[llvm-commits] CVS: llvm/lib/Transforms/Scalar/LoopSimplify.cpp
Chris Lattner
lattner at cs.uiuc.edu
Sun Oct 12 19:38:01 PDT 2003
Changes in directory llvm/lib/Transforms/Scalar:
LoopSimplify.cpp updated: 1.21 -> 1.22
---
Log message:
Add support to the loop canonicalization pass to make it transform loops to
have a SINGLE backedge. This is useful to, for example, the -indvars pass.
This implements testcase LoopSimplify/single-backedge.ll and closes PR#34
---
Diffs of the changes: (+163 -21)
Index: llvm/lib/Transforms/Scalar/LoopSimplify.cpp
diff -u llvm/lib/Transforms/Scalar/LoopSimplify.cpp:1.21 llvm/lib/Transforms/Scalar/LoopSimplify.cpp:1.22
--- llvm/lib/Transforms/Scalar/LoopSimplify.cpp:1.21 Sun Oct 12 16:52:27 2003
+++ llvm/lib/Transforms/Scalar/LoopSimplify.cpp Sun Oct 12 19:37:13 2003
@@ -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 @@
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 @@
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 @@
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 @@
// 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 @@
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 @@
// 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) {
- // Get all of the dominators of the predecessor...
- const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
- 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.
+ // 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(Pred);
+ for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
+ PDE = PredDoms.end(); PDI != PDE; ++PDI) {
+ BasicBlock *PredDom = *PDI;
+
+ // 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)) {
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