops;
for (unsigned i = 0; i != L->SubLoops.size(); ++i) {
Loop *Child = L->SubLoops[i];
assert(Child->getParentLoop() == L && "Not proper child loop?");
if (Loop *ContainingLoop = ContainingLoops[Child->getHeader()]) {
// If there is already a loop which contains this loop, move this loop
// into the containing loop.
MoveSiblingLoopInto(Child, ContainingLoop);
--i; // The loop got removed from the SubLoops list.
} else {
// This is currently considered to be a top-level loop. Check to see if
// any of the contained blocks are loop headers for subloops we have
// already processed.
for (unsigned b = 0, e = Child->Blocks.size(); b != e; ++b) {
Loop *&BlockLoop = ContainingLoops[Child->Blocks[b]];
if (BlockLoop == 0) { // Child block not processed yet...
BlockLoop = Child;
} else if (BlockLoop != Child) {
Loop *SubLoop = BlockLoop;
// Reparent all of the blocks which used to belong to BlockLoops
for (unsigned j = 0, e = SubLoop->Blocks.size(); j != e; ++j)
ContainingLoops[SubLoop->Blocks[j]] = Child;
// There is already a loop which contains this block, that means
// that we should reparent the loop which the block is currently
// considered to belong to to be a child of this loop.
MoveSiblingLoopInto(SubLoop, Child);
--i; // We just shrunk the SubLoops list.
// Now that we know all of the blocks that make up this loop, see if there are
// any branches to outside of the loop... building the ExitBlocks list.
for (std::vector<BasicBlock*>::iterator BI = L->Blocks.begin(),
BE = L->Blocks.end(); BI != BE; ++BI)
for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I)
if (!L->contains(*I)) // Not in current loop?
L->ExitBlocks.push_back(*I); // It must be an exit block...
return L;
/// MoveSiblingLoopInto - This method moves the NewChild loop to live inside of
/// the NewParent Loop, instead of being a sibling of it.
void LoopInfo::MoveSiblingLoopInto(Loop *NewChild, Loop *NewParent) {
Loop *OldParent = NewChild->getParentLoop();
assert(OldParent && OldParent == NewParent->getParentLoop() &&
NewChild != NewParent && "Not sibling loops!");
// Remove NewChild from being a child of OldParent
std::vector<Loop*>::iterator I =
std::find(OldParent->SubLoops.begin(), OldParent->SubLoops.end(), NewChild);
assert(I != OldParent->SubLoops.end() && "Parent fields incorrect??");
OldParent->SubLoops.erase(I); // Remove from parent's subloops list
NewChild->ParentLoop = 0;
InsertLoopInto(NewChild, NewParent);
/// InsertLoopInto - This inserts loop L into the specified parent loop. If the
/// parent loop contains a loop which should contain L, the loop gets inserted
/// into L instead.
void LoopInfo::InsertLoopInto(Loop *L, Loop *Parent) {
BasicBlock *LHeader = L->getHeader();
assert(Parent->contains(LHeader) && "This loop should not be inserted here!");
// Check to see if it belongs in a child loop...
for (unsigned i = 0, e = Parent->SubLoops.size(); i != e; ++i)
if (Parent->SubLoops[i]->contains(LHeader)) {
InsertLoopInto(L, Parent->SubLoops[i]);
// If not, insert it here!
L->ParentLoop = Parent;
/// getLoopPreheader - If there is a preheader for this loop, return it. A
/// loop has a preheader if there is only one edge to the header of the loop
/// from outside of the loop. If this is the case, the block branching to the
/// header of the loop is the preheader node. The "preheaders" pass can be
/// "Required" to ensure that there is always a preheader node for every loop.
/// This method returns null if there is no preheader for the loop (either
/// because the loop is dead or because multiple blocks branch to the header
/// node of this loop).
BasicBlock *Loop::getLoopPreheader() const {
// Keep track of nodes outside the loop branching to the header...
BasicBlock *Out = 0;
// Loop over the predecessors of the header node...
BasicBlock *Header = getHeader();
for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
PI != PE; ++PI)
if (!contains(*PI)) { // If the block is not in the loop...
if (Out && Out != *PI)
return 0; // Multiple predecessors outside the loop
Out = *PI;
// Make sure there is only one exit out of the preheader...
succ_iterator SI = succ_begin(Out);
if (SI != succ_end(Out))
return 0; // Multiple exits from the block, must not be a preheader.
// If there is exactly one preheader, return it. If there was zero, then Out
// is still null.
return Out;
/// addBasicBlockToLoop - This function is used by other analyses to update loop
/// information. NewBB is set to be a new member of the current loop. Because
/// of this, it is added as a member of all parent loops, and is added to the
/// specified LoopInfo object as being in the current basic block. It is not
/// valid to replace the loop header with this method.
void Loop::addBasicBlockToLoop(BasicBlock *NewBB, LoopInfo &LI) {
assert(LI[getHeader()] == this && "Incorrect LI specified for this loop!");
assert(NewBB && "Cannot add a null basic block to the loop!");
assert(LI[NewBB] == 0 && "BasicBlock already in the loop!");
// Add the loop mapping to the LoopInfo object...
LI.BBMap[NewBB] = this;
// Add the basic block to this loop and all parent loops...
Loop *L = this;
while (L) {
L = L->getParentLoop();
/// changeExitBlock - This method is used to update loop information. All
/// instances of the specified Old basic block are removed from the exit list
/// and replaced with New.
void Loop::changeExitBlock(BasicBlock *Old, BasicBlock *New) {
assert(Old != New && "Cannot changeExitBlock to the same thing!");
assert(Old && New && "Cannot changeExitBlock to or from a null node!");
assert(hasExitBlock(Old) && "Old exit block not found!");
I = std::find(ExitBlocks.begin(), ExitBlocks.end(), Old);
while (I != ExitBlocks.end()) {
*I = New;
I = std::find(I+1, ExitBlocks.end(), Old);