/// processOneIterationLoop -- Eliminate loop if loop body is executed
/// only once. For example,
/// for (i = 0; i < N; ++i) {
/// if ( i == X) {
/// ...
/// }
/// }
///
bool LoopIndexSplit::processOneIterationLoop() {
SplitCondition = NULL;
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *Header = L->getHeader();
BranchInst *BR = dyn_cast<BranchInst>(Header->getTerminator());
if (!BR) return false;
if (!isa<BranchInst>(Latch->getTerminator())) return false;
if (BR->isUnconditional()) return false;
SplitCondition = dyn_cast<ICmpInst>(BR->getCondition());
if (!SplitCondition) return false;
if (SplitCondition == ExitCondition) return false;
if (SplitCondition->getPredicate() != ICmpInst::ICMP_EQ) return false;
if (BR->getOperand(1) != Latch) return false;
if (!IVBasedValues.count(SplitCondition->getOperand(0))
&& !IVBasedValues.count(SplitCondition->getOperand(1)))
return false;
// If IV is used outside the loop then this loop traversal is required.
// FIXME: Calculate and use last IV value.
if (isUsedOutsideLoop(IVIncrement, L))
return false;
// If BR operands are not IV or not loop invariants then skip this loop.
Value *OPV = SplitCondition->getOperand(0);
Value *SplitValue = SplitCondition->getOperand(1);
if (!L->isLoopInvariant(SplitValue))
std::swap(OPV, SplitValue);
if (!L->isLoopInvariant(SplitValue))
return false;
Instruction *OPI = dyn_cast<Instruction>(OPV);
if (!OPI)
return false;
if (OPI->getParent() != Header || isUsedOutsideLoop(OPI, L))
return false;
Value *StartValue = IVStartValue;
Value *ExitValue = IVExitValue;;
if (OPV != IndVar) {
// If BR operand is IV based then use this operand to calculate
// effective conditions for loop body.
BinaryOperator *BOPV = dyn_cast<BinaryOperator>(OPV);
if (!BOPV)
return false;
if (BOPV->getOpcode() != Instruction::Add)
return false;
StartValue = BinaryOperator::CreateAdd(OPV, StartValue, "" , BR);
ExitValue = BinaryOperator::CreateAdd(OPV, ExitValue, "" , BR);
}
if (!cleanBlock(Header))
return false;
if (!cleanBlock(Latch))
return false;
// If the merge point for BR is not loop latch then skip this loop.
if (BR->getSuccessor(0) != Latch) {
DominanceFrontier::iterator DF0 = DF->find(BR->getSuccessor(0));
assert (DF0 != DF->end() && "Unable to find dominance frontier");
if (!DF0->second.count(Latch))
return false;
}
if (BR->getSuccessor(1) != Latch) {
DominanceFrontier::iterator DF1 = DF->find(BR->getSuccessor(1));
assert (DF1 != DF->end() && "Unable to find dominance frontier");
if (!DF1->second.count(Latch))
return false;
}
// Now, Current loop L contains compare instruction
// that compares induction variable, IndVar, against loop invariant. And
// entire (i.e. meaningful) loop body is dominated by this compare
// instruction. In such case eliminate
// loop structure surrounding this loop body. For example,
// for (int i = start; i < end; ++i) {
// if ( i == somevalue) {
// loop_body
// }
// }
// can be transformed into
// if (somevalue >= start && somevalue < end) {
// i = somevalue;
// loop_body
// }
// Replace index variable with split value in loop body. Loop body is executed
// only when index variable is equal to split value.
IndVar->replaceAllUsesWith(SplitValue);
// Replace split condition in header.
// Transform
// SplitCondition : icmp eq i32 IndVar, SplitValue
// into
// c1 = icmp uge i32 SplitValue, StartValue
// c2 = icmp ult i32 SplitValue, ExitValue
// and i32 c1, c2
Instruction *C1 = new ICmpInst(BR, ExitCondition->isSigned() ?
ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE,
SplitValue, StartValue, "lisplit");
CmpInst::Predicate C2P = ExitCondition->getPredicate();
BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
if (LatchBR->getOperand(1) != Header)
C2P = CmpInst::getInversePredicate(C2P);
Instruction *C2 = new ICmpInst(BR, C2P, SplitValue, ExitValue, "lisplit");
Instruction *NSplitCond = BinaryOperator::CreateAnd(C1, C2, "lisplit", BR);
SplitCondition->replaceAllUsesWith(NSplitCond);
SplitCondition->eraseFromParent();
// Remove Latch to Header edge.
BasicBlock *LatchSucc = NULL;
Header->removePredecessor(Latch);
for (succ_iterator SI = succ_begin(Latch), E = succ_end(Latch);
SI != E; ++SI) {
if (Header != *SI)
LatchSucc = *SI;
}
// Clean up latch block.
Value *LatchBRCond = LatchBR->getCondition();
LatchBR->setUnconditionalDest(LatchSucc);
RecursivelyDeleteTriviallyDeadInstructions(LatchBRCond);
LPM->deleteLoopFromQueue(L);
// Update Dominator Info.
// Only CFG change done is to remove Latch to Header edge. This
// does not change dominator tree because Latch did not dominate
// Header.
if (DF) {
DominanceFrontier::iterator HeaderDF = DF->find(Header);
if (HeaderDF != DF->end())
DF->removeFromFrontier(HeaderDF, Header);
DominanceFrontier::iterator LatchDF = DF->find(Latch);
if (LatchDF != DF->end())
DF->removeFromFrontier(LatchDF, Header);
}
++NumIndexSplitRemoved;
return true;
}
