#define DEBUG_TYPE "loop-delete"
#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
-#include "llvm/ADT/Statistic.h"
-#include "llvm/ADT/SmallVector.h"
using namespace llvm;
STATISTIC(NumDeleted, "Number of loops deleted");
LoopDeletion() : LoopPass(ID) {
initializeLoopDeletionPass(*PassRegistry::getPassRegistry());
}
-
+
// Possibly eliminate loop L if it is dead.
bool runOnLoop(Loop* L, LPPassManager& LPM);
-
+
bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
SmallVector<BasicBlock*, 4>& exitBlocks,
bool &Changed, BasicBlock *Preheader);
AU.addRequired<ScalarEvolution>();
AU.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
-
+
AU.addPreserved<ScalarEvolution>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<LoopInfo>();
AU.addPreservedID(LoopSimplifyID);
AU.addPreservedID(LCSSAID);
- AU.addPreserved<DominanceFrontier>();
}
};
}
-
+
char LoopDeletion::ID = 0;
INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion",
"Delete dead loops", false, false)
SmallVector<BasicBlock*, 4>& exitingBlocks,
SmallVector<BasicBlock*, 4>& exitBlocks,
bool &Changed, BasicBlock *Preheader) {
- BasicBlock* exitingBlock = exitingBlocks[0];
BasicBlock* exitBlock = exitBlocks[0];
-
+
// Make sure that all PHI entries coming from the loop are loop invariant.
// 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
// of the loop.
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
- Value* incoming = P->getIncomingValueForBlock(exitingBlock);
+ Value* incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
+
+ // 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.
+ for (unsigned i = 1; i < exitingBlocks.size(); ++i) {
+ if (incoming != P->getIncomingValueForBlock(exitingBlocks[i]))
+ return false;
+ }
+
if (Instruction* I = dyn_cast<Instruction>(incoming))
if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator()))
return false;
-
+
++BI;
}
-
+
// Make sure that no instructions in the block have potential side-effects.
// This includes instructions that could write to memory, and loads that are
// marked volatile. This could be made more aggressive by using aliasing
return false;
}
}
-
+
return true;
}
/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the
-/// observable behavior of the program other than finite running time. Note
+/// observable behavior of the program other than finite running time. Note
/// 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.
/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
/// in order to make various safety checks work.
bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
- // We can only remove the loop if there is a preheader that we can
+ // We can only remove the loop if there is a preheader that we can
// branch from after removing it.
BasicBlock* preheader = L->getLoopPreheader();
if (!preheader)
return false;
-
+
// If LoopSimplify form is not available, stay out of trouble.
if (!L->hasDedicatedExits())
return false;
// they would already have been removed in earlier executions of this pass.
if (L->begin() != L->end())
return false;
-
+
SmallVector<BasicBlock*, 4> exitingBlocks;
L->getExitingBlocks(exitingBlocks);
-
+
SmallVector<BasicBlock*, 4> exitBlocks;
L->getUniqueExitBlocks(exitBlocks);
-
+
// 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)
return false;
-
- // Loops with multiple exits are too complicated to handle correctly.
- if (exitingBlocks.size() != 1)
- return false;
-
+
// Finally, we have to check that the loop really is dead.
bool Changed = false;
if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader))
return Changed;
-
+
// 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.
ScalarEvolution& SE = getAnalysis<ScalarEvolution>();
const SCEV *S = SE.getMaxBackedgeTakenCount(L);
if (isa<SCEVCouldNotCompute>(S))
return Changed;
-
+
// Now that we know the removal is safe, remove the loop by changing the
- // branch from the preheader to go to the single exit block.
+ // branch from the preheader to go to the single exit block.
BasicBlock* exitBlock = exitBlocks[0];
- BasicBlock* exitingBlock = exitingBlocks[0];
-
+
// 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.
// Rewrite phis in the exit block to get their inputs from
// the preheader instead of the exiting block.
+ BasicBlock* exitingBlock = exitingBlocks[0];
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
- P->replaceUsesOfWith(exitingBlock, preheader);
+ int j = P->getBasicBlockIndex(exitingBlock);
+ assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
+ P->setIncomingBlock(j, preheader);
+ for (unsigned i = 1; i < exitingBlocks.size(); ++i)
+ P->removeIncomingValue(exitingBlocks[i]);
++BI;
}
-
+
// Update the dominator tree and remove the instructions and blocks that will
// be deleted from the reference counting scheme.
DominatorTree& DT = getAnalysis<DominatorTree>();
- DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>();
- SmallPtrSet<DomTreeNode*, 8> ChildNodes;
+ SmallVector<DomTreeNode*, 8> ChildNodes;
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
LI != LE; ++LI) {
// Move all of the block's children to be children of the preheader, which
// allows us to remove the domtree entry for the block.
- ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
- for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
+ ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
+ for (SmallVector<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
DE = ChildNodes.end(); DI != DE; ++DI) {
DT.changeImmediateDominator(*DI, DT[preheader]);
- if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT);
}
-
+
ChildNodes.clear();
DT.eraseNode(*LI);
- if (DF) DF->removeBlock(*LI);
// Remove the block from the reference counting scheme, so that we can
// delete it freely later.
(*LI)->dropAllReferences();
}
-
+
// Erase the instructions and the blocks without having to worry
// about ordering because we already dropped the references.
// NOTE: This iteration is safe because erasing the block does not remove its
for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(),
E = blocks.end(); I != E; ++I)
loopInfo.removeBlock(*I);
-
+
// The last step is to inform the loop pass manager that we've
// eliminated this loop.
LPM.deleteLoopFromQueue(L);
Changed = true;
-
+
++NumDeleted;
-
+
return Changed;
}
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