// 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.
+// 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/Function.h"
-#include "llvm/Instructions.h"
-#include "llvm/Type.h"
-#include "llvm/Support/CFG.h"
-#include "llvm/Support/Compiler.h"
-#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CFG.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Type.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
+#define DEBUG_TYPE "break-crit-edges"
+
STATISTIC(NumBroken, "Number of blocks inserted");
namespace {
- struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
+ struct BreakCriticalEdges : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
- BreakCriticalEdges() : FunctionPass(&ID) {}
+ BreakCriticalEdges() : FunctionPass(ID) {
+ initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
+ }
- virtual bool runOnFunction(Function &F);
+ bool runOnFunction(Function &F) override {
+ auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
+ auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
+ auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
+ auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
+ unsigned N =
+ SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
+ NumBroken += N;
+ return N > 0;
+ }
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addPreserved<DominatorTree>();
- AU.addPreserved<DominanceFrontier>();
- AU.addPreserved<LoopInfo>();
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addPreserved<DominatorTreeWrapperPass>();
+ AU.addPreserved<LoopInfoWrapperPass>();
// 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");
+INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
+ "Break critical edges in CFG", false, false)
-// Publically exposed interface to pass...
-const PassInfo *const llvm::BreakCriticalEdgesID = &X;
+// 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)
- 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;
+/// 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(ArrayRef<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->front() : 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);
}
- 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;
+/// 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,
+ const CriticalEdgeSplittingOptions &Options) {
+ if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
+ return nullptr;
+
+ 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 pad block is non-trivial. Don't do
+ // it in this generic function.
+ if (DestBB->isEHPad()) return nullptr;
+
// 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);
+ 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;
+ Function::iterator FBBI = TIBB->getIterator();
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);
+ {
+ 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) {
+ if (Options.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);
-
+ DestBB->removePredecessor(TIBB, Options.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 true;
+ // If we have nothing to update, just return.
+ auto *DT = Options.DT;
+ auto *LI = Options.LI;
+ if (!DT && !LI)
+ return NewBB;
// Now update analysis information. Since the only predecessor of NewBB is
// the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
// 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);
-
+ // 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 (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
+ if (DT) {
DomTreeNode *TINode = DT->getNode(TIBB);
// The new block is not the immediate dominator for any other nodes, but
//
if (TINode) { // Don't break unreachable code!
DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
- DomTreeNode *DestBBNode = 0;
-
+ DomTreeNode *DestBBNode = nullptr;
+
// If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
if (!OtherPreds.empty()) {
DestBBNode = DT->getNode(DestBB);
}
OtherPreds.clear();
}
-
+
// If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
// doesn't dominate anything.
if (NewBBDominatesDestBB) {
}
}
- // 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!
- LLVM_UNREACHABLE("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 (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->getHeader())) {
+ DestLoop->addBasicBlockToLoop(NewBB, *LI);
+ } 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->getHeader())) {
+ TIL->addBasicBlockToLoop(NewBB, *LI);
+ } else if (DestLoop->contains(TIL)) {
// Edge from an inner loop to an outer loop. Add to the outer loop.
- DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+ DestLoop->addBasicBlockToLoop(NewBB, *LI);
} else {
// Edge from two loops with no containment relation. Because these
// are natural loops, we know that the destination block must be the
assert(DestLoop->getHeader() == DestBB &&
"Should not create irreducible loops!");
if (Loop *P = DestLoop->getParentLoop())
- P->addBasicBlockToLoop(NewBB, LI->getBase());
+ P->addBasicBlockToLoop(NewBB, *LI);
}
}
+
+ // If TIBB is in a loop and DestBB is outside of that loop, we may need
+ // to update LoopSimplify form and LCSSA form.
+ if (!TIL->contains(DestBB)) {
+ assert(!TIL->contains(NewBB) &&
+ "Split point for loop exit is contained in loop!");
+
+ // Update LCSSA form in the newly created exit block.
+ if (Options.PreserveLCSSA) {
+ createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);
+ }
+
+ // The only that we can break LoopSimplify form by splitting a critical
+ // edge is if after the split there exists some edge from TIL to DestBB
+ // *and* the only edge into DestBB from outside of TIL is that of
+ // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB
+ // is the new exit block and it has no non-loop predecessors. If the
+ // second isn't true, then DestBB was not in LoopSimplify form prior to
+ // the split as it had a non-loop predecessor. In both of these cases,
+ // the predecessor must be directly in TIL, not in a subloop, or again
+ // LoopSimplify doesn't hold.
+ SmallVector<BasicBlock *, 4> LoopPreds;
+ for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E;
+ ++I) {
+ BasicBlock *P = *I;
+ if (P == NewBB)
+ continue; // The new block is known.
+ if (LI->getLoopFor(P) != TIL) {
+ // No need to re-simplify, it wasn't to start with.
+ LoopPreds.clear();
+ break;
+ }
+ LoopPreds.push_back(P);
+ }
+ if (!LoopPreds.empty()) {
+ assert(!DestBB->isEHPad() && "We don't split edges to EH pads!");
+ BasicBlock *NewExitBB = SplitBlockPredecessors(
+ DestBB, LoopPreds, "split", DT, LI, Options.PreserveLCSSA);
+ if (Options.PreserveLCSSA)
+ createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB);
+ }
+ }
+ }
}
- return true;
+
+ return NewBB;
}
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