llvm-6502/lib/Analysis/LoopInfo.cpp
Dan Gohman a342026504 Introduce a new LoopInfo utility function makeLoopInvariant, which
works similar to isLoopInvariant, except that it will do trivial
hoisting to try to make the value loop invariant if it isn't already.
This makes it easier for transformation passes to clear trivial
instructions out of the way (the regular LICM pass doesn't run
until relatively late). This is code factored out of LoopSimplify
and other places.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75578 91177308-0d34-0410-b5e6-96231b3b80d8
2009-07-14 01:06:29 +00:00

289 lines
10 KiB
C++

//===- 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/Assembly/Writer.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Streams.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include <algorithm>
using namespace llvm;
char LoopInfo::ID = 0;
static RegisterPass<LoopInfo>
X("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 isLoopInvariant(I);
return true; // All non-instructions are loop invariant
}
/// isLoopInvariant - Return true if the specified instruction is
/// loop-invariant.
///
bool Loop::isLoopInvariant(Instruction *I) const {
return !contains(I->getParent());
}
/// 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, Instruction *InsertPt) const {
if (Instruction *I = dyn_cast<Instruction>(V))
return makeLoopInvariant(I);
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, Instruction *InsertPt) const {
// Test if the value is already loop-invariant.
if (isLoopInvariant(I))
return true;
// Don't hoist instructions with side-effects.
if (I->isTrapping())
return false;
// Don't hoist PHI nodes.
if (isa<PHINode>(I))
return false;
// Don't hoist allocation instructions.
if (isa<AllocationInst>(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), InsertPt))
return false;
// Hoist.
I->moveBefore(InsertPt);
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;
typedef GraphTraits<Inverse<BasicBlock*> > InvBlockTraits;
InvBlockTraits::ChildIteratorType PI = InvBlockTraits::child_begin(H);
assert(PI != InvBlockTraits::child_end(H) &&
"Loop must have at least one backedge!");
Backedge = *PI++;
if (PI == InvBlockTraits::child_end(H)) return 0; // dead loop
Incoming = *PI++;
if (PI != InvBlockTraits::child_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 =
dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
if (CI->isNullValue())
if (Instruction *Inc =
dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
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;
}
/// getCanonicalInductionVariableIncrement - Return the LLVM value that holds
/// the canonical induction variable value for the "next" iteration of the
/// loop. This always succeeds if getCanonicalInductionVariable succeeds.
///
Instruction *Loop::getCanonicalInductionVariableIncrement() const {
if (PHINode *PN = getCanonicalInductionVariable()) {
bool P1InLoop = contains(PN->getIncomingBlock(1));
return cast<Instruction>(PN->getIncomingValue(P1InLoop));
}
return 0;
}
/// getTripCount - Return a loop-invariant LLVM value indicating the number of
/// times the loop will be executed. Note that this means that the backedge
/// of the loop executes N-1 times. If the trip-count cannot be determined,
/// this returns null.
///
/// The IndVarSimplify pass transforms loops to have a form that this
/// function easily understands.
///
Value *Loop::getTripCount() const {
// Canonical loops will end with a 'cmp ne I, V', where I is the incremented
// canonical induction variable and V is the trip count of the loop.
Instruction *Inc = getCanonicalInductionVariableIncrement();
if (Inc == 0) return 0;
PHINode *IV = cast<PHINode>(Inc->getOperand(0));
BasicBlock *BackedgeBlock =
IV->getIncomingBlock(contains(IV->getIncomingBlock(1)));
if (BranchInst *BI = dyn_cast<BranchInst>(BackedgeBlock->getTerminator()))
if (BI->isConditional()) {
if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
if (ICI->getOperand(0) == Inc) {
if (BI->getSuccessor(0) == getHeader()) {
if (ICI->getPredicate() == ICmpInst::ICMP_NE)
return ICI->getOperand(1);
} else if (ICI->getPredicate() == ICmpInst::ICMP_EQ) {
return ICI->getOperand(1);
}
}
}
}
return 0;
}
/// getSmallConstantTripCount - Returns the trip count of this loop as a
/// normal unsigned value, if possible. Returns 0 if the trip count is unknown
/// of not constant. Will also return 0 if the trip count is very large
/// (>= 2^32)
unsigned Loop::getSmallConstantTripCount() const {
Value* TripCount = this->getTripCount();
if (TripCount) {
if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCount)) {
// Guard against huge trip counts.
if (TripCountC->getValue().getActiveBits() <= 32) {
return (unsigned)TripCountC->getZExtValue();
}
}
}
return 0;
}
/// getSmallConstantTripMultiple - Returns the largest constant divisor of the
/// trip count of this loop as a normal unsigned value, if possible. This
/// means that the actual trip count is always a multiple of the returned
/// value (don't forget the trip count could very well be zero as well!).
///
/// Returns 1 if the trip count is unknown or not guaranteed to be the
/// multiple of a constant (which is also the case if the trip count is simply
/// constant, use getSmallConstantTripCount for that case), Will also return 1
/// if the trip count is very large (>= 2^32).
unsigned Loop::getSmallConstantTripMultiple() const {
Value* TripCount = this->getTripCount();
// This will hold the ConstantInt result, if any
ConstantInt *Result = NULL;
if (TripCount) {
// See if the trip count is constant itself
Result = dyn_cast<ConstantInt>(TripCount);
// if not, see if it is a multiplication
if (!Result)
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCount)) {
switch (BO->getOpcode()) {
case BinaryOperator::Mul:
Result = dyn_cast<ConstantInt>(BO->getOperand(1));
break;
default:
break;
}
}
}
// Guard against huge trip counts.
if (Result && Result->getValue().getActiveBits() <= 32) {
return (unsigned)Result->getZExtValue();
} else {
return 1;
}
}
/// isLCSSAForm - Return true if the Loop is in LCSSA form
bool Loop::isLCSSAForm() 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) {
BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
if (PHINode *P = dyn_cast<PHINode>(*UI)) {
UserBB = P->getIncomingBlock(UI);
}
// Check the current block, as a fast-path. Most values are used in
// the same block they are defined in.
if (UserBB != BB && !LoopBBs.count(UserBB))
return false;
}
}
return true;
}
//===----------------------------------------------------------------------===//
// LoopInfo implementation
//
bool LoopInfo::runOnFunction(Function &) {
releaseMemory();
LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
return false;
}
void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<DominatorTree>();
}