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/Statistic.h"
20 Statistic<> NumBroken("break-crit-edges", "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>();
29 AU.addPreservedID(LoopPreheadersID); // No preheaders deleted.
33 RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
34 "Break critical edges in CFG");
37 // Publically exposed interface to pass...
38 const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
39 Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }
42 // isCriticalEdge - Return true if the specified edge is a critical edge.
43 // Critical edges are edges from a block with multiple successors to a block
44 // with multiple predecessors.
46 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
47 assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
48 if (TI->getNumSuccessors() == 1) return false;
50 const BasicBlock *Dest = TI->getSuccessor(SuccNum);
51 pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
53 // If there is more than one predecessor, this is a critical edge...
54 assert(I != E && "No preds, but we have an edge to the block?");
55 ++I; // Skip one edge due to the incoming arc from TI.
59 // SplitCriticalEdge - Insert a new node node to split the critical edge. This
60 // will update DominatorSet, ImmediateDominator and DominatorTree information if
61 // it is available, thus calling this pass will not invalidate either of them.
63 void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
64 assert(isCriticalEdge(TI, SuccNum) &&
65 "Cannot break a critical edge, if it isn't a critical edge");
66 BasicBlock *TIBB = TI->getParent();
68 // Create a new basic block, linking it into the CFG.
69 BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
70 BasicBlock *DestBB = TI->getSuccessor(SuccNum);
71 // Create our unconditional branch...
72 BranchInst *BI = new BranchInst(DestBB);
73 NewBB->getInstList().push_back(BI);
75 // Branch to the new block, breaking the edge...
76 TI->setSuccessor(SuccNum, NewBB);
78 // Insert the block into the function... right after the block TI lives in.
79 Function &F = *TIBB->getParent();
80 F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
82 // If there are any PHI nodes in DestBB, we need to update them so that they
83 // merge incoming values from NewBB instead of from TIBB.
85 for (BasicBlock::iterator I = DestBB->begin();
86 PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
87 // We no longer enter through TIBB, now we come in through NewBB.
88 PN->replaceUsesOfWith(TIBB, NewBB);
91 // If we don't have a pass object, we can't update anything...
94 // Now update analysis information. These are the analyses that we are
95 // currently capable of updating...
98 // Should we update DominatorSet information?
99 if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
100 // The blocks that dominate the new one are the blocks that dominate TIBB
101 // plus the new block itself.
102 DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
103 DomSet.insert(NewBB); // A block always dominates itself.
104 DS->addBasicBlock(NewBB, DomSet);
107 // Should we update ImmdediateDominator information?
108 if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
109 // TIBB is the new immediate dominator for NewBB. NewBB doesn't dominate
111 ID->addNewBlock(NewBB, TIBB);
114 // Should we update DominatorTree information?
115 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
116 DominatorTree::Node *TINode = DT->getNode(TIBB);
118 // The new block is not the immediate dominator for any other nodes, but
119 // TINode is the immediate dominator for the new node.
121 if (TINode) // Don't break unreachable code!
122 DT->createNewNode(NewBB, TINode);
126 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
127 // edges as they are found.
129 bool BreakCriticalEdges::runOnFunction(Function &F) {
130 bool Changed = false;
131 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
132 TerminatorInst *TI = I->getTerminator();
133 if (TI->getNumSuccessors() > 1)
134 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
135 if (isCriticalEdge(TI, i)) {
136 SplitCriticalEdge(TI, i, this);