Transforms/Utils/BreakCriticalEdges.cpp

//===- 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.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/BreakCriticalEdges.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/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;

#define DEBUG_TYPE "break-crit-edges"

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());
    }

    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;
    }

    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;
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();
}

PreservedAnalyses BreakCriticalEdgesPass::run(Function &F,
                                              FunctionAnalysisManager &AM) {
  auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
  auto *LI = AM.getCachedResult<LoopAnalysis>(F);
  unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
  NumBroken += N;
  if (N == 0)
    return PreservedAnalyses::all();
  PreservedAnalyses PA;
  PA.preserve<DominatorTreeAnalysis>();
  PA.preserve<LoopAnalysis>();
  return PA;
}

//===----------------------------------------------------------------------===//
//    Implementation of the external critical edge manipulation functions
//===----------------------------------------------------------------------===//

/// 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);
  }
}

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(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->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.
  {
    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 (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, Options.DontDeleteUselessPHIs);

      // We found another edge to DestBB, go to NewBB instead.
      TI->setSuccessor(i, NewBB);
    }
  }

  // If we have nothing to update, just return.
  auto *DT = Options.DT;
  auto *LI = Options.LI;
  if (!DT && !LI)
    return NewBB;

  if (DT) {
    // Update the DominatorTree.
    //       ---> NewBB -----\
    //      /                 V
    //  TIBB -------\\------> DestBB
    //
    // First, inform the DT about the new path from TIBB to DestBB via NewBB,
    // then delete the old edge from TIBB to DestBB. By doing this in that order
    // DestBB stays reachable in the DT the whole time and its subtree doesn't
    // get disconnected.
    SmallVector<DominatorTree::UpdateType, 3> Updates;
    Updates.push_back({DominatorTree::Insert, TIBB, NewBB});
    Updates.push_back({DominatorTree::Insert, NewBB, DestBB});
    if (llvm::find(successors(TIBB), DestBB) == succ_end(TIBB))
      Updates.push_back({DominatorTree::Delete, TIBB, DestBB});

    DT->applyUpdates(Updates);
  }

  // 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);
        } else if (TIL->contains(DestLoop)) {
          // Edge from an outer loop to an inner loop.  Add to the outer loop.
          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);
        } 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);
        }
      }

      // 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 NewBB;
}