forked from OSchip/llvm-project
270 lines
10 KiB
C++
270 lines
10 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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#include "llvm/Transforms/Scalar/LoopDeletion.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/LoopPassManager.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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using namespace llvm;
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#define DEBUG_TYPE "loop-delete"
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STATISTIC(NumDeleted, "Number of loops deleted");
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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 LoopDeletionPass::isLoopDead(Loop *L, ScalarEvolution &SE,
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SmallVectorImpl<BasicBlock *> &exitingBlocks,
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SmallVectorImpl<BasicBlock *> &exitBlocks,
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bool &Changed, BasicBlock *Preheader) {
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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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bool AllEntriesInvariant = true;
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bool AllOutgoingValuesSame = true;
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while (PHINode *P = dyn_cast<PHINode>(BI)) {
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Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
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// Make sure all exiting blocks produce the same incoming value for the exit
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// block. If there are different incoming values for different exiting
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// blocks, then it is impossible to statically determine which value should
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// be used.
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AllOutgoingValuesSame =
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all_of(makeArrayRef(exitingBlocks).slice(1), [&](BasicBlock *BB) {
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return incoming == P->getIncomingValueForBlock(BB);
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});
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if (!AllOutgoingValuesSame)
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break;
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if (Instruction *I = dyn_cast<Instruction>(incoming))
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if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) {
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AllEntriesInvariant = false;
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break;
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}
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++BI;
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}
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if (Changed)
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SE.forgetLoopDispositions(L);
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if (!AllEntriesInvariant || !AllOutgoingValuesSame)
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return false;
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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 (Instruction &I : **LI) {
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if (I.mayHaveSideEffects())
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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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/// Remove dead loops, by which we mean loops that do not impact the observable
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/// behavior of the program other than finite running time. Note we do ensure
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/// that this never remove a loop that might be infinite, as doing so could
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/// change the halting/non-halting nature of a program. NOTE: This entire
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/// process relies pretty heavily on LoopSimplify and LCSSA in order to make
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/// various safety checks work.
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bool LoopDeletionPass::runImpl(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
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LoopInfo &loopInfo) {
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assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
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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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// If LoopSimplify form is not available, stay out of trouble.
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if (!L->hasDedicatedExits())
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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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// Finally, we have to check that the loop really is dead.
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bool Changed = false;
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if (!isLoopDead(L, SE, exitingBlocks, exitBlocks, Changed, preheader))
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return Changed;
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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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const SCEV *S = SE.getMaxBackedgeTakenCount(L);
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if (isa<SCEVCouldNotCompute>(S))
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return Changed;
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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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// 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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// Tell ScalarEvolution that the loop is deleted. Do this before
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// deleting the loop so that ScalarEvolution can look at the loop
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// to determine what it needs to clean up.
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SE.forgetLoop(L);
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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 *exitingBlock = exitingBlocks[0];
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BasicBlock::iterator BI = exitBlock->begin();
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while (PHINode *P = dyn_cast<PHINode>(BI)) {
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int j = P->getBasicBlockIndex(exitingBlock);
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assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
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P->setIncomingBlock(j, preheader);
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for (unsigned i = 1; i < exitingBlocks.size(); ++i)
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P->removeIncomingValue(exitingBlocks[i]);
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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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SmallVector<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(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
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for (DomTreeNode *ChildNode : ChildNodes) {
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DT.changeImmediateDominator(ChildNode, DT[preheader]);
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}
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ChildNodes.clear();
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DT.eraseNode(*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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(*LI)->eraseFromParent();
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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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SmallPtrSet<BasicBlock *, 8> blocks;
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blocks.insert(L->block_begin(), L->block_end());
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for (BasicBlock *BB : blocks)
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loopInfo.removeBlock(BB);
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// The last step is to update LoopInfo now that we've eliminated this loop.
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loopInfo.markAsRemoved(L);
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Changed = true;
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++NumDeleted;
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return Changed;
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}
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PreservedAnalyses LoopDeletionPass::run(Loop &L, AnalysisManager<Loop> &AM) {
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auto &FAM = AM.getResult<FunctionAnalysisManagerLoopProxy>(L).getManager();
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Function *F = L.getHeader()->getParent();
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auto &DT = *FAM.getCachedResult<DominatorTreeAnalysis>(*F);
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auto &SE = *FAM.getCachedResult<ScalarEvolutionAnalysis>(*F);
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auto &LI = *FAM.getCachedResult<LoopAnalysis>(*F);
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bool Changed = runImpl(&L, DT, SE, LI);
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if (!Changed)
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return PreservedAnalyses::all();
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return getLoopPassPreservedAnalyses();
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}
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namespace {
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class LoopDeletionLegacyPass : public LoopPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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LoopDeletionLegacyPass() : LoopPass(ID) {
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initializeLoopDeletionLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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// Possibly eliminate loop L if it is dead.
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bool runOnLoop(Loop *L, LPPassManager &) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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getLoopAnalysisUsage(AU);
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}
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};
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}
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char LoopDeletionLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(LoopDeletionLegacyPass, "loop-deletion",
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"Delete dead loops", false, false)
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INITIALIZE_PASS_DEPENDENCY(LoopPass)
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INITIALIZE_PASS_END(LoopDeletionLegacyPass, "loop-deletion",
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"Delete dead loops", false, false)
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Pass *llvm::createLoopDeletionPass() { return new LoopDeletionLegacyPass(); }
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bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &) {
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if (skipLoop(L))
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return false;
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DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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LoopDeletionPass Impl;
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return Impl.runImpl(L, DT, SE, loopInfo);
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}
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