1 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
3 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by
4 // inserting a dummy basic block. This pass may be "required" by passes that
5 // cannot deal with critical edges. For this usage, the structure type is
6 // forward declared. This pass obviously invalidates the CFG, but can update
7 // forward dominator (set, immediate dominators, and tree) information.
9 //===----------------------------------------------------------------------===//
11 #include "llvm/Transforms/Scalar.h"
12 #include "llvm/Analysis/Dominators.h"
13 #include "llvm/Function.h"
14 #include "llvm/iTerminators.h"
15 #include "llvm/iPHINode.h"
16 #include "llvm/Support/CFG.h"
17 #include "Support/StatisticReporter.h"
20 Statistic<> NumBroken("break-crit-edges\t- Number of blocks inserted");
22 struct BreakCriticalEdges : public FunctionPass {
23 virtual bool runOnFunction(Function &F);
25 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
26 AU.addPreserved<DominatorSet>();
27 AU.addPreserved<ImmediateDominators>();
28 AU.addPreserved<DominatorTree>();
32 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
33 "Break critical edges in CFG");
36 // Publically exposed interface to pass...
37 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
38 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
41 // isCriticalEdge - Return true if the specified edge is a critical edge.
42 // Critical edges are edges from a block with multiple successors to a block
43 // with multiple predecessors.
45 static bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
46 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
47 assert (TI->getNumSuccessors() > 1);
49 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
50 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
52 // If there is more than one predecessor, this is a critical edge...
53 assert(I != E && "No preds, but we have an edge to the block?");
54 ++I; // Skip one edge due to the incoming arc from TI.
58 // SplitCriticalEdge - Insert a new node node to split the critical edge. This
59 // will update DominatorSet, ImmediateDominator and DominatorTree information if
60 // it is available, thus calling this pass will not invalidate either of them.
62 static void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
63 assert(isCriticalEdge(TI, SuccNum) &&
64 "Cannot break a critical edge, if it isn't a critical edge");
65 BasicBlock *TIBB = TI->getParent();
67 // Create a new basic block, linking it into the CFG.
68 BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
69 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
70 // Create our unconditional branch...
71 BranchInst *BI = new BranchInst(DestBB);
72 NewBB->getInstList().push_back(BI);
74 // Branch to the new block, breaking the edge...
75 TI->setSuccessor(SuccNum, NewBB);
77 // Insert the block into the function... right after the block TI lives in.
78 Function &F = *TIBB->getParent();
79 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
81 // If there are any PHI nodes in DestBB, we need to update them so that they
82 // merge incoming values from NewBB instead of from TIBB.
84 for (BasicBlock::iterator I = DestBB->begin();
85 PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
86 // We no longer enter through TIBB, now we come in through NewBB.
87 PN->replaceUsesOfWith(TIBB, NewBB);
90 // Now if we have a pass object, update analysis information. Currently we
91 // update DominatorSet and DominatorTree information if it's available.
94 // Should we update DominatorSet information?
95 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
96 // The blocks that dominate the new one are the blocks that dominate TIBB
97 // plus the new block itself.
98 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
99 DomSet.insert(NewBB); // A block always dominates itself.
100 DS->addBasicBlock(NewBB, DomSet);
103 // Should we update ImmdediateDominator information?
104 if (ImmediateDominators *ID =
105 P->getAnalysisToUpdate<ImmediateDominators>()) {
106 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
108 ID->addNewBlock(NewBB, TIBB);
111 // Should we update DominatorTree information?
112 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
113 DominatorTree::Node *TINode = DT->getNode(TIBB);
115 // The new block is not the immediate dominator for any other nodes, but
116 // TINode is the immediate dominator for the new node.
118 DT->createNewNode(NewBB, TINode);
123 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
124 // edges as they are found.
126 bool BreakCriticalEdges::runOnFunction(Function &F) {
127 bool Changed = false;
128 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
129 TerminatorInst *TI = I->getTerminator();
130 if (TI->getNumSuccessors() > 1)
131 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
132 if (isCriticalEdge(TI, i)) {
133 SplitCriticalEdge(TI, i, this);