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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
// dominator trees.
//
//===----------------------------------------------------------------------===//
#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/Analysis/ProfileInfo.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumBroken, "Number of blocks inserted");
namespace {
struct BreakCriticalEdges : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
BreakCriticalEdges() : FunctionPass(ID) {
initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addPreserved<DominatorTree>();
AU.addPreserved<LoopInfo>();
AU.addPreserved<ProfileInfo>();
// No loop canonicalization guarantees are broken by this pass.
AU.addPreservedID(LoopSimplifyID);
}
};
}
char BreakCriticalEdges::ID = 0;
INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
"Break critical edges in CFG", false, false)
// Publicly exposed interface to pass...
char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
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 && !isa<IndirectBrInst>(TI))
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);
const_pred_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) {
const BasicBlock *P = *I;
if (P != 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(P);
++I;
}
return false;
}
/// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
/// may require new PHIs in the new exit block. This function inserts the
/// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
/// is the new loop exit block, and DestBB is the old loop exit, now the
/// successor of SplitBB.
static void createPHIsForSplitLoopExit(SmallVectorImpl<BasicBlock *> &Preds,
BasicBlock *SplitBB,
BasicBlock *DestBB) {
// SplitBB shouldn't have anything non-trivial in it yet.
assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");
// For each PHI in the destination block.
for (BasicBlock::iterator I = DestBB->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
unsigned Idx = PN->getBasicBlockIndex(SplitBB);
Value *V = PN->getIncomingValue(Idx);
// If the input is a PHI which already satisfies LCSSA, don't create
// a new one.
if (const PHINode *VP = dyn_cast<PHINode>(V))
if (VP->getParent() == SplitBB)
continue;
// Otherwise a new PHI is needed. Create one and populate it.
PHINode *NewPN =
PHINode::Create(PN->getType(), Preds.size(), "split",
SplitBB->isLandingPad() ?
SplitBB->begin() : SplitBB->getTerminator());
for (unsigned i = 0, e = Preds.size(); i != e; ++i)
NewPN->addIncoming(V, Preds[i]);
// Update the original PHI.
PN->setIncomingValue(Idx, NewPN);
}
}
/// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
/// split the critical edge. This will update DominatorTree information if it
/// is available, thus calling this pass will not invalidate either of them.
/// This returns the new block if the edge was split, null otherwise.
///
/// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
/// specified successor will be merged into the same critical edge block.
/// This is most commonly interesting with switch instructions, which may
/// have many edges to any one destination. This ensures that all edges to that
/// dest go to one block instead of each going to a different block, but isn't
/// the standard definition of a "critical edge".
///
/// It is invalid to call this function on a critical edge that starts at an
/// IndirectBrInst. Splitting these edges will almost always create an invalid
/// program because the address of the new block won't be the one that is jumped
/// to.
///
BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
Pass *P, bool MergeIdenticalEdges,
bool DontDeleteUselessPhis,
bool SplitLandingPads) {
if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;
assert(!isa<IndirectBrInst>(TI) &&
"Cannot split critical edge from IndirectBrInst");
BasicBlock *TIBB = TI->getParent();
BasicBlock *DestBB = TI->getSuccessor(SuccNum);
// Splitting the critical edge to a landing pad block is non-trivial. Don't do
// it in this generic function.
if (DestBB->isLandingPad()) return 0;
// Create a new basic block, linking it into the CFG.
BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
// Create our unconditional branch.
BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
NewBI->setDebugLoc(TI->getDebugLoc());
// 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.
{
unsigned BBIdx = 0;
for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++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.
PHINode *PN = cast<PHINode>(I);
// Reuse the previous value of BBIdx if it lines up. In cases where we
// have multiple phi nodes with *lots* of predecessors, this is a speed
// win because we don't have to scan the PHI looking for TIBB. This
// happens because the BB list of PHI nodes are usually in the same
// order.
if (PN->getIncomingBlock(BBIdx) != TIBB)
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, DontDeleteUselessPhis);
// 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 NewBB;
DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
// If we have nothing to update, just return.
if (DT == 0 && LI == 0 && PI == 0)
return NewBB;
// 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;
// If there is a PHI in the block, loop over predecessors with it, which is
// faster than iterating pred_begin/end.
if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingBlock(i) != NewBB)
OtherPreds.push_back(PN->getIncomingBlock(i));
} else {
for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
I != E; ++I) {
BasicBlock *P = *I;
if (P != NewBB)
OtherPreds.push_back(P);
}
}
bool NewBBDominatesDestBB = true;
// Should we update DominatorTree information?
if (DT) {
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);
}
}
}
// Update LoopInfo if it is around.
if (LI) {
if (Loop *TIL = LI->getLoopFor(TIBB)) {
// 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 *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)) {
// Edge from an outer loop to an inner loop. Add to the outer loop.
TIL->addBasicBlockToLoop(NewBB, LI->getBase());
} else if (DestLoop->contains(TIL)) {
// 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());
}
}
// If TIBB is in a loop and DestBB is outside of that loop, split the
// other exit blocks of the loop that also have predecessors outside
// the loop, to maintain a LoopSimplify guarantee.
if (!TIL->contains(DestBB) &&
P->mustPreserveAnalysisID(LoopSimplifyID)) {
assert(!TIL->contains(NewBB) &&
"Split point for loop exit is contained in loop!");
// Update LCSSA form in the newly created exit block.
if (P->mustPreserveAnalysisID(LCSSAID)) {
SmallVector<BasicBlock *, 1> OrigPred;
OrigPred.push_back(TIBB);
createPHIsForSplitLoopExit(OrigPred, NewBB, DestBB);
}
// For each unique exit block...
// FIXME: This code is functionally equivalent to the corresponding
// loop in LoopSimplify.
SmallVector<BasicBlock *, 4> ExitBlocks;
TIL->getExitBlocks(ExitBlocks);
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
// Collect all the preds that are inside the loop, and note
// whether there are any preds outside the loop.
SmallVector<BasicBlock *, 4> Preds;
bool HasPredOutsideOfLoop = false;
BasicBlock *Exit = ExitBlocks[i];
for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
I != E; ++I) {
BasicBlock *P = *I;
if (TIL->contains(P)) {
if (isa<IndirectBrInst>(P->getTerminator())) {
Preds.clear();
break;
}
Preds.push_back(P);
} else {
HasPredOutsideOfLoop = true;
}
}
// If there are any preds not in the loop, we'll need to split
// the edges. The Preds.empty() check is needed because a block
// may appear multiple times in the list. We can't use
// getUniqueExitBlocks above because that depends on LoopSimplify
// form, which we're in the process of restoring!
if (!Preds.empty() && HasPredOutsideOfLoop) {
if (!Exit->isLandingPad()) {
BasicBlock *NewExitBB =
SplitBlockPredecessors(Exit, Preds, "split", P);
if (P->mustPreserveAnalysisID(LCSSAID))
createPHIsForSplitLoopExit(Preds, NewExitBB, Exit);
} else if (SplitLandingPads) {
SmallVector<BasicBlock*, 8> NewBBs;
SplitLandingPadPredecessors(Exit, Preds,
".split1", ".split2",
P, NewBBs);
if (P->mustPreserveAnalysisID(LCSSAID))
createPHIsForSplitLoopExit(Preds, NewBBs[0], Exit);
}
}
}
}
// LCSSA form was updated above for the case where LoopSimplify is
// available, which means that all predecessors of loop exit blocks
// are within the loop. Without LoopSimplify form, it would be
// necessary to insert a new phi.
assert((!P->mustPreserveAnalysisID(LCSSAID) ||
P->mustPreserveAnalysisID(LoopSimplifyID)) &&
"SplitCriticalEdge doesn't know how to update LCCSA form "
"without LoopSimplify!");
}
}
// Update ProfileInfo if it is around.
if (PI)
PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);
return NewBB;
}