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* Significant changes to the preheader insertion pass:
- Now we perform loop exit-block splitting to ensure exit blocks are
always dominated by the loop header.
- We now preserve dominance frontier information
- This fixes bug: LICM/2003-02-26-LoopExitNotDominated.ll
llvm-svn: 5652
This commit is contained in:
Chris Lattner
parent
db73d0ceab
commit
b078b28fa5
@ -1,7 +1,20 @@
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//===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
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//
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// Insert Loop pre-headers into the CFG for each function in the module. This
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// pass updates loop information and dominator information.
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// Insert Loop pre-headers and exit blocks into the CFG for each function in the
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// module. This pass updates loop information and dominator information.
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//
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// Loop pre-header insertion guarantees that there is a single, non-critical
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// entry edge from outside of the loop to the loop header. This simplifies a
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// number of analyses and transformations, such as LICM.
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//
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// Loop exit-block insertion guarantees that all exit blocks from the loop
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// (blocks which are outside of the loop that have predecessors inside of the
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// loop) are dominated by the loop header. This simplifies transformations such
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// as store-sinking that is built into LICM.
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//
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// Note that the simplifycfg pass will clean up blocks which are split out but
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// end up being unneccesary, so usage of this pass does not neccesarily
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// pessimize generated code.
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//
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//===----------------------------------------------------------------------===//
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@ -13,6 +26,7 @@
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#include "llvm/iPHINode.h"
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#include "llvm/Constant.h"
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#include "llvm/Support/CFG.h"
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#include "Support/SetOperations.h"
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#include "Support/Statistic.h"
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namespace {
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@ -24,15 +38,20 @@ namespace {
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// We need loop information to identify the loops...
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorSet>();
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AU.addPreserved<LoopInfo>();
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AU.addPreserved<DominatorSet>();
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AU.addPreserved<ImmediateDominators>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<DominanceFrontier>();
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AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
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}
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private:
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bool ProcessLoop(Loop *L);
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BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
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const std::vector<BasicBlock*> &Preds);
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void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
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void InsertPreheaderForLoop(Loop *L);
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};
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@ -71,12 +90,80 @@ bool Preheaders::ProcessLoop(Loop *L) {
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Changed = true;
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}
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DominatorSet &DS = getAnalysis<DominatorSet>();
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BasicBlock *Header = L->getHeader();
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for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
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if (!DS.dominates(Header, L->getExitBlocks()[i])) {
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RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
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NumInserted++;
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Changed = true;
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}
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const std::vector<Loop*> &SubLoops = L->getSubLoops();
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for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
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Changed |= ProcessLoop(SubLoops[i]);
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return Changed;
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}
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/// SplitBlockPredecessors - Split the specified block into two blocks. We want
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/// to move the predecessors specified in the Preds list to point to the new
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/// block, leaving the remaining predecessors pointing to BB. This method
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/// updates the SSA PHINode's, but no other analyses.
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///
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BasicBlock *Preheaders::SplitBlockPredecessors(BasicBlock *BB,
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const char *Suffix,
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const std::vector<BasicBlock*> &Preds) {
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// Create new basic block, insert right before the original block...
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BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);
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// The preheader first gets an unconditional branch to the loop header...
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BranchInst *BI = new BranchInst(BB);
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NewBB->getInstList().push_back(BI);
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// For every PHI node in the block, insert a PHI node into NewBB where the
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// incoming values from the out of loop edges are moved to NewBB. We have two
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// possible cases here. If the loop is dead, we just insert dummy entries
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// into the PHI nodes for the new edge. If the loop is not dead, we move the
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// incoming edges in BB into new PHI nodes in NewBB.
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//
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if (!Preds.empty()) { // Is the loop not obviously dead?
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for (BasicBlock::iterator I = BB->begin();
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PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
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// Create the new PHI node, insert it into NewBB at the end of the block
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PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
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// Move all of the edges from blocks outside the loop to the new PHI
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for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
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Value *V = PN->removeIncomingValue(Preds[i]);
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NewPHI->addIncoming(V, Preds[i]);
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}
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// Add an incoming value to the PHI node in the loop for the preheader
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// edge
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PN->addIncoming(NewPHI, NewBB);
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}
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// Now that the PHI nodes are updated, actually move the edges from
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// Preds to point to NewBB instead of BB.
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//
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for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
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TerminatorInst *TI = Preds[i]->getTerminator();
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for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
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if (TI->getSuccessor(s) == BB)
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TI->setSuccessor(s, NewBB);
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}
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} else { // Otherwise the loop is dead...
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for (BasicBlock::iterator I = BB->begin();
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PHINode *PN = dyn_cast<PHINode>(&*I); ++I)
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// Insert dummy values as the incoming value...
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PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
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}
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return NewBB;
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}
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/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
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/// preheader, this method is called to insert one. This method has two phases:
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@ -94,55 +181,10 @@ void Preheaders::InsertPreheaderForLoop(Loop *L) {
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assert(OutsideBlocks.size() != 1 && "Loop already has a preheader!");
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// Create new basic block, insert right before the header of the loop...
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BasicBlock *NewBB = new BasicBlock(Header->getName()+".preheader", Header);
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// The preheader first gets an unconditional branch to the loop header...
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BranchInst *BI = new BranchInst(Header);
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NewBB->getInstList().push_back(BI);
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// Split out the loop pre-header
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BasicBlock *NewBB =
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SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
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// For every PHI node in the loop body, insert a PHI node into NewBB where
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// the incoming values from the out of loop edges are moved to NewBB. We
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// have two possible cases here. If the loop is dead, we just insert dummy
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// entries into the PHI nodes for the new edge. If the loop is not dead, we
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// move the incoming edges in Header into new PHI nodes in NewBB.
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//
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if (!OutsideBlocks.empty()) { // Is the loop not obviously dead?
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for (BasicBlock::iterator I = Header->begin();
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PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
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// Create the new PHI node, insert it into NewBB at the end of the block
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PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
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// Move all of the edges from blocks outside the loop to the new PHI
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for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
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Value *V = PN->removeIncomingValue(OutsideBlocks[i]);
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NewPHI->addIncoming(V, OutsideBlocks[i]);
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}
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// Add an incoming value to the PHI node in the loop for the preheader
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// edge
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PN->addIncoming(NewPHI, NewBB);
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}
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// Now that the PHI nodes are updated, actually move the edges from
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// OutsideBlocks to point to NewBB instead of Header.
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//
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for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
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TerminatorInst *TI = OutsideBlocks[i]->getTerminator();
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for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
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if (TI->getSuccessor(s) == Header)
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TI->setSuccessor(s, NewBB);
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}
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} else { // Otherwise the loop is dead...
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for (BasicBlock::iterator I = Header->begin();
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PHINode *PN = dyn_cast<PHINode>(&*I); ++I)
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// Insert dummy values as the incoming value...
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PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
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}
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//===--------------------------------------------------------------------===//
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// Update analysis results now that we have preformed the transformation
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//
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@ -151,20 +193,20 @@ void Preheaders::InsertPreheaderForLoop(Loop *L) {
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if (Loop *Parent = L->getParentLoop())
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Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
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// Update dominator information if it is around...
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if (DominatorSet *DS = getAnalysisToUpdate<DominatorSet>()) {
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DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
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{
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// The blocks that dominate NewBB are the blocks that dominate Header,
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// minus Header, plus NewBB.
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DominatorSet::DomSetType DomSet = DS->getDominators(Header);
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DominatorSet::DomSetType DomSet = DS.getDominators(Header);
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DomSet.insert(NewBB); // We dominate ourself
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DomSet.erase(Header); // Header does not dominate us...
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DS->addBasicBlock(NewBB, DomSet);
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DS.addBasicBlock(NewBB, DomSet);
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// The newly created basic block dominates all nodes dominated by Header.
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for (Function::iterator I = Header->getParent()->begin(),
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E = Header->getParent()->end(); I != E; ++I)
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if (DS->dominates(Header, I))
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DS->addDominator(I, NewBB);
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if (DS.dominates(Header, I))
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DS.addDominator(I, NewBB);
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}
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// Update immediate dominator information if we have it...
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@ -188,4 +230,139 @@ void Preheaders::InsertPreheaderForLoop(Loop *L) {
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// Change the header node so that PNHode is the new immediate dominator
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DT->changeImmediateDominator(HeaderNode, PHNode);
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}
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// Update dominance frontier information...
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if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
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// The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
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// everything that Header does, and it strictly dominates Header in
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// addition.
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assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
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DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
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NewDFSet.erase(Header);
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DF->addBasicBlock(NewBB, NewDFSet);
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// Now we must loop over all of the dominance frontiers in the function,
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// replacing occurances of Header with NewBB in some cases. If a block
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// dominates a (now) predecessor of NewBB, but did not strictly dominate
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// Header, it will have Header in it's DF set, but should now have NewBB in
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// its set.
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for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
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// Get all of the dominators of the predecessor...
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const DominatorSet::DomSetType &PredDoms =
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DS.getDominators(OutsideBlocks[i]);
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for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
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PDE = PredDoms.end(); PDI != PDE; ++PDI) {
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BasicBlock *PredDom = *PDI;
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// If the loop header is in DF(PredDom), then PredDom didn't dominate
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// the header but did dominate a predecessor outside of the loop. Now
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// we change this entry to include the preheader in the DF instead of
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// the header.
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DominanceFrontier::iterator DFI = DF->find(PredDom);
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assert(DFI != DF->end() && "No dominance frontier for node?");
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if (DFI->second.count(Header)) {
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DF->removeFromFrontier(DFI, Header);
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DF->addToFrontier(DFI, NewBB);
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}
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}
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}
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}
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}
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void Preheaders::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
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DominatorSet &DS = getAnalysis<DominatorSet>();
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assert(!DS.dominates(L->getHeader(), Exit) &&
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"Loop already dominates exit block??");
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std::vector<BasicBlock*> LoopBlocks;
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for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
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if (L->contains(*I))
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LoopBlocks.push_back(*I);
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BasicBlock *NewBB =
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SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
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// Update dominator information... The blocks that dominate NewBB are the
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// intersection of the dominators of predecessors, plus the block itself.
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// The newly created basic block does not dominate anything except itself.
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//
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DominatorSet::DomSetType NewBBDomSet = DS.getDominators(LoopBlocks[0]);
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for (unsigned i = 1, e = LoopBlocks.size(); i != e; ++i)
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set_intersect(NewBBDomSet, DS.getDominators(LoopBlocks[i]));
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NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
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DS.addBasicBlock(NewBB, NewBBDomSet);
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// Update immediate dominator information if we have it...
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BasicBlock *NewBBIDom = 0;
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if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
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// This block does not strictly dominate anything, so it is not an immediate
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// dominator. To find the immediate dominator of the new exit node, we
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// trace up the immediate dominators of a predecessor until we find a basic
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// block that dominates the exit block.
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//
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BasicBlock *Dom = LoopBlocks[0]; // Some random predecessor...
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while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
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assert(Dom != 0 && "No shared dominator found???");
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Dom = ID->get(Dom);
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}
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// Set the immediate dominator now...
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ID->addNewBlock(NewBB, Dom);
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NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
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}
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// Update DominatorTree information if it is active.
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if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
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// NewBB doesn't dominate anything, so just create a node and link it into
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// its immediate dominator. If we don't have ImmediateDominator info
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// around, calculate the idom as above.
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DominatorTree::Node *NewBBIDomNode;
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if (NewBBIDom) {
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NewBBIDomNode = DT->getNode(NewBBIDom);
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} else {
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NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
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while (!NewBBDomSet.count(NewBBIDomNode->getNode())) {
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NewBBIDomNode = NewBBIDomNode->getIDom();
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assert(NewBBIDomNode && "No shared dominator found??");
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}
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}
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// Create the new dominator tree node...
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DT->createNewNode(NewBB, NewBBIDomNode);
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}
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// Update dominance frontier information...
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if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
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// DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
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// does dominate itself (and there is an edge (NewBB -> Exit)).
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DominanceFrontier::DomSetType NewDFSet;
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NewDFSet.insert(Exit);
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DF->addBasicBlock(NewBB, NewDFSet);
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// Now we must loop over all of the dominance frontiers in the function,
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// replacing occurances of Exit with NewBB in some cases. If a block
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// dominates a (now) predecessor of NewBB, but did not strictly dominate
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// Exit, it will have Exit in it's DF set, but should now have NewBB in its
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// set.
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for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
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// Get all of the dominators of the predecessor...
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const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
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for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
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PDE = PredDoms.end(); PDI != PDE; ++PDI) {
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BasicBlock *PredDom = *PDI;
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// Make sure to only rewrite blocks that are part of the loop...
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if (L->contains(PredDom)) {
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// If the exit node is in DF(PredDom), then PredDom didn't dominate
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// Exit but did dominate a predecessor inside of the loop. Now we
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// change this entry to include NewBB in the DF instead of Exit.
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DominanceFrontier::iterator DFI = DF->find(PredDom);
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assert(DFI != DF->end() && "No dominance frontier for node?");
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if (DFI->second.count(Exit)) {
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DF->removeFromFrontier(DFI, Exit);
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DF->addToFrontier(DFI, NewBB);
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}
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}
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}
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}
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}
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}
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