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+//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
+// inserting a dummy basic block. This pass may be "required" by passes that
+// cannot deal with critical edges. For this usage, the structure type is
+// forward declared. This pass obviously invalidates the CFG, but can update
+// forward dominator (set, immediate dominators, tree, and frontier)
+// information.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "break-crit-edges"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Type.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
+
+STATISTIC(NumBroken, "Number of blocks inserted");
+
+namespace {
+ struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ BreakCriticalEdges() : FunctionPass(&ID) {}
+
+ virtual bool runOnFunction(Function &F);
+
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addPreserved<DominatorTree>();
+ AU.addPreserved<DominanceFrontier>();
+ AU.addPreserved<LoopInfo>();
+
+ // No loop canonicalization guarantees are broken by this pass.
+ AU.addPreservedID(LoopSimplifyID);
+ }
+ };
+}
+
+char BreakCriticalEdges::ID = 0;
+static RegisterPass<BreakCriticalEdges>
+X("break-crit-edges", "Break critical edges in CFG");
+
+// Publically exposed interface to pass...
+const PassInfo *const llvm::BreakCriticalEdgesID = &X;
+FunctionPass *llvm::createBreakCriticalEdgesPass() {
+ return new BreakCriticalEdges();
+}
+
+// runOnFunction - Loop over all of the edges in the CFG, breaking critical
+// edges as they are found.
+//
+bool BreakCriticalEdges::runOnFunction(Function &F) {
+ bool Changed = false;
+ for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+ TerminatorInst *TI = I->getTerminator();
+ if (TI->getNumSuccessors() > 1)
+ for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
+ if (SplitCriticalEdge(TI, i, this)) {
+ ++NumBroken;
+ Changed = true;
+ }
+ }
+
+ return Changed;
+}
+
+//===----------------------------------------------------------------------===//
+// Implementation of the external critical edge manipulation functions
+//===----------------------------------------------------------------------===//
+
+// isCriticalEdge - Return true if the specified edge is a critical edge.
+// Critical edges are edges from a block with multiple successors to a block
+// with multiple predecessors.
+//
+bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
+ bool AllowIdenticalEdges) {
+ assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
+ if (TI->getNumSuccessors() == 1) return false;
+
+ const BasicBlock *Dest = TI->getSuccessor(SuccNum);
+ pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
+
+ // If there is more than one predecessor, this is a critical edge...
+ assert(I != E && "No preds, but we have an edge to the block?");
+ const BasicBlock *FirstPred = *I;
+ ++I; // Skip one edge due to the incoming arc from TI.
+ if (!AllowIdenticalEdges)
+ return I != E;
+
+ // If AllowIdenticalEdges is true, then we allow this edge to be considered
+ // non-critical iff all preds come from TI's block.
+ while (I != E) {
+ if (*I != FirstPred)
+ return true;
+ // Note: leave this as is until no one ever compiles with either gcc 4.0.1
+ // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
+ E = pred_end(*I);
+ ++I;
+ }
+ return false;
+}
+
+/// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
+/// split the critical edge. This will update DominatorTree and
+/// DominatorFrontier information if it is available, thus calling this pass
+/// will not invalidate any of them. This returns true if the edge was split,
+/// false otherwise. This ensures that all edges to that dest go to one block
+/// instead of each going to a different block.
+//
+bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
+ bool MergeIdenticalEdges) {
+ if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
+ BasicBlock *TIBB = TI->getParent();
+ BasicBlock *DestBB = TI->getSuccessor(SuccNum);
+
+ // Create a new basic block, linking it into the CFG.
+ BasicBlock *NewBB = BasicBlock::Create(TIBB->getName() + "." +
+ DestBB->getName() + "_crit_edge");
+ // Create our unconditional branch...
+ BranchInst::Create(DestBB, NewBB);
+
+ // Branch to the new block, breaking the edge.
+ TI->setSuccessor(SuccNum, NewBB);
+
+ // Insert the block into the function... right after the block TI lives in.
+ Function &F = *TIBB->getParent();
+ Function::iterator FBBI = TIBB;
+ F.getBasicBlockList().insert(++FBBI, NewBB);
+
+ // If there are any PHI nodes in DestBB, we need to update them so that they
+ // merge incoming values from NewBB instead of from TIBB.
+ //
+ for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
+ PHINode *PN = cast<PHINode>(I);
+ // We no longer enter through TIBB, now we come in through NewBB. Revector
+ // exactly one entry in the PHI node that used to come from TIBB to come
+ // from NewBB.
+ int BBIdx = PN->getBasicBlockIndex(TIBB);
+ PN->setIncomingBlock(BBIdx, NewBB);
+ }
+
+ // If there are any other edges from TIBB to DestBB, update those to go
+ // through the split block, making those edges non-critical as well (and
+ // reducing the number of phi entries in the DestBB if relevant).
+ if (MergeIdenticalEdges) {
+ for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
+ if (TI->getSuccessor(i) != DestBB) continue;
+
+ // Remove an entry for TIBB from DestBB phi nodes.
+ DestBB->removePredecessor(TIBB);
+
+ // We found another edge to DestBB, go to NewBB instead.
+ TI->setSuccessor(i, NewBB);
+ }
+ }
+
+
+
+ // If we don't have a pass object, we can't update anything...
+ if (P == 0) return true;
+
+ // Now update analysis information. Since the only predecessor of NewBB is
+ // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
+ // anything, as there are other successors of DestBB. However, if all other
+ // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
+ // loop header) then NewBB dominates DestBB.
+ SmallVector<BasicBlock*, 8> OtherPreds;
+
+ for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
+ if (*I != NewBB)
+ OtherPreds.push_back(*I);
+
+ bool NewBBDominatesDestBB = true;
+
+ // Should we update DominatorTree information?
+ if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
+ DomTreeNode *TINode = DT->getNode(TIBB);
+
+ // The new block is not the immediate dominator for any other nodes, but
+ // TINode is the immediate dominator for the new node.
+ //
+ if (TINode) { // Don't break unreachable code!
+ DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
+ DomTreeNode *DestBBNode = 0;
+
+ // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
+ if (!OtherPreds.empty()) {
+ DestBBNode = DT->getNode(DestBB);
+ while (!OtherPreds.empty() && NewBBDominatesDestBB) {
+ if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
+ NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
+ OtherPreds.pop_back();
+ }
+ OtherPreds.clear();
+ }
+
+ // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
+ // doesn't dominate anything.
+ if (NewBBDominatesDestBB) {
+ if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
+ DT->changeImmediateDominator(DestBBNode, NewBBNode);
+ }
+ }
+ }
+
+ // Should we update DominanceFrontier information?
+ if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
+ // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
+ if (!OtherPreds.empty()) {
+ // FIXME: IMPLEMENT THIS!
+ assert(0 && "Requiring domfrontiers but not idom/domtree/domset."
+ " not implemented yet!");
+ }
+
+ // Since the new block is dominated by its only predecessor TIBB,
+ // it cannot be in any block's dominance frontier. If NewBB dominates
+ // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
+ // just {DestBB}.
+ DominanceFrontier::DomSetType NewDFSet;
+ if (NewBBDominatesDestBB) {
+ DominanceFrontier::iterator I = DF->find(DestBB);
+ if (I != DF->end()) {
+ DF->addBasicBlock(NewBB, I->second);
+
+ if (I->second.count(DestBB)) {
+ // However NewBB's frontier does not include DestBB.
+ DominanceFrontier::iterator NF = DF->find(NewBB);
+ DF->removeFromFrontier(NF, DestBB);
+ }
+ }
+ else
+ DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
+ } else {
+ DominanceFrontier::DomSetType NewDFSet;
+ NewDFSet.insert(DestBB);
+ DF->addBasicBlock(NewBB, NewDFSet);
+ }
+ }
+
+ // Update LoopInfo if it is around.
+ if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
+ // If one or the other blocks were not in a loop, the new block is not
+ // either, and thus LI doesn't need to be updated.
+ if (Loop *TIL = LI->getLoopFor(TIBB))
+ if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
+ if (TIL == DestLoop) {
+ // Both in the same loop, the NewBB joins loop.
+ DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+ } else if (TIL->contains(DestLoop->getHeader())) {
+ // Edge from an outer loop to an inner loop. Add to the outer loop.
+ TIL->addBasicBlockToLoop(NewBB, LI->getBase());
+ } else if (DestLoop->contains(TIL->getHeader())) {
+ // Edge from an inner loop to an outer loop. Add to the outer loop.
+ DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+ } else {
+ // Edge from two loops with no containment relation. Because these
+ // are natural loops, we know that the destination block must be the
+ // header of its loop (adding a branch into a loop elsewhere would
+ // create an irreducible loop).
+ assert(DestLoop->getHeader() == DestBB &&
+ "Should not create irreducible loops!");
+ if (Loop *P = DestLoop->getParentLoop())
+ P->addBasicBlockToLoop(NewBB, LI->getBase());
+ }
+ }
+ }
+ return true;
+}
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