389 lines
16 KiB
C++
389 lines
16 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/IR/Dominators.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Scalar/LoopPassManager.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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/// This function deletes dead loops. The caller of this function needs to
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/// guarantee that the loop is infact dead. Here we handle two kinds of dead
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/// loop. The first kind (\p isLoopDead) is where only invariant values from
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/// within the loop are used outside of it. The second kind (\p
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/// isLoopNeverExecuted) is where the loop is provably never executed. We can
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/// always remove never executed loops since they will not cause any difference
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/// to program behaviour.
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///
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/// This also updates the relevant analysis information in \p DT, \p SE, and \p
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/// LI. It also updates the loop PM if an updater struct is provided.
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// TODO: This function will be used by loop-simplifyCFG as well. So, move this
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// to LoopUtils.cpp
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static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
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LoopInfo &LI, LPMUpdater *Updater = nullptr);
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/// Determines if a loop is dead.
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///
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/// This assumes that we've already checked for unique exit and exiting blocks,
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/// and that the code is in LCSSA form.
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static bool isLoopDead(Loop *L, ScalarEvolution &SE,
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SmallVectorImpl<BasicBlock *> &ExitingBlocks,
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BasicBlock *ExitBlock, bool &Changed,
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BasicBlock *Preheader) {
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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.
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for (auto &I : L->blocks())
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if (any_of(*I, [](Instruction &I) { return I.mayHaveSideEffects(); }))
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return false;
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return true;
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}
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/// This function returns true if there is no viable path from the
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/// entry block to the header of \p L. Right now, it only does
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/// a local search to save compile time.
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static bool isLoopNeverExecuted(Loop *L) {
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using namespace PatternMatch;
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auto *Preheader = L->getLoopPreheader();
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// TODO: We can relax this constraint, since we just need a loop
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// predecessor.
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assert(Preheader && "Needs preheader!");
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if (Preheader == &Preheader->getParent()->getEntryBlock())
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return false;
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// All predecessors of the preheader should have a constant conditional
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// branch, with the loop's preheader as not-taken.
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for (auto *Pred: predecessors(Preheader)) {
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BasicBlock *Taken, *NotTaken;
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ConstantInt *Cond;
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if (!match(Pred->getTerminator(),
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m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
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return false;
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if (!Cond->getZExtValue())
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std::swap(Taken, NotTaken);
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if (Taken == Preheader)
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return false;
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}
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assert(!pred_empty(Preheader) &&
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"Preheader should have predecessors at this point!");
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// All the predecessors have the loop preheader as not-taken target.
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return true;
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}
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/// Remove a loop if it is dead.
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///
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/// A loop is considered dead if it does not impact the observable behavior of
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/// the program other than finite running time. This never removes a loop that
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/// might be infinite (unless it is never executed), as doing so could change
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/// the halting/non-halting nature of a program.
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///
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/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
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/// order to make various safety checks work.
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///
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/// \returns true if any changes were made. This may mutate the loop even if it
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/// is unable to delete it due to hoisting trivially loop invariant
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/// instructions out of the loop.
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static bool deleteLoopIfDead(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
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LoopInfo &LI, LPMUpdater *Updater = nullptr) {
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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 branch from
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// after removing it. Also, if LoopSimplify form is not available, stay out
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// of trouble.
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BasicBlock *Preheader = L->getLoopPreheader();
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if (!Preheader || !L->hasDedicatedExits()) {
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DEBUG(dbgs()
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<< "Deletion requires Loop with preheader and dedicated exits.\n");
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return false;
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}
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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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DEBUG(dbgs() << "Loop contains subloops.\n");
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return false;
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}
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BasicBlock *ExitBlock = L->getUniqueExitBlock();
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if (ExitBlock && isLoopNeverExecuted(L)) {
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DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
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// Set incoming value to undef for phi nodes in the exit 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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for (unsigned i = 0; i < P->getNumIncomingValues(); i++)
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P->setIncomingValue(i, UndefValue::get(P->getType()));
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BI++;
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}
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deleteDeadLoop(L, DT, SE, LI, Updater);
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++NumDeleted;
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return true;
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}
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// The remaining checks below are for a loop being dead because all statements
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// in the loop are invariant.
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SmallVector<BasicBlock *, 4> ExitingBlocks;
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L->getExitingBlocks(ExitingBlocks);
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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 (!ExitBlock) {
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DEBUG(dbgs() << "Deletion requires single exit block\n");
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return false;
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}
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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, ExitBlock, Changed, Preheader)) {
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DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
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return Changed;
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}
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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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DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
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return Changed;
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}
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DEBUG(dbgs() << "Loop is invariant, delete it!");
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deleteDeadLoop(L, DT, SE, LI, Updater);
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++NumDeleted;
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return true;
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}
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static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
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LoopInfo &LI, LPMUpdater *Updater) {
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assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
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auto *Preheader = L->getLoopPreheader();
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assert(Preheader && "Preheader should exist!");
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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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//
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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.
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// If we have an LPM updater, tell it about the loop being removed.
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if (Updater)
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Updater->markLoopAsDeleted(*L);
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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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auto *ExitBlock = L->getUniqueExitBlock();
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assert(ExitBlock && "Should have a unique exit block!");
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assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
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auto *OldBr = dyn_cast<BranchInst>(Preheader->getTerminator());
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assert(OldBr && "Preheader must end with a branch");
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assert(OldBr->isUnconditional() && "Preheader must have a single successor");
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// Connect the preheader to the exit block. Keep the old edge to the header
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// around to perform the dominator tree update in two separate steps
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// -- #1 insertion of the edge preheader -> exit and #2 deletion of the edge
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// preheader -> header.
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//
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//
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// 0. Preheader 1. Preheader 2. Preheader
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// | | | |
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// V | V |
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// Header <--\ | Header <--\ | Header <--\
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// | | | | | | | | | | |
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// | V | | | V | | | V |
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// | Body --/ | | Body --/ | | Body --/
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// V V V V V
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// Exit Exit Exit
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//
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// By doing this is two separate steps we can perform the dominator tree
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// update without using the batch update API.
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//
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// Even when the loop is never executed, we cannot remove the edge from the
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// source block to the exit block. Consider the case where the unexecuted loop
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// branches back to an outer loop. If we deleted the loop and removed the edge
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// coming to this inner loop, this will break the outer loop structure (by
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// deleting the backedge of the outer loop). If the outer loop is indeed a
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// non-loop, it will be deleted in a future iteration of loop deletion pass.
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IRBuilder<> Builder(OldBr);
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Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
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// Remove the old branch. The conditional branch becomes a new terminator.
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OldBr->eraseFromParent();
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// Update the dominator tree by informing it about the new edge from the
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// preheader to the exit.
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DT.insertEdge(Preheader, ExitBlock);
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// Rewrite phis in the exit block to get their inputs from the Preheader
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// 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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// Set the zero'th element of Phi to be from the preheader and remove all
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// other incoming values. Given the loop has dedicated exits, all other
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// incoming values must be from the exiting blocks.
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int PredIndex = 0;
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P->setIncomingBlock(PredIndex, Preheader);
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// Removes all incoming values from all other exiting blocks (including
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// duplicate values from an exiting block).
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// Nuke all entries except the zero'th entry which is the preheader entry.
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// NOTE! We need to remove Incoming Values in the reverse order as done
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// below, to keep the indices valid for deletion (removeIncomingValues
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// updates getNumIncomingValues and shifts all values down into the operand
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// being deleted).
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for (unsigned i = 0, e = P->getNumIncomingValues() - 1; i != e; ++i)
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P->removeIncomingValue(e-i, false);
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assert((P->getNumIncomingValues() == 1 &&
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P->getIncomingBlock(PredIndex) == Preheader) &&
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"Should have exactly one value and that's from the preheader!");
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++BI;
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}
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// Disconnect the loop body by branching directly to its exit.
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Builder.SetInsertPoint(Preheader->getTerminator());
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Builder.CreateBr(ExitBlock);
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// Remove the old branch.
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Preheader->getTerminator()->eraseFromParent();
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// Inform the dominator tree about the removed edge.
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DT.deleteEdge(Preheader, L->getHeader());
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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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for (auto *Block : L->blocks())
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Block->dropAllReferences();
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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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LI.removeBlock(BB);
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// The last step is to update LoopInfo now that we've eliminated this loop.
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LI.markAsRemoved(L);
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}
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PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR,
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LPMUpdater &Updater) {
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DEBUG(dbgs() << "Analyzing Loop for deletion: ");
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DEBUG(L.dump());
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if (!deleteLoopIfDead(&L, AR.DT, AR.SE, AR.LI, &Updater))
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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 &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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DEBUG(dbgs() << "Analyzing Loop for deletion: ");
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DEBUG(L->dump());
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return deleteLoopIfDead(L, DT, SE, LI);
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}
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