//===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
//
// Insert Loop pre-headers and exit blocks into the CFG for each function in the
// module. This pass updates loop information and dominator information.
//
// Loop pre-header insertion guarantees that there is a single, non-critical
// entry edge from outside of the loop to the loop header. This simplifies a
// number of analyses and transformations, such as LICM.
//
// Loop exit-block insertion guarantees that all exit blocks from the loop
// (blocks which are outside of the loop that have predecessors inside of the
// loop) are dominated by the loop header. This simplifies transformations such
// as store-sinking that is built into LICM.
//
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unneccesary, so usage of this pass does not neccesarily
// pessimize generated code.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Function.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
#include "Support/SetOperations.h"
#include "Support/Statistic.h"
namespace {
Statistic<> NumInserted("preheaders", "Number of pre-header nodes inserted");
struct Preheaders : public FunctionPass {
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
// We need loop information to identify the loops...
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorSet>();
AU.addPreserved<LoopInfo>();
AU.addPreserved<DominatorSet>();
AU.addPreserved<ImmediateDominators>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<DominanceFrontier>();
AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
}
private:
bool ProcessLoop(Loop *L);
BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
const std::vector<BasicBlock*> &Preds);
void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
void InsertPreheaderForLoop(Loop *L);
};
RegisterOpt<Preheaders> X("preheaders", "Natural loop pre-header insertion");
}
// Publically exposed interface to pass...
const PassInfo *LoopPreheadersID = X.getPassInfo();
Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }
/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
/// it in any convenient order) inserting preheaders...
///
bool Preheaders::runOnFunction(Function &F) {
bool Changed = false;
LoopInfo &LI = getAnalysis<LoopInfo>();
for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);
return Changed;
}
/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
/// all loops have preheaders.
///
bool Preheaders::ProcessLoop(Loop *L) {
bool Changed = false;
// Does the loop already have a preheader? If so, don't modify the loop...
if (L->getLoopPreheader() == 0) {
InsertPreheaderForLoop(L);
NumInserted++;
Changed = true;
}
DominatorSet &DS = getAnalysis<DominatorSet>();
BasicBlock *Header = L->getHeader();
for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
if (!DS.dominates(Header, L->getExitBlocks()[i])) {
RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
"RewriteLoopExitBlock failed?");
NumInserted++;
Changed = true;
}
const std::vector<Loop*> &SubLoops = L->getSubLoops();
for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
Changed |= ProcessLoop(SubLoops[i]);
return Changed;
}
/// SplitBlockPredecessors - Split the specified block into two blocks. We want
/// to move the predecessors specified in the Preds list to point to the new
/// block, leaving the remaining predecessors pointing to BB. This method
/// updates the SSA PHINode's, but no other analyses.
///
BasicBlock *Preheaders::SplitBlockPredecessors(BasicBlock *BB,
const char *Suffix,
const std::vector<BasicBlock*> &Preds) {
// Create new basic block, insert right before the original block...
BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);
// The preheader first gets an unconditional branch to the loop header...
BranchInst *BI = new BranchInst(BB);
NewBB->getInstList().push_back(BI);
// For every PHI node in the block, insert a PHI node into NewBB where the
// incoming values from the out of loop edges are moved to NewBB. We have two
// possible cases here. If the loop is dead, we just insert dummy entries
// into the PHI nodes for the new edge. If the loop is not dead, we move the
// incoming edges in BB into new PHI nodes in NewBB.
//
if (!Preds.empty()) { // Is the loop not obviously dead?
for (BasicBlock::iterator I = BB->begin();
PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
// Create the new PHI node, insert it into NewBB at the end of the block
PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
// Move all of the edges from blocks outside the loop to the new PHI
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
Value *V = PN->removeIncomingValue(Preds[i]);
NewPHI->addIncoming(V, Preds[i]);
}
// Add an incoming value to the PHI node in the loop for the preheader
// edge
PN->addIncoming(NewPHI, NewBB);
}
// Now that the PHI nodes are updated, actually move the edges from
// Preds to point to NewBB instead of BB.
//
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
TerminatorInst *TI = Preds[i]->getTerminator();
for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
if (TI->getSuccessor(s) == BB)
TI->setSuccessor(s, NewBB);
}
} else { // Otherwise the loop is dead...
for (BasicBlock::iterator I = BB->begin();
PHINode *PN = dyn_cast<PHINode>(&*I); ++I)
// Insert dummy values as the incoming value...
PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
}
return NewBB;
}
/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one. This method has two phases:
/// preheader insertion and analysis updating.
///
void Preheaders::InsertPreheaderForLoop(Loop *L) {
BasicBlock *Header = L->getHeader();
// Compute the set of predecessors of the loop that are not in the loop.
std::vector<BasicBlock*> OutsideBlocks;
for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
PI != PE; ++PI)
if (!L->contains(*PI)) // Coming in from outside the loop?
OutsideBlocks.push_back(*PI); // Keep track of it...
// Split out the loop pre-header
BasicBlock *NewBB =
SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
//===--------------------------------------------------------------------===//
// Update analysis results now that we have preformed the transformation
//
// We know that we have loop information to update... update it now.
if (Loop *Parent = L->getParentLoop())
Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
{
// The blocks that dominate NewBB are the blocks that dominate Header,
// minus Header, plus NewBB.
DominatorSet::DomSetType DomSet = DS.getDominators(Header);
DomSet.insert(NewBB); // We dominate ourself
DomSet.erase(Header); // Header does not dominate us...
DS.addBasicBlock(NewBB, DomSet);
// The newly created basic block dominates all nodes dominated by Header.
for (Function::iterator I = Header->getParent()->begin(),
E = Header->getParent()->end(); I != E; ++I)
if (DS.dominates(Header, I))
DS.addDominator(I, NewBB);
}
// Update immediate dominator information if we have it...
if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
// Whatever i-dominated the header node now immediately dominates NewBB
ID->addNewBlock(NewBB, ID->get(Header));
// The preheader now is the immediate dominator for the header node...
ID->setImmediateDominator(Header, NewBB);
}
// Update DominatorTree information if it is active.
if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
// The immediate dominator of the preheader is the immediate dominator of
// the old header.
//
DominatorTree::Node *HeaderNode = DT->getNode(Header);
DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
HeaderNode->getIDom());
// Change the header node so that PNHode is the new immediate dominator
DT->changeImmediateDominator(HeaderNode, PHNode);
}
// Update dominance frontier information...
if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
// The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
// everything that Header does, and it strictly dominates Header in
// addition.
assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
NewDFSet.erase(Header);
DF->addBasicBlock(NewBB, NewDFSet);
// Now we must loop over all of the dominance frontiers in the function,
// replacing occurances of Header with NewBB in some cases. If a block
// dominates a (now) predecessor of NewBB, but did not strictly dominate
// Header, it will have Header in it's DF set, but should now have NewBB in
// its set.
for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
// Get all of the dominators of the predecessor...
const DominatorSet::DomSetType &PredDoms =
DS.getDominators(OutsideBlocks[i]);
for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
PDE = PredDoms.end(); PDI != PDE; ++PDI) {
BasicBlock *PredDom = *PDI;
// If the loop header is in DF(PredDom), then PredDom didn't dominate
// the header but did dominate a predecessor outside of the loop. Now
// we change this entry to include the preheader in the DF instead of
// the header.
DominanceFrontier::iterator DFI = DF->find(PredDom);
assert(DFI != DF->end() && "No dominance frontier for node?");
if (DFI->second.count(Header)) {
DF->removeFromFrontier(DFI, Header);
DF->addToFrontier(DFI, NewBB);
}
}
}
}
}
void Preheaders::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
DominatorSet &DS = getAnalysis<DominatorSet>();
assert(!DS.dominates(L->getHeader(), Exit) &&
"Loop already dominates exit block??");
assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
!= L->getExitBlocks().end() && "Not a current exit block!");
std::vector<BasicBlock*> LoopBlocks;
for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
if (L->contains(*I))
LoopBlocks.push_back(*I);
assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
// Update Loop Information - we know that the new block will be in the parent
// loop of L.
if (Loop *Parent = L->getParentLoop())
Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
L->changeExitBlock(Exit, NewBB); // Update exit block information
// 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]));
NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
DS.addBasicBlock(NewBB, NewBBDomSet);
// Update immediate dominator information if we have it...
BasicBlock *NewBBIDom = 0;
if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
// This block does not strictly dominate anything, so it is not an immediate
// dominator. To find the immediate dominator of the new exit node, we
// 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...
while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
assert(Dom != 0 && "No shared dominator found???");
Dom = ID->get(Dom);
}
// Set the immediate dominator now...
ID->addNewBlock(NewBB, Dom);
NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
}
// Update DominatorTree information if it is active.
if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
// NewBB doesn't dominate anything, so just create a node and link it into
// its immediate dominator. If we don't have ImmediateDominator info
// around, calculate the idom as above.
DominatorTree::Node *NewBBIDomNode;
if (NewBBIDom) {
NewBBIDomNode = DT->getNode(NewBBIDom);
} else {
NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
while (!NewBBDomSet.count(NewBBIDomNode->getNode())) {
NewBBIDomNode = NewBBIDomNode->getIDom();
assert(NewBBIDomNode && "No shared dominator found??");
}
}
// Create the new dominator tree node...
DT->createNewNode(NewBB, NewBBIDomNode);
}
// 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)).
DominanceFrontier::DomSetType NewDFSet;
NewDFSet.insert(Exit);
DF->addBasicBlock(NewBB, NewDFSet);
// Now we must loop over all of the dominance frontiers in the function,
// replacing occurances 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.
DominanceFrontier::iterator DFI = DF->find(PredDom);
assert(DFI != DF->end() && "No dominance frontier for node?");
if (DFI->second.count(Exit)) {
DF->removeFromFrontier(DFI, Exit);
DF->addToFrontier(DFI, NewBB);
}
}
}
}
}
}