/// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
/// loop with no side effects (including infinite loops).
///
/// If true, we return true and set ExitBB to the block we
/// exit through.
///
static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
BasicBlock *&ExitBB,
std::set<BasicBlock*> &Visited) {
if (!Visited.insert(BB).second) {
// Already visited. Without more analysis, this could indicate an infinte loop.
return false;
} else if (!L->contains(BB)) {
// Otherwise, this is a loop exit, this is fine so long as this is the
// first exit.
if (ExitBB != 0) return false;
ExitBB = BB;
return true;
}
// Otherwise, this is an unvisited intra-loop node. Check all successors.
for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
// Check to see if the successor is a trivial loop exit.
if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
return false;
}
// Okay, everything after this looks good, check to make sure that this block
// doesn't include any side effects.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (I->mayHaveSideEffects())
return false;
return true;
}
/// If there's a single exit block, sink any loop-invariant values that
/// were defined in the preheader but not used inside the loop into the
/// exit block to reduce register pressure in the loop.
void IndVarSimplify::SinkUnusedInvariants(Loop *L) {
BasicBlock *ExitBlock = L->getExitBlock();
if (!ExitBlock) return;
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader) return;
Instruction *InsertPt = ExitBlock->getFirstNonPHI();
BasicBlock::iterator I = Preheader->getTerminator();
while (I != Preheader->begin()) {
--I;
// New instructions were inserted at the end of the preheader.
if (isa<PHINode>(I))
break;
// Don't move instructions which might have side effects, since the side
// effects need to complete before instructions inside the loop. Also
// don't move instructions which might read memory, since the loop may
// modify memory. Note that it's okay if the instruction might have
// undefined behavior: LoopSimplify guarantees that the preheader
// dominates the exit block.
if (I->mayHaveSideEffects() || I->mayReadFromMemory())
continue;
// Don't sink static AllocaInsts out of the entry block, which would
// turn them into dynamic allocas!
if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
if (AI->isStaticAlloca())
continue;
// Determine if there is a use in or before the loop (direct or
// otherwise).
bool UsedInLoop = false;
for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
BasicBlock *UseBB = cast<Instruction>(UI)->getParent();
if (PHINode *P = dyn_cast<PHINode>(UI)) {
unsigned i =
PHINode::getIncomingValueNumForOperand(UI.getOperandNo());
UseBB = P->getIncomingBlock(i);
}
if (UseBB == Preheader || L->contains(UseBB)) {
UsedInLoop = true;
break;
}
}
// If there is, the def must remain in the preheader.
if (UsedInLoop)
continue;
// Otherwise, sink it to the exit block.
Instruction *ToMove = I;
bool Done = false;
if (I != Preheader->begin())
--I;
else
Done = true;
ToMove->moveBefore(InsertPt);
if (Done)
break;
InsertPt = ToMove;
}
}
/// isLoopDead - Determined if a loop is dead. This assumes that we've already
/// checked for unique exit and exiting blocks, and that the code is in LCSSA
/// form.
bool LoopDeletion::isLoopDead(Loop *L, ScalarEvolution &SE,
SmallVectorImpl<BasicBlock *> &exitingBlocks,
SmallVectorImpl<BasicBlock *> &exitBlocks,
bool &Changed, BasicBlock *Preheader) {
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
// sufficient to guarantee that no loop-variant values are used outside
// of the loop.
BasicBlock::iterator BI = exitBlock->begin();
bool AllEntriesInvariant = true;
bool AllOutgoingValuesSame = true;
while (PHINode *P = dyn_cast<PHINode>(BI)) {
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.
AllOutgoingValuesSame =
all_of(makeArrayRef(exitingBlocks).slice(1), [&](BasicBlock *BB) {
return incoming == P->getIncomingValueForBlock(BB);
});
if (!AllOutgoingValuesSame)
break;
if (Instruction *I = dyn_cast<Instruction>(incoming))
if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) {
AllEntriesInvariant = false;
break;
}
++BI;
}
if (Changed)
SE.forgetLoopDispositions(L);
if (!AllEntriesInvariant || !AllOutgoingValuesSame)
return false;
// 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
// information to identify readonly and readnone calls.
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) {
if (BI->mayHaveSideEffects())
return false;
}
}
return true;
}
/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
/// trivial: that is, that the condition controls whether or not the loop does
/// anything at all. If this is a trivial condition, unswitching produces no
/// code duplications (equivalently, it produces a simpler loop and a new empty
/// loop, which gets deleted).
///
/// If this is a trivial condition, return true, otherwise return false. When
/// returning true, this sets Cond and Val to the condition that controls the
/// trivial condition: when Cond dynamically equals Val, the loop is known to
/// exit. Finally, this sets LoopExit to the BB that the loop exits to when
/// Cond == Val.
///
bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
BasicBlock **LoopExit) {
BasicBlock *Header = currentLoop->getHeader();
TerminatorInst *HeaderTerm = Header->getTerminator();
LLVMContext &Context = Header->getContext();
BasicBlock *LoopExitBB = 0;
if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
// If the header block doesn't end with a conditional branch on Cond, we
// can't handle it.
if (!BI->isConditional() || BI->getCondition() != Cond)
return false;
// Check to see if a successor of the branch is guaranteed to
// exit through a unique exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this.
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(0)))) {
if (Val) *Val = ConstantInt::getTrue(Context);
} else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
BI->getSuccessor(1)))) {
if (Val) *Val = ConstantInt::getFalse(Context);
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
// If this isn't a switch on Cond, we can't handle it.
if (SI->getCondition() != Cond) return false;
// Check to see if a successor of the switch is guaranteed to go to the
// latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this. Note that we can't trivially unswitch on the default case.
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
SI->getSuccessor(i)))) {
// Okay, we found a trivial case, remember the value that is trivial.
if (Val) *Val = SI->getCaseValue(i);
break;
}
}
// If we didn't find a single unique LoopExit block, or if the loop exit block
// contains phi nodes, this isn't trivial.
if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
return false; // Can't handle this.
if (LoopExit) *LoopExit = LoopExitBB;
// We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any
// side-effecting instructions (e.g. stores, calls, volatile loads) in the
// part of the loop that the code *would* execute. We already checked the
// tail, check the header now.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
if (I->mayHaveSideEffects())
return false;
return true;
}
/// IsLoopDead - Determined if a loop is dead. This assumes that we've already
/// checked for unique exit and exiting blocks, and that the code is in LCSSA
/// form.
bool LoopDeletion::IsLoopDead(Loop* L,
SmallVector<BasicBlock*, 4>& exitingBlocks,
SmallVector<BasicBlock*, 4>& exitBlocks,
bool &Changed, BasicBlock *Preheader) {
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
// sufficient to guarantee that no loop-variant values are used outside
// of the loop.
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
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
// information to identify readonly and readnone calls.
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) {
if (BI->mayHaveSideEffects())
return false;
}
}
return true;
}
