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//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
// The LLVM Compiler Infrastructure
// This file 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/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include <algorithm>
using namespace llvm;
// Always verify loopinfo if expensive checking is enabled.
#ifdef XDEBUG
static bool VerifyLoopInfo = true;
static bool VerifyLoopInfo = false;
static cl::opt<bool,true>
VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
cl::desc("Verify loop info (time consuming)"));
char LoopInfo::ID = 0;
INITIALIZE_PASS_BEGIN(LoopInfo, "loops", "Natural Loop Information", true, true)
INITIALIZE_PASS_END(LoopInfo, "loops", "Natural Loop Information", true, true)
// Loop implementation
/// isLoopInvariant - Return true if the specified value is loop invariant
bool Loop::isLoopInvariant(Value *V) const {
if (Instruction *I = dyn_cast<Instruction>(V))
return !contains(I);
return true; // All non-instructions are loop invariant
/// hasLoopInvariantOperands - Return true if all the operands of the
/// specified instruction are loop invariant.
bool Loop::hasLoopInvariantOperands(Instruction *I) const {
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (!isLoopInvariant(I->getOperand(i)))
return false;
return true;
/// makeLoopInvariant - If the given value is an instruciton inside of the
/// loop and it can be hoisted, do so to make it trivially loop-invariant.
/// Return true if the value after any hoisting is loop invariant. This
/// function can be used as a slightly more aggressive replacement for
/// isLoopInvariant.
/// If InsertPt is specified, it is the point to hoist instructions to.
/// If null, the terminator of the loop preheader is used.
bool Loop::makeLoopInvariant(Value *V, bool &Changed,
Instruction *InsertPt) const {
if (Instruction *I = dyn_cast<Instruction>(V))
return makeLoopInvariant(I, Changed, InsertPt);
return true; // All non-instructions are loop-invariant.
/// makeLoopInvariant - If the given instruction is inside of the
/// loop and it can be hoisted, do so to make it trivially loop-invariant.
/// Return true if the instruction after any hoisting is loop invariant. This
/// function can be used as a slightly more aggressive replacement for
/// isLoopInvariant.
/// If InsertPt is specified, it is the point to hoist instructions to.
/// If null, the terminator of the loop preheader is used.
bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
Instruction *InsertPt) const {
// Test if the value is already loop-invariant.
if (isLoopInvariant(I))
return true;
if (!isSafeToSpeculativelyExecute(I))
return false;
if (I->mayReadFromMemory())
return false;
// The landingpad instruction is immobile.
if (isa<LandingPadInst>(I))
return false;
// Determine the insertion point, unless one was given.
if (!InsertPt) {
BasicBlock *Preheader = getLoopPreheader();
// Without a preheader, hoisting is not feasible.
if (!Preheader)
return false;
InsertPt = Preheader->getTerminator();
// Don't hoist instructions with loop-variant operands.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
return false;
// Hoist.
Changed = true;
return true;
/// getCanonicalInductionVariable - Check to see if the loop has a canonical
/// induction variable: an integer recurrence that starts at 0 and increments
/// by one each time through the loop. If so, return the phi node that
/// corresponds to it.
/// The IndVarSimplify pass transforms loops to have a canonical induction
/// variable.
PHINode *Loop::getCanonicalInductionVariable() const {
BasicBlock *H = getHeader();
BasicBlock *Incoming = 0, *Backedge = 0;
pred_iterator PI = pred_begin(H);
assert(PI != pred_end(H) &&
"Loop must have at least one backedge!");
Backedge = *PI++;
if (PI == pred_end(H)) return 0; // dead loop
Incoming = *PI++;
if (PI != pred_end(H)) return 0; // multiple backedges?
if (contains(Incoming)) {
if (contains(Backedge))
return 0;
std::swap(Incoming, Backedge);
} else if (!contains(Backedge))
return 0;
// Loop over all of the PHI nodes, looking for a canonical indvar.
for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
if (ConstantInt *CI =
if (CI->isNullValue())
if (Instruction *Inc =
if (Inc->getOpcode() == Instruction::Add &&
Inc->getOperand(0) == PN)
if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
if (CI->equalsInt(1))
return PN;
return 0;
/// isLCSSAForm - Return true if the Loop is in LCSSA form
bool Loop::isLCSSAForm(DominatorTree &DT) const {
// Sort the blocks vector so that we can use binary search to do quick
// lookups.
SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
BasicBlock *BB = *BI;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI) {
User *U = *UI;
BasicBlock *UserBB = cast<Instruction>(U)->getParent();
if (PHINode *P = dyn_cast<PHINode>(U))
UserBB = P->getIncomingBlock(UI);
// Check the current block, as a fast-path, before checking whether
// the use is anywhere in the loop. Most values are used in the same
// block they are defined in. Also, blocks not reachable from the
// entry are special; uses in them don't need to go through PHIs.
if (UserBB != BB &&
!LoopBBs.count(UserBB) &&
return false;
return true;
/// isLoopSimplifyForm - Return true if the Loop is in the form that
/// the LoopSimplify form transforms loops to, which is sometimes called
/// normal form.
bool Loop::isLoopSimplifyForm() const {
// Normal-form loops have a preheader, a single backedge, and all of their
// exits have all their predecessors inside the loop.
return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
/// isSafeToClone - Return true if the loop body is safe to clone in practice.
/// Routines that reform the loop CFG and split edges often fail on indirectbr.
bool Loop::isSafeToClone() const {
// Return false if any loop blocks contain indirectbrs.
for (Loop::block_iterator I = block_begin(), E = block_end(); I != E; ++I) {
if (isa<IndirectBrInst>((*I)->getTerminator()))
return false;
return true;
/// hasDedicatedExits - Return true if no exit block for the loop
/// has a predecessor that is outside the loop.
bool Loop::hasDedicatedExits() const {
// Sort the blocks vector so that we can use binary search to do quick
// lookups.
SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
// Each predecessor of each exit block of a normal loop is contained
// within the loop.
SmallVector<BasicBlock *, 4> ExitBlocks;
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
for (pred_iterator PI = pred_begin(ExitBlocks[i]),
PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
if (!LoopBBs.count(*PI))
return false;
// All the requirements are met.
return true;
/// getUniqueExitBlocks - Return all unique successor blocks of this loop.
/// These are the blocks _outside of the current loop_ which are branched to.
/// This assumes that loop exits are in canonical form.
Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
assert(hasDedicatedExits() &&
"getUniqueExitBlocks assumes the loop has canonical form exits!");
// Sort the blocks vector so that we can use binary search to do quick
// lookups.
SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
std::sort(LoopBBs.begin(), LoopBBs.end());
SmallVector<BasicBlock *, 32> switchExitBlocks;
for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
BasicBlock *current = *BI;
for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
// If block is inside the loop then it is not a exit block.
if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
pred_iterator PI = pred_begin(*I);
BasicBlock *firstPred = *PI;
// If current basic block is this exit block's first predecessor
// then only insert exit block in to the output ExitBlocks vector.
// This ensures that same exit block is not inserted twice into
// ExitBlocks vector.
if (current != firstPred)
// If a terminator has more then two successors, for example SwitchInst,
// then it is possible that there are multiple edges from current block
// to one exit block.
if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
// In case of multiple edges from current block to exit block, collect
// only one edge in ExitBlocks. Use switchExitBlocks to keep track of
// duplicate edges.
if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
== switchExitBlocks.end()) {
/// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
/// block, return that block. Otherwise return null.
BasicBlock *Loop::getUniqueExitBlock() const {
SmallVector<BasicBlock *, 8> UniqueExitBlocks;
if (UniqueExitBlocks.size() == 1)
return UniqueExitBlocks[0];
return 0;
void Loop::dump() const {
// UnloopUpdater implementation
namespace {
/// Find the new parent loop for all blocks within the "unloop" whose last
/// backedges has just been removed.
class UnloopUpdater {
Loop *Unloop;
LoopInfo *LI;
LoopBlocksDFS DFS;
// Map unloop's immediate subloops to their nearest reachable parents. Nested
// loops within these subloops will not change parents. However, an immediate
// subloop's new parent will be the nearest loop reachable from either its own
// exits *or* any of its nested loop's exits.
DenseMap<Loop*, Loop*> SubloopParents;
// Flag the presence of an irreducible backedge whose destination is a block
// directly contained by the original unloop.
bool FoundIB;
UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
void updateBlockParents();
void removeBlocksFromAncestors();
void updateSubloopParents();
Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
} // end anonymous namespace
/// updateBlockParents - Update the parent loop for all blocks that are directly
/// contained within the original "unloop".
void UnloopUpdater::updateBlockParents() {
if (Unloop->getNumBlocks()) {
// Perform a post order CFG traversal of all blocks within this loop,
// propagating the nearest loop from sucessors to predecessors.
LoopBlocksTraversal Traversal(DFS, LI);
for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
POE = Traversal.end(); POI != POE; ++POI) {
Loop *L = LI->getLoopFor(*POI);
Loop *NL = getNearestLoop(*POI, L);
if (NL != L) {
// For reducible loops, NL is now an ancestor of Unloop.
assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
"uninitialized successor");
LI->changeLoopFor(*POI, NL);
else {
// Or the current block is part of a subloop, in which case its parent
// is unchanged.
assert((FoundIB || Unloop->contains(L)) && "uninitialized successor");
// Each irreducible loop within the unloop induces a round of iteration using
// the DFS result cached by Traversal.
bool Changed = FoundIB;
for (unsigned NIters = 0; Changed; ++NIters) {
assert(NIters < Unloop->getNumBlocks() && "runaway iterative algorithm");
// Iterate over the postorder list of blocks, propagating the nearest loop
// from successors to predecessors as before.
Changed = false;
for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
POE = DFS.endPostorder(); POI != POE; ++POI) {
Loop *L = LI->getLoopFor(*POI);
Loop *NL = getNearestLoop(*POI, L);
if (NL != L) {
assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
"uninitialized successor");
LI->changeLoopFor(*POI, NL);
Changed = true;
/// removeBlocksFromAncestors - Remove unloop's blocks from all ancestors below
/// their new parents.
void UnloopUpdater::removeBlocksFromAncestors() {
// Remove all unloop's blocks (including those in nested subloops) from
// ancestors below the new parent loop.
for (Loop::block_iterator BI = Unloop->block_begin(),
BE = Unloop->block_end(); BI != BE; ++BI) {
Loop *OuterParent = LI->getLoopFor(*BI);
if (Unloop->contains(OuterParent)) {
while (OuterParent->getParentLoop() != Unloop)
OuterParent = OuterParent->getParentLoop();
OuterParent = SubloopParents[OuterParent];
// Remove blocks from former Ancestors except Unloop itself which will be
// deleted.
for (Loop *OldParent = Unloop->getParentLoop(); OldParent != OuterParent;
OldParent = OldParent->getParentLoop()) {
assert(OldParent && "new loop is not an ancestor of the original");
/// updateSubloopParents - Update the parent loop for all subloops directly
/// nested within unloop.
void UnloopUpdater::updateSubloopParents() {
while (!Unloop->empty()) {
Loop *Subloop = *llvm::prior(Unloop->end());
assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
if (SubloopParents[Subloop])
/// getNearestLoop - Return the nearest parent loop among this block's
/// successors. If a successor is a subloop header, consider its parent to be
/// the nearest parent of the subloop's exits.
/// For subloop blocks, simply update SubloopParents and return NULL.
Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
// Initially for blocks directly contained by Unloop, NearLoop == Unloop and
// is considered uninitialized.
Loop *NearLoop = BBLoop;
Loop *Subloop = 0;
if (NearLoop != Unloop && Unloop->contains(NearLoop)) {
Subloop = NearLoop;
// Find the subloop ancestor that is directly contained within Unloop.
while (Subloop->getParentLoop() != Unloop) {
Subloop = Subloop->getParentLoop();
assert(Subloop && "subloop is not an ancestor of the original loop");
// Get the current nearest parent of the Subloop exits, initially Unloop.
if (!SubloopParents.count(Subloop))
SubloopParents[Subloop] = Unloop;
NearLoop = SubloopParents[Subloop];
succ_iterator I = succ_begin(BB), E = succ_end(BB);
if (I == E) {
assert(!Subloop && "subloop blocks must have a successor");
NearLoop = 0; // unloop blocks may now exit the function.
for (; I != E; ++I) {
if (*I == BB)
continue; // self loops are uninteresting
Loop *L = LI->getLoopFor(*I);
if (L == Unloop) {
// This successor has not been processed. This path must lead to an
// irreducible backedge.
assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
FoundIB = true;
if (L != Unloop && Unloop->contains(L)) {
// Successor is in a subloop.
if (Subloop)
continue; // Branching within subloops. Ignore it.
// BB branches from the original into a subloop header.
assert(L->getParentLoop() == Unloop && "cannot skip into nested loops");
// Get the current nearest parent of the Subloop's exits.
L = SubloopParents[L];
// L could be Unloop if the only exit was an irreducible backedge.
if (L == Unloop) {
// Handle critical edges from Unloop into a sibling loop.
if (L && !L->contains(Unloop)) {
L = L->getParentLoop();
// Remember the nearest parent loop among successors or subloop exits.
if (NearLoop == Unloop || !NearLoop || NearLoop->contains(L))
NearLoop = L;
if (Subloop) {
SubloopParents[Subloop] = NearLoop;
return BBLoop;
return NearLoop;
// LoopInfo implementation
bool LoopInfo::runOnFunction(Function &) {
LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
return false;
/// updateUnloop - The last backedge has been removed from a loop--now the
/// "unloop". Find a new parent for the blocks contained within unloop and
/// update the loop tree. We don't necessarily have valid dominators at this
/// point, but LoopInfo is still valid except for the removal of this loop.
/// Note that Unloop may now be an empty loop. Calling Loop::getHeader without
/// checking first is illegal.
void LoopInfo::updateUnloop(Loop *Unloop) {
// First handle the special case of no parent loop to simplify the algorithm.
if (!Unloop->getParentLoop()) {
// Since BBLoop had no parent, Unloop blocks are no longer in a loop.
for (Loop::block_iterator I = Unloop->block_begin(),
E = Unloop->block_end(); I != E; ++I) {
// Don't reparent blocks in subloops.
if (getLoopFor(*I) != Unloop)
// Blocks no longer have a parent but are still referenced by Unloop until
// the Unloop object is deleted.
LI.changeLoopFor(*I, 0);
// Remove the loop from the top-level LoopInfo object.
for (LoopInfo::iterator I = LI.begin();; ++I) {
assert(I != LI.end() && "Couldn't find loop");
if (*I == Unloop) {
// Move all of the subloops to the top-level.
while (!Unloop->empty())
// Update the parent loop for all blocks within the loop. Blocks within
// subloops will not change parents.
UnloopUpdater Updater(Unloop, this);
// Remove blocks from former ancestor loops.
// Add direct subloops as children in their new parent loop.
// Remove unloop from its parent loop.
Loop *ParentLoop = Unloop->getParentLoop();
for (Loop::iterator I = ParentLoop->begin();; ++I) {
assert(I != ParentLoop->end() && "Couldn't find loop");
if (*I == Unloop) {
void LoopInfo::verifyAnalysis() const {
// LoopInfo is a FunctionPass, but verifying every loop in the function
// each time verifyAnalysis is called is very expensive. The
// -verify-loop-info option can enable this. In order to perform some
// checking by default, LoopPass has been taught to call verifyLoop
// manually during loop pass sequences.
if (!VerifyLoopInfo) return;
DenseSet<const Loop*> Loops;
for (iterator I = begin(), E = end(); I != E; ++I) {
assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
// Verify that blocks are mapped to valid loops.
// FIXME: With an up-to-date DFS (see LoopIterator.h) and DominatorTree, we
// could also verify that the blocks are still in the correct loops.
for (DenseMap<BasicBlock*, Loop*>::const_iterator I = LI.BBMap.begin(),
E = LI.BBMap.end(); I != E; ++I) {
assert(Loops.count(I->second) && "orphaned loop");
assert(I->second->contains(I->first) && "orphaned block");
void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
void LoopInfo::print(raw_ostream &OS, const Module*) const {
// LoopBlocksDFS implementation
/// Traverse the loop blocks and store the DFS result.
/// Useful for clients that just want the final DFS result and don't need to
/// visit blocks during the initial traversal.
void LoopBlocksDFS::perform(LoopInfo *LI) {
LoopBlocksTraversal Traversal(*this, LI);
for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
POE = Traversal.end(); POI != POE; ++POI) ;