bool runOnFunction(Function &F);
private:
inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
+ inline bool canEliminateUnconditionalBranch(TerminatorInst *TI);
inline void eliminateUnconditionalBranch(BranchInst *BI);
inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
BasicBlock *NewBlock);
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; )
- if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
+ if (shouldEliminateUnconditionalBranch(I->getTerminator()) &&
+ canEliminateUnconditionalBranch(I->getTerminator())) {
eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
Changed = true;
} else {
return true;
}
+/// canEliminateUnconditionalBranch - Unfortunately, the general form of tail
+/// duplication can do very bad things to SSA form, by destroying arbitrary
+/// relationships between dominators and dominator frontiers as it processes the
+/// program. The right solution for this is to have an incrementally updating
+/// dominator data structure, which can gracefully react to arbitrary
+/// "addEdge/removeEdge" changes to the CFG. Implementing this is nontrivial,
+/// however, so we just disable the transformation in cases where it is not
+/// currently safe.
+///
+bool TailDup::canEliminateUnconditionalBranch(TerminatorInst *TI) {
+ // Basically, we refuse to make the transformation if any of the values
+ // computed in the 'tail' are used in any other basic blocks.
+ BasicBlock *Tail = TI->getSuccessor(0);
+
+ for (BasicBlock::iterator I = Tail->begin(), E = Tail->end(); I != E; ++I)
+ for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+ ++UI) {
+ Instruction *User = cast<Instruction>(*UI);
+ if (User->getParent() != Tail || isa<PHINode>(User))
+ return false;
+ }
+ return true;
+}
+
/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.