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//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file defines the LoopInfo class that is used to identify natural loops
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// and determine the loop depth of various nodes of the CFG. Note that the
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// loops identified may actually be several natural loops that share the same
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// header node... not just a single natural loop.
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Assembly/Writer.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SmallPtrSet.h"
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// Always verify loopinfo if expensive checking is enabled.
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static bool VerifyLoopInfo = true;
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static bool VerifyLoopInfo = false;
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static cl::opt<bool,true>
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VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
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cl::desc("Verify loop info (time consuming)"));
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char LoopInfo::ID = 0;
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INITIALIZE_PASS(LoopInfo, "loops", "Natural Loop Information", true, true);
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//===----------------------------------------------------------------------===//
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// Loop implementation
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/// isLoopInvariant - Return true if the specified value is loop invariant
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bool Loop::isLoopInvariant(Value *V) const {
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if (Instruction *I = dyn_cast<Instruction>(V))
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return isLoopInvariant(I);
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return true; // All non-instructions are loop invariant
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/// isLoopInvariant - Return true if the specified instruction is
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bool Loop::isLoopInvariant(Instruction *I) const {
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/// makeLoopInvariant - If the given value is an instruciton inside of the
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/// loop and it can be hoisted, do so to make it trivially loop-invariant.
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/// Return true if the value after any hoisting is loop invariant. This
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/// function can be used as a slightly more aggressive replacement for
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/// If InsertPt is specified, it is the point to hoist instructions to.
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/// If null, the terminator of the loop preheader is used.
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bool Loop::makeLoopInvariant(Value *V, bool &Changed,
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Instruction *InsertPt) const {
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if (Instruction *I = dyn_cast<Instruction>(V))
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return makeLoopInvariant(I, Changed, InsertPt);
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return true; // All non-instructions are loop-invariant.
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/// makeLoopInvariant - If the given instruction is inside of the
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/// loop and it can be hoisted, do so to make it trivially loop-invariant.
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/// Return true if the instruction after any hoisting is loop invariant. This
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/// function can be used as a slightly more aggressive replacement for
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/// If InsertPt is specified, it is the point to hoist instructions to.
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/// If null, the terminator of the loop preheader is used.
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bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
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Instruction *InsertPt) const {
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// Test if the value is already loop-invariant.
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if (isLoopInvariant(I))
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if (!I->isSafeToSpeculativelyExecute())
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if (I->mayReadFromMemory())
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// Determine the insertion point, unless one was given.
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BasicBlock *Preheader = getLoopPreheader();
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// Without a preheader, hoisting is not feasible.
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InsertPt = Preheader->getTerminator();
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// Don't hoist instructions with loop-variant operands.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
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I->moveBefore(InsertPt);
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/// getCanonicalInductionVariable - Check to see if the loop has a canonical
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/// induction variable: an integer recurrence that starts at 0 and increments
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/// by one each time through the loop. If so, return the phi node that
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/// corresponds to it.
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/// The IndVarSimplify pass transforms loops to have a canonical induction
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PHINode *Loop::getCanonicalInductionVariable() const {
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BasicBlock *H = getHeader();
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BasicBlock *Incoming = 0, *Backedge = 0;
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pred_iterator PI = pred_begin(H);
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assert(PI != pred_end(H) &&
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"Loop must have at least one backedge!");
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if (PI == pred_end(H)) return 0; // dead loop
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if (PI != pred_end(H)) return 0; // multiple backedges?
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if (contains(Incoming)) {
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if (contains(Backedge))
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std::swap(Incoming, Backedge);
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} else if (!contains(Backedge))
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// Loop over all of the PHI nodes, looking for a canonical indvar.
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for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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if (ConstantInt *CI =
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dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
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if (CI->isNullValue())
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if (Instruction *Inc =
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dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
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if (Inc->getOpcode() == Instruction::Add &&
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Inc->getOperand(0) == PN)
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if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
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if (CI->equalsInt(1))
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/// getTripCount - Return a loop-invariant LLVM value indicating the number of
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/// times the loop will be executed. Note that this means that the backedge
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/// of the loop executes N-1 times. If the trip-count cannot be determined,
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/// this returns null.
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/// The IndVarSimplify pass transforms loops to have a form that this
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/// function easily understands.
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Value *Loop::getTripCount() const {
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// Canonical loops will end with a 'cmp ne I, V', where I is the incremented
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// canonical induction variable and V is the trip count of the loop.
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PHINode *IV = getCanonicalInductionVariable();
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if (IV == 0 || IV->getNumIncomingValues() != 2) return 0;
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bool P0InLoop = contains(IV->getIncomingBlock(0));
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Value *Inc = IV->getIncomingValue(!P0InLoop);
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BasicBlock *BackedgeBlock = IV->getIncomingBlock(!P0InLoop);
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if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
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if (BI->isConditional()) {
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if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
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if (ICI->getOperand(0) == Inc) {
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if (BI->getSuccessor(0) == getHeader()) {
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if (ICI->getPredicate() == ICmpInst::ICMP_NE)
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return ICI->getOperand(1);
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} else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
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return ICI->getOperand(1);
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/// getSmallConstantTripCount - Returns the trip count of this loop as a
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/// normal unsigned value, if possible. Returns 0 if the trip count is unknown
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/// of not constant. Will also return 0 if the trip count is very large
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unsigned Loop::getSmallConstantTripCount() const {
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Value* TripCount = this->getTripCount();
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if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
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// Guard against huge trip counts.
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if (TripCountC->getValue().getActiveBits() <= 32) {
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return (unsigned)TripCountC->getZExtValue();
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/// getSmallConstantTripMultiple - Returns the largest constant divisor of the
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/// trip count of this loop as a normal unsigned value, if possible. This
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/// means that the actual trip count is always a multiple of the returned
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/// value (don't forget the trip count could very well be zero as well!).
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/// Returns 1 if the trip count is unknown or not guaranteed to be the
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/// multiple of a constant (which is also the case if the trip count is simply
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/// constant, use getSmallConstantTripCount for that case), Will also return 1
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/// if the trip count is very large (>= 2^32).
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unsigned Loop::getSmallConstantTripMultiple() const {
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Value* TripCount = this->getTripCount();
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// This will hold the ConstantInt result, if any
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ConstantInt *Result = NULL;
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// See if the trip count is constant itself
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Result = dyn_cast<ConstantInt>(TripCount);
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// if not, see if it is a multiplication
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if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
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switch (BO->getOpcode()) {
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case BinaryOperator::Mul:
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Result = dyn_cast<ConstantInt>(BO->getOperand(1));
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case BinaryOperator::Shl:
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if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
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if (CI->getValue().getActiveBits() <= 5)
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return 1u << CI->getZExtValue();
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// Guard against huge trip counts.
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if (Result && Result->getValue().getActiveBits() <= 32) {
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return (unsigned)Result->getZExtValue();
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/// isLCSSAForm - Return true if the Loop is in LCSSA form
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bool Loop::isLCSSAForm(DominatorTree &DT) const {
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// Sort the blocks vector so that we can use binary search to do quick
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SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
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for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
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BasicBlock *BB = *BI;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
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BasicBlock *UserBB = cast<Instruction>(U)->getParent();
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if (PHINode *P = dyn_cast<PHINode>(U))
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UserBB = P->getIncomingBlock(UI);
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// Check the current block, as a fast-path, before checking whether
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// the use is anywhere in the loop. Most values are used in the same
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// block they are defined in. Also, blocks not reachable from the
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// entry are special; uses in them don't need to go through PHIs.
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!LoopBBs.count(UserBB) &&
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DT.isReachableFromEntry(UserBB))
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/// isLoopSimplifyForm - Return true if the Loop is in the form that
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/// the LoopSimplify form transforms loops to, which is sometimes called
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bool Loop::isLoopSimplifyForm() const {
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// Normal-form loops have a preheader, a single backedge, and all of their
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// exits have all their predecessors inside the loop.
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return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
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/// hasDedicatedExits - Return true if no exit block for the loop
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/// has a predecessor that is outside the loop.
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bool Loop::hasDedicatedExits() const {
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// Sort the blocks vector so that we can use binary search to do quick
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SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
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// Each predecessor of each exit block of a normal loop is contained
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SmallVector<BasicBlock *, 4> ExitBlocks;
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getExitBlocks(ExitBlocks);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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for (pred_iterator PI = pred_begin(ExitBlocks[i]),
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PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
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if (!LoopBBs.count(*PI))
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// All the requirements are met.
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/// getUniqueExitBlocks - Return all unique successor blocks of this loop.
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/// These are the blocks _outside of the current loop_ which are branched to.
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/// This assumes that loop exits are in canonical form.
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Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
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assert(hasDedicatedExits() &&
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"getUniqueExitBlocks assumes the loop has canonical form exits!");
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// Sort the blocks vector so that we can use binary search to do quick
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SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
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std::sort(LoopBBs.begin(), LoopBBs.end());
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SmallVector<BasicBlock *, 32> switchExitBlocks;
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for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
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BasicBlock *current = *BI;
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switchExitBlocks.clear();
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for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
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// If block is inside the loop then it is not a exit block.
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if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
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pred_iterator PI = pred_begin(*I);
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BasicBlock *firstPred = *PI;
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// If current basic block is this exit block's first predecessor
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// then only insert exit block in to the output ExitBlocks vector.
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// This ensures that same exit block is not inserted twice into
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// ExitBlocks vector.
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if (current != firstPred)
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// If a terminator has more then two successors, for example SwitchInst,
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// then it is possible that there are multiple edges from current block
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// to one exit block.
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if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
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ExitBlocks.push_back(*I);
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// In case of multiple edges from current block to exit block, collect
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// only one edge in ExitBlocks. Use switchExitBlocks to keep track of
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if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
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== switchExitBlocks.end()) {
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switchExitBlocks.push_back(*I);
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ExitBlocks.push_back(*I);
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/// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
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/// block, return that block. Otherwise return null.
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BasicBlock *Loop::getUniqueExitBlock() const {
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SmallVector<BasicBlock *, 8> UniqueExitBlocks;
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getUniqueExitBlocks(UniqueExitBlocks);
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if (UniqueExitBlocks.size() == 1)
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return UniqueExitBlocks[0];
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void Loop::dump() const {
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//===----------------------------------------------------------------------===//
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// LoopInfo implementation
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bool LoopInfo::runOnFunction(Function &) {
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LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
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void LoopInfo::verifyAnalysis() const {
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// LoopInfo is a FunctionPass, but verifying every loop in the function
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// each time verifyAnalysis is called is very expensive. The
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// -verify-loop-info option can enable this. In order to perform some
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// checking by default, LoopPass has been taught to call verifyLoop
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// manually during loop pass sequences.
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if (!VerifyLoopInfo) return;
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for (iterator I = begin(), E = end(); I != E; ++I) {
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assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
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(*I)->verifyLoopNest();
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// TODO: check BBMap consistency.
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void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesAll();
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AU.addRequired<DominatorTree>();
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void LoopInfo::print(raw_ostream &OS, const Module*) const {