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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29925 91177308-0d34-0410-b5e6-96231b3b80d8
213 lines
8.3 KiB
C++
213 lines
8.3 KiB
C++
//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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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, tree, and frontier)
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// information.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Type.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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namespace {
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Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
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struct VISIBILITY_HIDDEN 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<ETForest>();
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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.addPreserved<LoopInfo>();
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// No loop canonicalization guarantees are broken by this pass.
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AU.addPreservedID(LoopSimplifyID);
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}
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};
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RegisterPass<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 *llvm::BreakCriticalEdgesID = X.getPassInfo();
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FunctionPass *llvm::createBreakCriticalEdgesPass() {
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return new BreakCriticalEdges();
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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 (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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//===----------------------------------------------------------------------===//
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// Implementation of the external critical edge manipulation functions
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//===----------------------------------------------------------------------===//
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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 llvm::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 - If this edge is a critical edge, insert a new node to
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// split the critical edge. This will update DominatorSet, ImmediateDominator,
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// DominatorTree, and DominatorFrontier information if it is available, thus
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// calling this pass will not invalidate either of them. This returns true if
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// the edge was split, false otherwise.
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//
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bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
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if (!isCriticalEdge(TI, SuccNum)) return false;
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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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new BranchInst(DestBB, NewBB);
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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(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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// We no longer enter through TIBB, now we come in through NewBB. Revector
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// exactly one entry in the PHI node that used to come from TIBB to come
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// from NewBB.
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int BBIdx = PN->getBasicBlockIndex(TIBB);
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PN->setIncomingBlock(BBIdx, 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 true;
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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 ImmediateDominator 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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// Update the forest?
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if (ETForest *EF = P->getAnalysisToUpdate<ETForest>())
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EF->addNewBlock(NewBB, TIBB);
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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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// Update LoopInfo if it is around.
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if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
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// If one or the other blocks were not in a loop, the new block is not
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// either, and thus LI doesn't need to be updated.
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if (Loop *TIL = LI->getLoopFor(TIBB))
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if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
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if (TIL == DestLoop) {
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// Both in the same loop, the NewBB joins loop.
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DestLoop->addBasicBlockToLoop(NewBB, *LI);
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} else if (TIL->contains(DestLoop->getHeader())) {
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// Edge from an outer loop to an inner loop. Add to the outer lopo.
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TIL->addBasicBlockToLoop(NewBB, *LI);
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} else if (DestLoop->contains(TIL->getHeader())) {
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// Edge from an inner loop to an outer loop. Add to the outer lopo.
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DestLoop->addBasicBlockToLoop(NewBB, *LI);
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} else {
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// Edge from two loops with no containment relation. Because these
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// are natural loops, we know that the destination block must be the
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// header of its loop (adding a branch into a loop elsewhere would
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// create an irreducible loop).
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assert(DestLoop->getHeader() == DestBB &&
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"Should not create irreducible loops!");
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if (Loop *P = DestLoop->getParentLoop())
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P->addBasicBlockToLoop(NewBB, *LI);
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}
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}
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}
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return true;
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}
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