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Avoid making a broken transformation!

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llvm-svn: 1115
This commit is contained in:
Chris Lattner 2001-11-03 21:30:22 +00:00
parent 2b011e7b50
commit 82c7401f23
1 changed files with 33 additions and 20 deletions
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53
lib/Transforms/Scalar/DCE.cpp
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@ -105,11 +105,13 @@ static void ReplaceUsesWithConstant(Instruction *I) {
// PropogatePredecessors - This gets "Succ" ready to have the predecessors from
// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
// have extra slots added to them to hold the merge edges from BB's
// predecessors.
// predecessors. This function returns true (failure) if the Succ BB already
// has a predecessor that is a predecessor of BB.
//
// Assumption: BB is the single predecessor of Succ.
// Assumption: Succ is the single successor for BB.
//
static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*BB->succ_begin() == Succ && "Succ is not successor of BB!");
assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
// If there is more than one predecessor, and there are PHI nodes in
@ -117,6 +119,15 @@ static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
//
const vector<BasicBlock*> BBPreds(BB->pred_begin(), BB->pred_end());
// Check to see if one of the predecessors of BB is already a predecessor of
// Succ. If so, we cannot do the transformation!
//
for (BasicBlock::pred_iterator PI = Succ->pred_begin(), PE = Succ->pred_end();
PI != PE; ++PI) {
if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end())
return true;
}
BasicBlock::iterator I = Succ->begin();
do { // Loop over all of the PHI nodes in the successor BB
PHINode *PN = cast<PHINode>(*I);
@ -131,6 +142,7 @@ static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
++I;
} while (isa<PHINode>(*I));
return false;
}
@ -182,28 +194,29 @@ bool opt::SimplifyCFG(Method::iterator &BBIt) {
// successor. If so, replace block references with successor.
BasicBlock::succ_iterator SI(BB->succ_begin());
if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ?
Instruction *I = BB->front();
if (I->isTerminator()) { // Terminator is the only instruction!
if (BB->front()->isTerminator()) { // Terminator is the only instruction!
BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor
//cerr << "Killing Trivial BB: \n" << BB;
if (Succ != BB) { // Arg, don't hurt infinite loops!
if (isa<PHINode>(Succ->front())) {
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
//
PropogatePredecessorsForPHIs(BB, Succ);
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
// Be careful though, if this transformation fails (returns true) then
// we cannot do this transformation!
//
if (!isa<PHINode>(Succ->front()) ||
!PropogatePredecessorsForPHIs(BB, Succ)) {
BB->replaceAllUsesWith(Succ);
BB = M->getBasicBlocks().remove(BBIt);
if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
Succ->setName(BB->getName());
delete BB; // Delete basic block
//cerr << "Method after removal: \n" << M;
return true;
}
BB->replaceAllUsesWith(Succ);
BB = M->getBasicBlocks().remove(BBIt);
if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
Succ->setName(BB->getName());
delete BB; // Delete basic block
//cerr << "Method after removal: \n" << M;
return true;
}
}
}