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e408e25132
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5872 91177308-0d34-0410-b5e6-96231b3b80d8
156 lines
5.9 KiB
C++
156 lines
5.9 KiB
C++
//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
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//
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// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
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// inserting a dummy basic block. This pass may be "required" by passes that
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// cannot deal with critical edges. For this usage, the structure type is
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// forward declared. This pass obviously invalidates the CFG, but can update
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// forward dominator (set, immediate dominators, and tree) information.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Function.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Support/CFG.h"
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#include "Support/Statistic.h"
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namespace {
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Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
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struct BreakCriticalEdges : public FunctionPass {
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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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(LoopPreheadersID); // No preheaders deleted.
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}
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};
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RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
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"Break critical edges in CFG");
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}
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// Publically exposed interface to pass...
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const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
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Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
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// isCriticalEdge - Return true if the specified edge is a critical edge.
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// Critical edges are edges from a block with multiple successors to a block
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// with multiple predecessors.
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//
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bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
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assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
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if (TI->getNumSuccessors() == 1) return false;
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const BasicBlock *Dest = TI->getSuccessor(SuccNum);
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pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
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// If there is more than one predecessor, this is a critical edge...
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assert(I != E && "No preds, but we have an edge to the block?");
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++I; // Skip one edge due to the incoming arc from TI.
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return I != E;
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}
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// SplitCriticalEdge - Insert a new node node to split the critical edge. This
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// will update DominatorSet, ImmediateDominator and DominatorTree information if
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// it is available, thus calling this pass will not invalidate either of them.
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//
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void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
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assert(isCriticalEdge(TI, SuccNum) &&
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"Cannot break a critical edge, if it isn't a critical edge");
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BasicBlock *TIBB = TI->getParent();
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BasicBlock *DestBB = TI->getSuccessor(SuccNum);
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// Create a new basic block, linking it into the CFG.
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BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
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DestBB->getName() + "_crit_edge");
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// Create our unconditional branch...
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BranchInst *BI = new BranchInst(DestBB);
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NewBB->getInstList().push_back(BI);
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// Branch to the new block, breaking the edge...
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TI->setSuccessor(SuccNum, NewBB);
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// Insert the block into the function... right after the block TI lives in.
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Function &F = *TIBB->getParent();
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F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
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// If there are any PHI nodes in DestBB, we need to update them so that they
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// merge incoming values from NewBB instead of from TIBB.
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//
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for (BasicBlock::iterator I = DestBB->begin();
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PHINode *PN = dyn_cast<PHINode>(I); ++I) {
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// We no longer enter through TIBB, now we come in through NewBB.
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PN->replaceUsesOfWith(TIBB, NewBB);
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}
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// If we don't have a pass object, we can't update anything...
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if (P == 0) return;
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// Now update analysis information. These are the analyses that we are
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// currently capable of updating...
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//
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// Should we update DominatorSet information?
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if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
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// The blocks that dominate the new one are the blocks that dominate TIBB
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// plus the new block itself.
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DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
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DomSet.insert(NewBB); // A block always dominates itself.
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DS->addBasicBlock(NewBB, DomSet);
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}
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// Should we update ImmdediateDominator information?
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if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
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// TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
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// anything.
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ID->addNewBlock(NewBB, TIBB);
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}
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// Should we update DominatorTree information?
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if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
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DominatorTree::Node *TINode = DT->getNode(TIBB);
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// The new block is not the immediate dominator for any other nodes, but
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// TINode is the immediate dominator for the new node.
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//
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if (TINode) // Don't break unreachable code!
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DT->createNewNode(NewBB, TINode);
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}
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// Should we update DominanceFrontier information?
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if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
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// Since the new block is dominated by its only predecessor TIBB,
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// it cannot be in any block's dominance frontier. Its dominance
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// frontier is {DestBB}.
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DominanceFrontier::DomSetType NewDFSet;
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NewDFSet.insert(DestBB);
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DF->addBasicBlock(NewBB, NewDFSet);
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}
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}
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// runOnFunction - Loop over all of the edges in the CFG, breaking critical
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// edges as they are found.
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//
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bool BreakCriticalEdges::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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TerminatorInst *TI = I->getTerminator();
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if (TI->getNumSuccessors() > 1)
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
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if (isCriticalEdge(TI, i)) {
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SplitCriticalEdge(TI, i, this);
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++NumBroken;
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Changed = true;
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}
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}
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return Changed;
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}
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