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Also, use SCEV to determine the trip count of the loop, which is more powerful and accurate that Loop::getTripCount. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51179 91177308-0d34-0410-b5e6-96231b3b80d8
285 lines
11 KiB
C++
285 lines
11 KiB
C++
//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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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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//===----------------------------------------------------------------------===//
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//
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// This file implements the Dead Loop Deletion Pass. This pass is responsible
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// for eliminating loops with non-infinite computable trip counts that have no
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// side effects or volatile instructions, and do not contribute to the
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// computation of the function's return value.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-delete"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/SmallVector.h"
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using namespace llvm;
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STATISTIC(NumDeleted, "Number of loops deleted");
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namespace {
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class VISIBILITY_HIDDEN LoopDeletion : public LoopPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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LoopDeletion() : LoopPass((intptr_t)&ID) { }
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// Possibly eliminate loop L if it is dead.
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bool runOnLoop(Loop* L, LPPassManager& LPM);
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bool SingleDominatingExit(Loop* L,
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SmallVector<BasicBlock*, 4>& exitingBlocks);
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bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
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SmallVector<BasicBlock*, 4>& exitBlocks);
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bool IsLoopInvariantInst(Instruction *I, Loop* L);
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virtual void getAnalysisUsage(AnalysisUsage& AU) const {
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AU.addRequired<ScalarEvolution>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<LoopInfo>();
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AU.addRequiredID(LoopSimplifyID);
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AU.addRequiredID(LCSSAID);
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AU.addPreserved<ScalarEvolution>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<LoopInfo>();
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AU.addPreservedID(LoopSimplifyID);
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AU.addPreservedID(LCSSAID);
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}
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};
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}
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char LoopDeletion::ID = 0;
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static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops");
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LoopPass* llvm::createLoopDeletionPass() {
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return new LoopDeletion();
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}
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/// SingleDominatingExit - Checks that there is only a single blocks that
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/// branches out of the loop, and that it also g the latch block. Loops
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/// with multiple or non-latch-dominating exiting blocks could be dead, but we'd
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/// have to do more extensive analysis to make sure, for instance, that the
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/// control flow logic involved was or could be made loop-invariant.
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bool LoopDeletion::SingleDominatingExit(Loop* L,
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SmallVector<BasicBlock*, 4>& exitingBlocks) {
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if (exitingBlocks.size() != 1)
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return false;
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BasicBlock* latch = L->getLoopLatch();
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if (!latch)
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return false;
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DominatorTree& DT = getAnalysis<DominatorTree>();
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return DT.dominates(exitingBlocks[0], latch);
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}
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/// IsLoopInvariantInst - Checks if an instruction is invariant with respect to
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/// a loop, which is defined as being true if all of its operands are defined
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/// outside of the loop. These instructions can be hoisted out of the loop
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/// if their results are needed. This could be made more aggressive by
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/// recursively checking the operands for invariance, but it's not clear that
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/// it's worth it.
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bool LoopDeletion::IsLoopInvariantInst(Instruction *I, Loop* L) {
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// PHI nodes are not loop invariant if defined in the loop.
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if (isa<PHINode>(I) && L->contains(I->getParent()))
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return false;
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// The instruction is loop invariant if all of its operands are loop-invariant
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (!L->isLoopInvariant(I->getOperand(i)))
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return false;
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// If we got this far, the instruction is loop invariant!
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return true;
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}
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/// IsLoopDead - Determined if a loop is dead. This assumes that we've already
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/// checked for unique exit and exiting blocks, and that the code is in LCSSA
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/// form.
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bool LoopDeletion::IsLoopDead(Loop* L,
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SmallVector<BasicBlock*, 4>& exitingBlocks,
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SmallVector<BasicBlock*, 4>& exitBlocks) {
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BasicBlock* exitingBlock = exitingBlocks[0];
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BasicBlock* exitBlock = exitBlocks[0];
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// Make sure that all PHI entries coming from the loop are loop invariant.
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// Because the code is in LCSSA form, any values used outside of the loop
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// must pass through a PHI in the exit block, meaning that this check is
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// sufficient to guarantee that no loop-variant values are used outside
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// of the loop.
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BasicBlock::iterator BI = exitBlock->begin();
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while (PHINode* P = dyn_cast<PHINode>(BI)) {
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Value* incoming = P->getIncomingValueForBlock(exitingBlock);
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if (Instruction* I = dyn_cast<Instruction>(incoming))
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if (!IsLoopInvariantInst(I, L))
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return false;
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BI++;
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}
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// Make sure that no instructions in the block have potential side-effects.
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// This includes instructions that could write to memory, and loads that are
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// marked volatile. This could be made more aggressive by using aliasing
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// information to identify readonly and readnone calls.
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for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
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LI != LE; ++LI) {
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for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
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BI != BE; ++BI) {
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if (BI->mayWriteToMemory())
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return false;
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else if (LoadInst* L = dyn_cast<LoadInst>(BI))
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if (L->isVolatile())
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return false;
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}
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}
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return true;
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}
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/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
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/// observable behavior of the program other than finite running time. Note
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/// we do ensure that this never remove a loop that might be infinite, as doing
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/// so could change the halting/non-halting nature of a program.
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/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
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/// in order to make various safety checks work.
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bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
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// We can only remove the loop if there is a preheader that we can
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// branch from after removing it.
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BasicBlock* preheader = L->getLoopPreheader();
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if (!preheader)
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return false;
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// We can't remove loops that contain subloops. If the subloops were dead,
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// they would already have been removed in earlier executions of this pass.
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if (L->begin() != L->end())
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return false;
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SmallVector<BasicBlock*, 4> exitingBlocks;
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L->getExitingBlocks(exitingBlocks);
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SmallVector<BasicBlock*, 4> exitBlocks;
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L->getUniqueExitBlocks(exitBlocks);
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// We require that the loop only have a single exit block. Otherwise, we'd
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// be in the situation of needing to be able to solve statically which exit
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// block will be branched to, or trying to preserve the branching logic in
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// a loop invariant manner.
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if (exitBlocks.size() != 1)
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return false;
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// Loops with multiple exits or exits that don't dominate the latch
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// are too complicated to handle correctly.
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if (!SingleDominatingExit(L, exitingBlocks))
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return false;
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// Finally, we have to check that the loop really is dead.
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if (!IsLoopDead(L, exitingBlocks, exitBlocks))
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return false;
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// Don't remove loops for which we can't solve the trip count.
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// They could be infinite, in which case we'd be changing program behavior.
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ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
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SCEVHandle S = SE.getIterationCount(L);
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if (isa<SCEVCouldNotCompute>(S))
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return false;
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// Now that we know the removal is safe, remove the loop by changing the
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// branch from the preheader to go to the single exit block.
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BasicBlock* exitBlock = exitBlocks[0];
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BasicBlock* exitingBlock = exitingBlocks[0];
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// Because we're deleting a large chunk of code at once, the sequence in which
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// we remove things is very important to avoid invalidation issues. Don't
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// mess with this unless you have good reason and know what you're doing.
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// Move simple loop-invariant expressions out of the loop, since they
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// might be needed by the exit phis.
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for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
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LI != LE; ++LI)
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for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
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BI != BE; ) {
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Instruction* I = BI++;
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if (I->getNumUses() > 0 && IsLoopInvariantInst(I, L))
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I->moveBefore(preheader->getTerminator());
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}
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// Connect the preheader directly to the exit block.
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TerminatorInst* TI = preheader->getTerminator();
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TI->replaceUsesOfWith(L->getHeader(), exitBlock);
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// Rewrite phis in the exit block to get their inputs from
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// the preheader instead of the exiting block.
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BasicBlock::iterator BI = exitBlock->begin();
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while (PHINode* P = dyn_cast<PHINode>(BI)) {
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P->replaceUsesOfWith(exitingBlock, preheader);
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BI++;
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}
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// Update the dominator tree and remove the instructions and blocks that will
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// be deleted from the reference counting scheme.
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DominatorTree& DT = getAnalysis<DominatorTree>();
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SmallPtrSet<DomTreeNode*, 8> ChildNodes;
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for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
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LI != LE; ++LI) {
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// Move all of the block's children to be children of the preheader, which
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// allows us to remove the domtree entry for the block.
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ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
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for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
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DE = ChildNodes.end(); DI != DE; ++DI)
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DT.changeImmediateDominator(*DI, DT[preheader]);
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ChildNodes.clear();
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DT.eraseNode(*LI);
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// Remove instructions that we're deleting from ScalarEvolution.
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for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
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BI != BE; ++BI)
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SE.deleteValueFromRecords(BI);
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SE.deleteValueFromRecords(*LI);
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// Remove the block from the reference counting scheme, so that we can
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// delete it freely later.
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(*LI)->dropAllReferences();
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}
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// Erase the instructions and the blocks without having to worry
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// about ordering because we already dropped the references.
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// NOTE: This iteration is safe because erasing the block does not remove its
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// entry from the loop's block list. We do that in the next section.
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for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
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LI != LE; ++LI) {
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for (Value::use_iterator UI = (*LI)->use_begin(), UE = (*LI)->use_end();
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UI != UE; ++UI)
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(*UI)->dump();
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(*LI)->eraseFromParent();
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}
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// Finally, the blocks from loopinfo. This has to happen late because
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// otherwise our loop iterators won't work.
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LoopInfo& loopInfo = getAnalysis<LoopInfo>();
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SmallPtrSet<BasicBlock*, 8> blocks;
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blocks.insert(L->block_begin(), L->block_end());
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for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
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E = blocks.end(); I != E; ++I)
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loopInfo.removeBlock(*I);
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// The last step is to inform the loop pass manager that we've
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// eliminated this loop.
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LPM.deleteLoopFromQueue(L);
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NumDeleted++;
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return true;
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}
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