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Add support to the loop canonicalization pass to make it transform loops to

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have a SINGLE backedge.  This is useful to, for example, the -indvars pass.

This implements testcase LoopSimplify/single-backedge.ll and closes PR#34

llvm-svn: 9065
This commit is contained in:
Chris Lattner 2003-10-13 00:37:13 +00:00
parent 85b4d229d1
commit 7519b1216b
1 changed files with 163 additions and 21 deletions
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176
lib/Transforms/Scalar/LoopSimplify.cpp
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@ -13,6 +13,8 @@
// loop) are dominated by the loop header. This simplifies transformations such
// as store-sinking that are built into LICM.
//
// This pass also guarantees that loops will have exactly one backedge.
//
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unnecessary, so usage of this pass should not pessimize
// generated code.
@ -59,6 +61,10 @@ namespace {
const std::vector<BasicBlock*> &Preds);
void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
void InsertPreheaderForLoop(Loop *L);
void InsertUniqueBackedgeBlock(Loop *L);
void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
std::vector<BasicBlock*> &PredBlocks);
};
RegisterOpt<LoopSimplify>
@ -111,6 +117,15 @@ bool LoopSimplify::ProcessLoop(Loop *L) {
Changed = true;
}
// The preheader may have more than two predecessors at this point (from the
// preheader and from the backedges). To simplify the loop more, insert an
// extra back-edge block in the loop so that there is exactly one backedge.
if (L->getNumBackEdges() != 1) {
InsertUniqueBackedgeBlock(L);
NumInserted++;
Changed = true;
}
const std::vector<Loop*> &SubLoops = L->getSubLoops();
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= ProcessLoop(SubLoops[i]);
@ -333,13 +348,136 @@ void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
if (I->hasExitBlock(Exit))
I->changeExitBlock(Exit, NewBB); // Update exit block information
// Update dominator information (set, immdom, domtree, and domfrontier)
UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks);
}
/// InsertUniqueBackedgeBlock - This method is called when the specified loop
/// has more than one backedge in it. If this occurs, revector all of these
/// backedges to target a new basic block and have that block branch to the loop
/// header. This ensures that loops have exactly one backedge.
///
void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
// Get information about the loop
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
Function *F = Header->getParent();
// Figure out which basic blocks contain back-edges to the loop header.
std::vector<BasicBlock*> BackedgeBlocks;
for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
if (*I != Preheader) BackedgeBlocks.push_back(*I);
// Create and insert the new backedge block...
BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
Instruction *BETerminator = new BranchInst(Header);
BEBlock->getInstList().push_back(BETerminator);
// Move the new backedge block to right after the last backedge block.
Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
// Now that the block has been inserted into the function, create PHI nodes in
// the backedge block which correspond to any PHI nodes in the header block.
for (BasicBlock::iterator I = Header->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
BETerminator);
NewPN->op_reserve(2*BackedgeBlocks.size());
// Loop over the PHI node, moving all entries except the one for the
// preheader over to the new PHI node.
unsigned PreheaderIdx = ~0U;
bool HasUniqueIncomingValue = true;
Value *UniqueValue = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *IBB = PN->getIncomingBlock(i);
Value *IV = PN->getIncomingValue(i);
if (IBB == Preheader) {
PreheaderIdx = i;
} else {
NewPN->addIncoming(IV, IBB);
if (HasUniqueIncomingValue) {
if (UniqueValue == 0)
UniqueValue = IV;
else if (UniqueValue != IV)
HasUniqueIncomingValue = false;
}
}
}
// Delete all of the incoming values from the old PN except the preheader's
assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
if (PreheaderIdx != 0) {
PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
}
PN->op_erase(PN->op_begin()+2, PN->op_end());
// Finally, add the newly constructed PHI node as the entry for the BEBlock.
PN->addIncoming(NewPN, BEBlock);
// As an optimization, if all incoming values in the new PhiNode (which is a
// subset of the incoming values of the old PHI node) have the same value,
// eliminate the PHI Node.
if (HasUniqueIncomingValue) {
NewPN->replaceAllUsesWith(UniqueValue);
BEBlock->getInstList().erase(NewPN);
}
}
// Now that all of the PHI nodes have been inserted and adjusted, modify the
// backedge blocks to just to the BEBlock instead of the header.
for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
if (TI->getSuccessor(Op) == Header)
TI->setSuccessor(Op, BEBlock);
}
//===--- Update all analyses which we must preserve now -----------------===//
// Update Loop Information - we know that this block is now in the current
// loop and all parent loops.
L->addBasicBlockToLoop(BEBlock, getAnalysis<LoopInfo>());
// Replace any instances of Exit with NewBB in this and any nested loops...
for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
if (I->hasExitBlock(Header))
I->changeExitBlock(Header, BEBlock); // Update exit block information
// Update dominator information (set, immdom, domtree, and domfrontier)
UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks);
}
/// UpdateDomInfoForRevectoredPreds - This method is used to update the four
/// different kinds of dominator information (dominator sets, immediate
/// dominators, dominator trees, and dominance frontiers) after a new block has
/// been added to the CFG.
///
/// This only supports the case when an existing block (known as "Exit"), had
/// some of its predecessors factored into a new basic block. This
/// transformation inserts a new basic block ("NewBB"), with a single
/// unconditional branch to Exit, and moves some predecessors of "Exit" to now
/// branch to NewBB. These predecessors are listed in PredBlocks, even though
/// they are the same as pred_begin(NewBB)/pred_end(NewBB).
///
void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
std::vector<BasicBlock*> &PredBlocks) {
assert(succ_begin(NewBB) != succ_end(NewBB) &&
++succ_begin(NewBB) == succ_end(NewBB) &&
"NewBB should have a single successor!");
DominatorSet &DS = getAnalysis<DominatorSet>();
// Update dominator information... The blocks that dominate NewBB are the
// intersection of the dominators of predecessors, plus the block itself.
// The newly created basic block does not dominate anything except itself.
//
DominatorSet::DomSetType NewBBDomSet = DS.getDominators(LoopBlocks[0]);
for (unsigned i = 1, e = LoopBlocks.size(); i != e; ++i)
set_intersect(NewBBDomSet, DS.getDominators(LoopBlocks[i]));
DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]);
for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i)
set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i]));
NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
DS.addBasicBlock(NewBB, NewBBDomSet);
@ -351,7 +489,7 @@ void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
// trace up the immediate dominators of a predecessor until we find a basic
// block that dominates the exit block.
//
BasicBlock *Dom = LoopBlocks[0]; // Some random predecessor...
BasicBlock *Dom = PredBlocks[0]; // Some random predecessor...
while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
assert(Dom != 0 && "No shared dominator found???");
Dom = ID->get(Dom);
@ -371,7 +509,7 @@ void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
if (NewBBIDom) {
NewBBIDomNode = DT->getNode(NewBBIDom);
} else {
NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred
while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
NewBBIDomNode = NewBBIDomNode->getIDom();
assert(NewBBIDomNode && "No shared dominator found??");
@ -385,27 +523,31 @@ void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
// Update dominance frontier information...
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
// DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
// does dominate itself (and there is an edge (NewBB -> Exit)).
// does dominate itself (and there is an edge (NewBB -> Exit)). Exit is the
// single successor of NewBB.
DominanceFrontier::DomSetType NewDFSet;
BasicBlock *Exit = *succ_begin(NewBB);
NewDFSet.insert(Exit);
DF->addBasicBlock(NewBB, NewDFSet);
// Now we must loop over all of the dominance frontiers in the function,
// replacing occurrences of Exit with NewBB in some cases. If a block
// dominates a (now) predecessor of NewBB, but did not strictly dominate
// Exit, it will have Exit in it's DF set, but should now have NewBB in its
// set.
for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
// replacing occurrences of Exit with NewBB in some cases. All blocks that
// dominate a block in PredBlocks and contained Exit in their dominance
// frontier must be updated to contain NewBB instead. This only occurs if
// there is more than one block in PredBlocks.
//
if (PredBlocks.size() > 1) {
for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) {
BasicBlock *Pred = PredBlocks[i];
// Get all of the dominators of the predecessor...
const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred);
for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
PDE = PredDoms.end(); PDI != PDE; ++PDI) {
BasicBlock *PredDom = *PDI;
// Make sure to only rewrite blocks that are part of the loop...
if (L->contains(PredDom)) {
// If the exit node is in DF(PredDom), then PredDom didn't dominate
// Exit but did dominate a predecessor inside of the loop. Now we
// change this entry to include NewBB in the DF instead of Exit.
// If the Exit node is in DF(PredDom), then PredDom didn't dominate
// Exit but did dominate a predecessor of it. Now we change this
// entry to include NewBB in the DF instead of Exit.
DominanceFrontier::iterator DFI = DF->find(PredDom);
assert(DFI != DF->end() && "No dominance frontier for node?");
if (DFI->second.count(Exit)) {