2008-04-30 04:06:54 +08:00
|
|
|
//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
|
2008-04-29 08:38:34 +08:00
|
|
|
//
|
|
|
|
// The LLVM Compiler Infrastructure
|
|
|
|
//
|
|
|
|
// This file is distributed under the University of Illinois Open Source
|
|
|
|
// License. See LICENSE.TXT for details.
|
|
|
|
//
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
//
|
2008-05-09 01:46:35 +08:00
|
|
|
// This file implements the Dead Loop Deletion Pass. This pass is responsible
|
|
|
|
// for eliminating loops with non-infinite computable trip counts that have no
|
|
|
|
// side effects or volatile instructions, and do not contribute to the
|
|
|
|
// computation of the function's return value.
|
2008-04-29 08:38:34 +08:00
|
|
|
//
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
|
|
|
|
#include "llvm/Transforms/Scalar.h"
|
2012-12-04 00:50:05 +08:00
|
|
|
#include "llvm/ADT/SmallVector.h"
|
|
|
|
#include "llvm/ADT/Statistic.h"
|
|
|
|
#include "llvm/Analysis/LoopPass.h"
|
2008-05-16 12:32:45 +08:00
|
|
|
#include "llvm/Analysis/ScalarEvolution.h"
|
2014-01-13 17:26:24 +08:00
|
|
|
#include "llvm/IR/Dominators.h"
|
2008-04-29 08:38:34 +08:00
|
|
|
using namespace llvm;
|
|
|
|
|
2014-04-22 10:55:47 +08:00
|
|
|
#define DEBUG_TYPE "loop-delete"
|
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
STATISTIC(NumDeleted, "Number of loops deleted");
|
|
|
|
|
|
|
|
namespace {
|
2009-09-02 14:11:42 +08:00
|
|
|
class LoopDeletion : public LoopPass {
|
2008-04-29 08:38:34 +08:00
|
|
|
public:
|
|
|
|
static char ID; // Pass ID, replacement for typeid
|
2010-10-20 01:21:58 +08:00
|
|
|
LoopDeletion() : LoopPass(ID) {
|
|
|
|
initializeLoopDeletionPass(*PassRegistry::getPassRegistry());
|
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
// Possibly eliminate loop L if it is dead.
|
2014-03-05 17:10:37 +08:00
|
|
|
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2014-03-05 17:10:37 +08:00
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
2014-01-13 21:07:17 +08:00
|
|
|
AU.addRequired<DominatorTreeWrapperPass>();
|
2015-01-17 22:16:18 +08:00
|
|
|
AU.addRequired<LoopInfoWrapperPass>();
|
2010-07-27 02:11:16 +08:00
|
|
|
AU.addRequired<ScalarEvolution>();
|
2008-04-29 08:38:34 +08:00
|
|
|
AU.addRequiredID(LoopSimplifyID);
|
|
|
|
AU.addRequiredID(LCSSAID);
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
AU.addPreserved<ScalarEvolution>();
|
2014-01-13 21:07:17 +08:00
|
|
|
AU.addPreserved<DominatorTreeWrapperPass>();
|
2015-01-17 22:16:18 +08:00
|
|
|
AU.addPreserved<LoopInfoWrapperPass>();
|
2008-04-29 08:38:34 +08:00
|
|
|
AU.addPreservedID(LoopSimplifyID);
|
|
|
|
AU.addPreservedID(LCSSAID);
|
|
|
|
}
|
2013-03-19 07:31:30 +08:00
|
|
|
|
|
|
|
private:
|
2013-07-14 12:42:23 +08:00
|
|
|
bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks,
|
|
|
|
SmallVectorImpl<BasicBlock *> &exitBlocks,
|
2013-03-19 07:31:30 +08:00
|
|
|
bool &Changed, BasicBlock *Preheader);
|
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
};
|
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-13 08:00:25 +08:00
|
|
|
char LoopDeletion::ID = 0;
|
2010-10-13 03:48:12 +08:00
|
|
|
INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
|
|
|
|
"Delete dead loops", false, false)
|
2014-01-13 21:07:17 +08:00
|
|
|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
|
2015-01-17 22:16:18 +08:00
|
|
|
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
|
2010-10-13 03:48:12 +08:00
|
|
|
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
|
|
|
|
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
|
|
|
|
INITIALIZE_PASS_DEPENDENCY(LCSSA)
|
|
|
|
INITIALIZE_PASS_END(LoopDeletion, "loop-deletion",
|
2010-10-08 06:25:06 +08:00
|
|
|
"Delete dead loops", false, false)
|
2008-04-29 08:38:34 +08:00
|
|
|
|
2013-03-19 07:31:30 +08:00
|
|
|
Pass *llvm::createLoopDeletionPass() {
|
2008-04-30 04:06:54 +08:00
|
|
|
return new LoopDeletion();
|
2008-04-29 08:38:34 +08:00
|
|
|
}
|
|
|
|
|
2013-03-19 07:31:30 +08:00
|
|
|
/// isLoopDead - Determined if a loop is dead. This assumes that we've already
|
2008-04-30 04:59:33 +08:00
|
|
|
/// checked for unique exit and exiting blocks, and that the code is in LCSSA
|
|
|
|
/// form.
|
2013-03-19 07:31:30 +08:00
|
|
|
bool LoopDeletion::isLoopDead(Loop *L,
|
2013-07-14 12:42:23 +08:00
|
|
|
SmallVectorImpl<BasicBlock *> &exitingBlocks,
|
|
|
|
SmallVectorImpl<BasicBlock *> &exitBlocks,
|
2009-07-15 09:25:43 +08:00
|
|
|
bool &Changed, BasicBlock *Preheader) {
|
2013-03-19 07:31:30 +08:00
|
|
|
BasicBlock *exitBlock = exitBlocks[0];
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
// Make sure that all PHI entries coming from the loop are loop invariant.
|
2008-04-30 04:59:33 +08:00
|
|
|
// Because the code is in LCSSA form, any values used outside of the loop
|
|
|
|
// must pass through a PHI in the exit block, meaning that this check is
|
|
|
|
// sufficient to guarantee that no loop-variant values are used outside
|
|
|
|
// of the loop.
|
2008-04-29 08:38:34 +08:00
|
|
|
BasicBlock::iterator BI = exitBlock->begin();
|
2013-03-19 07:31:30 +08:00
|
|
|
while (PHINode *P = dyn_cast<PHINode>(BI)) {
|
|
|
|
Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
|
2011-02-23 06:25:39 +08:00
|
|
|
|
|
|
|
// Make sure all exiting blocks produce the same incoming value for the exit
|
|
|
|
// block. If there are different incoming values for different exiting
|
|
|
|
// blocks, then it is impossible to statically determine which value should
|
|
|
|
// be used.
|
2013-03-19 07:31:30 +08:00
|
|
|
for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) {
|
2011-02-23 06:25:39 +08:00
|
|
|
if (incoming != P->getIncomingValueForBlock(exitingBlocks[i]))
|
|
|
|
return false;
|
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2013-03-19 07:31:30 +08:00
|
|
|
if (Instruction *I = dyn_cast<Instruction>(incoming))
|
2009-07-15 09:25:43 +08:00
|
|
|
if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
|
2008-04-29 08:38:34 +08:00
|
|
|
return false;
|
2011-02-23 06:25:39 +08:00
|
|
|
|
2010-06-22 23:08:57 +08:00
|
|
|
++BI;
|
2008-04-29 08:38:34 +08:00
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
// Make sure that no instructions in the block have potential side-effects.
|
2008-04-30 04:59:33 +08:00
|
|
|
// This includes instructions that could write to memory, and loads that are
|
|
|
|
// marked volatile. This could be made more aggressive by using aliasing
|
|
|
|
// information to identify readonly and readnone calls.
|
2008-04-29 08:38:34 +08:00
|
|
|
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
|
|
|
|
LI != LE; ++LI) {
|
|
|
|
for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
|
|
|
|
BI != BE; ++BI) {
|
2009-05-06 14:49:50 +08:00
|
|
|
if (BI->mayHaveSideEffects())
|
2008-04-29 08:38:34 +08:00
|
|
|
return false;
|
|
|
|
}
|
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
return true;
|
|
|
|
}
|
|
|
|
|
2008-04-29 14:34:55 +08:00
|
|
|
/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
|
2012-07-24 18:51:42 +08:00
|
|
|
/// observable behavior of the program other than finite running time. Note
|
2008-04-29 14:34:55 +08:00
|
|
|
/// we do ensure that this never remove a loop that might be infinite, as doing
|
|
|
|
/// so could change the halting/non-halting nature of a program.
|
2008-04-30 04:59:33 +08:00
|
|
|
/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
|
|
|
|
/// in order to make various safety checks work.
|
2013-03-19 07:31:30 +08:00
|
|
|
bool LoopDeletion::runOnLoop(Loop *L, LPPassManager &LPM) {
|
2014-02-06 08:07:05 +08:00
|
|
|
if (skipOptnoneFunction(L))
|
|
|
|
return false;
|
|
|
|
|
2012-07-24 18:51:42 +08:00
|
|
|
// We can only remove the loop if there is a preheader that we can
|
2008-04-29 08:38:34 +08:00
|
|
|
// branch from after removing it.
|
2013-03-19 07:31:30 +08:00
|
|
|
BasicBlock *preheader = L->getLoopPreheader();
|
2008-04-29 08:38:34 +08:00
|
|
|
if (!preheader)
|
|
|
|
return false;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2009-11-06 05:47:04 +08:00
|
|
|
// If LoopSimplify form is not available, stay out of trouble.
|
|
|
|
if (!L->hasDedicatedExits())
|
|
|
|
return false;
|
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
// We can't remove loops that contain subloops. If the subloops were dead,
|
|
|
|
// they would already have been removed in earlier executions of this pass.
|
|
|
|
if (L->begin() != L->end())
|
|
|
|
return false;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
SmallVector<BasicBlock*, 4> exitingBlocks;
|
|
|
|
L->getExitingBlocks(exitingBlocks);
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
SmallVector<BasicBlock*, 4> exitBlocks;
|
|
|
|
L->getUniqueExitBlocks(exitBlocks);
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
// We require that the loop only have a single exit block. Otherwise, we'd
|
|
|
|
// be in the situation of needing to be able to solve statically which exit
|
|
|
|
// block will be branched to, or trying to preserve the branching logic in
|
|
|
|
// a loop invariant manner.
|
|
|
|
if (exitBlocks.size() != 1)
|
2008-04-30 04:59:33 +08:00
|
|
|
return false;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:38:34 +08:00
|
|
|
// Finally, we have to check that the loop really is dead.
|
2009-07-15 09:25:43 +08:00
|
|
|
bool Changed = false;
|
2013-03-19 07:31:30 +08:00
|
|
|
if (!isLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
|
2009-07-15 09:25:43 +08:00
|
|
|
return Changed;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
// Don't remove loops for which we can't solve the trip count.
|
|
|
|
// They could be infinite, in which case we'd be changing program behavior.
|
2013-03-19 07:31:30 +08:00
|
|
|
ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
|
2009-10-24 01:10:01 +08:00
|
|
|
const SCEV *S = SE.getMaxBackedgeTakenCount(L);
|
2008-05-16 12:32:45 +08:00
|
|
|
if (isa<SCEVCouldNotCompute>(S))
|
2009-07-15 09:25:43 +08:00
|
|
|
return Changed;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-30 04:59:33 +08:00
|
|
|
// Now that we know the removal is safe, remove the loop by changing the
|
2012-07-24 18:51:42 +08:00
|
|
|
// branch from the preheader to go to the single exit block.
|
2013-03-19 07:31:30 +08:00
|
|
|
BasicBlock *exitBlock = exitBlocks[0];
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// Because we're deleting a large chunk of code at once, the sequence in which
|
|
|
|
// we remove things is very important to avoid invalidation issues. Don't
|
|
|
|
// mess with this unless you have good reason and know what you're doing.
|
2009-07-09 03:14:29 +08:00
|
|
|
|
|
|
|
// Tell ScalarEvolution that the loop is deleted. Do this before
|
|
|
|
// deleting the loop so that ScalarEvolution can look at the loop
|
|
|
|
// to determine what it needs to clean up.
|
2009-10-31 23:04:55 +08:00
|
|
|
SE.forgetLoop(L);
|
2009-07-09 03:14:29 +08:00
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// Connect the preheader directly to the exit block.
|
2013-03-19 07:31:30 +08:00
|
|
|
TerminatorInst *TI = preheader->getTerminator();
|
2008-04-30 04:59:33 +08:00
|
|
|
TI->replaceUsesOfWith(L->getHeader(), exitBlock);
|
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// Rewrite phis in the exit block to get their inputs from
|
|
|
|
// the preheader instead of the exiting block.
|
2013-03-19 07:31:30 +08:00
|
|
|
BasicBlock *exitingBlock = exitingBlocks[0];
|
2008-04-29 08:38:34 +08:00
|
|
|
BasicBlock::iterator BI = exitBlock->begin();
|
2013-03-19 07:31:30 +08:00
|
|
|
while (PHINode *P = dyn_cast<PHINode>(BI)) {
|
2011-06-21 18:02:43 +08:00
|
|
|
int j = P->getBasicBlockIndex(exitingBlock);
|
|
|
|
assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
|
|
|
|
P->setIncomingBlock(j, preheader);
|
2011-02-23 06:25:39 +08:00
|
|
|
for (unsigned i = 1; i < exitingBlocks.size(); ++i)
|
|
|
|
P->removeIncomingValue(exitingBlocks[i]);
|
2010-06-22 23:08:57 +08:00
|
|
|
++BI;
|
2008-04-29 08:38:34 +08:00
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-30 04:59:33 +08:00
|
|
|
// Update the dominator tree and remove the instructions and blocks that will
|
|
|
|
// be deleted from the reference counting scheme.
|
2014-01-13 21:07:17 +08:00
|
|
|
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
2011-01-13 03:12:45 +08:00
|
|
|
SmallVector<DomTreeNode*, 8> ChildNodes;
|
2008-04-29 08:38:34 +08:00
|
|
|
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
|
|
|
|
LI != LE; ++LI) {
|
2008-04-29 08:45:15 +08:00
|
|
|
// Move all of the block's children to be children of the preheader, which
|
|
|
|
// allows us to remove the domtree entry for the block.
|
2011-01-13 03:12:45 +08:00
|
|
|
ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
|
2013-07-04 09:31:24 +08:00
|
|
|
for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(),
|
2009-02-24 09:21:53 +08:00
|
|
|
DE = ChildNodes.end(); DI != DE; ++DI) {
|
2008-04-29 08:38:34 +08:00
|
|
|
DT.changeImmediateDominator(*DI, DT[preheader]);
|
2009-02-24 09:21:53 +08:00
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-30 04:59:33 +08:00
|
|
|
ChildNodes.clear();
|
2008-04-29 08:38:34 +08:00
|
|
|
DT.eraseNode(*LI);
|
2009-02-24 09:21:53 +08:00
|
|
|
|
2008-05-16 12:32:45 +08:00
|
|
|
// Remove the block from the reference counting scheme, so that we can
|
|
|
|
// delete it freely later.
|
2008-04-29 08:38:34 +08:00
|
|
|
(*LI)->dropAllReferences();
|
|
|
|
}
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// Erase the instructions and the blocks without having to worry
|
|
|
|
// about ordering because we already dropped the references.
|
2008-04-30 04:59:33 +08:00
|
|
|
// NOTE: This iteration is safe because erasing the block does not remove its
|
|
|
|
// entry from the loop's block list. We do that in the next section.
|
2008-04-29 08:38:34 +08:00
|
|
|
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
|
2008-05-29 16:15:48 +08:00
|
|
|
LI != LE; ++LI)
|
2008-04-29 08:38:34 +08:00
|
|
|
(*LI)->eraseFromParent();
|
2009-02-24 01:10:29 +08:00
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// Finally, the blocks from loopinfo. This has to happen late because
|
|
|
|
// otherwise our loop iterators won't work.
|
2015-01-17 22:16:18 +08:00
|
|
|
LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
|
2008-04-29 08:38:34 +08:00
|
|
|
SmallPtrSet<BasicBlock*, 8> blocks;
|
|
|
|
blocks.insert(L->block_begin(), L->block_end());
|
2014-08-25 07:23:06 +08:00
|
|
|
for (BasicBlock *BB : blocks)
|
|
|
|
loopInfo.removeBlock(BB);
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2008-04-29 08:45:15 +08:00
|
|
|
// The last step is to inform the loop pass manager that we've
|
|
|
|
// eliminated this loop.
|
2008-04-29 08:38:34 +08:00
|
|
|
LPM.deleteLoopFromQueue(L);
|
2009-07-15 09:25:43 +08:00
|
|
|
Changed = true;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2010-06-22 23:08:57 +08:00
|
|
|
++NumDeleted;
|
2012-07-24 18:51:42 +08:00
|
|
|
|
2009-07-15 09:25:43 +08:00
|
|
|
return Changed;
|
2008-04-29 08:38:34 +08:00
|
|
|
}
|