llvm/lib/Transforms/Utils/SSAUpdaterBulk.cpp

//===- SSAUpdaterBulk.cpp - Unstructured SSA Update Tool ------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the SSAUpdaterBulk class.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/SSAUpdaterBulk.h"
#include "llvm/Analysis/IteratedDominanceFrontier.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"

using namespace llvm;

#define DEBUG_TYPE "ssaupdaterbulk"

/// Add a new variable to the SSA rewriter. This needs to be called before
/// AddAvailableValue or AddUse calls.
void SSAUpdaterBulk::AddVariable(unsigned Var, StringRef Name, Type *Ty) {
  assert(Rewrites.find(Var) == Rewrites.end() && "Variable added twice!");
  RewriteInfo RI(Name, Ty);
  Rewrites[Var] = RI;
}

/// Indicate that a rewritten value is available in the specified block with the
/// specified value.
void SSAUpdaterBulk::AddAvailableValue(unsigned Var, BasicBlock *BB, Value *V) {
  assert(Rewrites.find(Var) != Rewrites.end() && "Should add variable first!");
  Rewrites[Var].Defines[BB] = V;
}

/// Record a use of the symbolic value. This use will be updated with a
/// rewritten value when RewriteAllUses is called.
void SSAUpdaterBulk::AddUse(unsigned Var, Use *U) {
  assert(Rewrites.find(Var) != Rewrites.end() && "Should add variable first!");
  Rewrites[Var].Uses.insert(U);
}

/// Return true if the SSAUpdater already has a value for the specified variable
/// in the specified block.
bool SSAUpdaterBulk::HasValueForBlock(unsigned Var, BasicBlock *BB) {
  return Rewrites.count(Var) ? Rewrites[Var].Defines.count(BB) : false;
}

// Compute value at the given block BB. We either should already know it, or we
// should be able to recursively reach it going up dominator tree.
Value *SSAUpdaterBulk::computeValueAt(BasicBlock *BB, RewriteInfo &R,
                                      DominatorTree *DT) {
  if (!R.Defines.count(BB)) {
    if (DT->isReachableFromEntry(BB) && PredCache.get(BB).size()) {
      BasicBlock *IDom = DT->getNode(BB)->getIDom()->getBlock();
      Value *V = computeValueAt(IDom, R, DT);
      R.Defines[BB] = V;
    } else
      R.Defines[BB] = UndefValue::get(R.Ty);
  }
  return R.Defines[BB];
}

/// Given sets of UsingBlocks and DefBlocks, compute the set of LiveInBlocks.
/// This is basically a subgraph limited by DefBlocks and UsingBlocks.
static void
ComputeLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &UsingBlocks,
                    const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
                    SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) {
  // To determine liveness, we must iterate through the predecessors of blocks
  // where the def is live.  Blocks are added to the worklist if we need to
  // check their predecessors.  Start with all the using blocks.
  SmallVector<BasicBlock *, 64> LiveInBlockWorklist(UsingBlocks.begin(),
                                                    UsingBlocks.end());

  // Now that we have a set of blocks where the phi is live-in, recursively add
  // their predecessors until we find the full region the value is live.
  while (!LiveInBlockWorklist.empty()) {
    BasicBlock *BB = LiveInBlockWorklist.pop_back_val();

    // The block really is live in here, insert it into the set.  If already in
    // the set, then it has already been processed.
    if (!LiveInBlocks.insert(BB).second)
      continue;

    // Since the value is live into BB, it is either defined in a predecessor or
    // live into it to.  Add the preds to the worklist unless they are a
    // defining block.
    for (BasicBlock *P : predecessors(BB)) {
      // The value is not live into a predecessor if it defines the value.
      if (DefBlocks.count(P))
        continue;

      // Otherwise it is, add to the worklist.
      LiveInBlockWorklist.push_back(P);
    }
  }
}

/// Helper function for finding a block which should have a value for the given
/// user. For PHI-nodes this block is the corresponding predecessor, for other
/// instructions it's their parent block.
static BasicBlock *getUserBB(Use *U) {
  auto *User = cast<Instruction>(U->getUser());

  if (auto *UserPN = dyn_cast<PHINode>(User))
    return UserPN->getIncomingBlock(*U);
  else
    return User->getParent();
}

/// Perform all the necessary updates, including new PHI-nodes insertion and the
/// requested uses update.
void SSAUpdaterBulk::RewriteAllUses(DominatorTree *DT,
                                    SmallVectorImpl<PHINode *> *InsertedPHIs) {
  for (auto P : Rewrites) {
    // Compute locations for new phi-nodes.
    // For that we need to initialize DefBlocks from definitions in R.Defines,
    // UsingBlocks from uses in R.Uses, then compute LiveInBlocks, and then use
    // this set for computing iterated dominance frontier (IDF).
    // The IDF blocks are the blocks where we need to insert new phi-nodes.
    ForwardIDFCalculator IDF(*DT);
    RewriteInfo &R = P.second;
    SmallPtrSet<BasicBlock *, 2> DefBlocks;
    for (auto Def : R.Defines)
      DefBlocks.insert(Def.first);
    IDF.setDefiningBlocks(DefBlocks);

    SmallPtrSet<BasicBlock *, 2> UsingBlocks;
    for (auto U : R.Uses)
      UsingBlocks.insert(getUserBB(U));

    SmallVector<BasicBlock *, 32> IDFBlocks;
    SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
    ComputeLiveInBlocks(UsingBlocks, DefBlocks, LiveInBlocks);
    IDF.resetLiveInBlocks();
    IDF.setLiveInBlocks(LiveInBlocks);
    IDF.calculate(IDFBlocks);

    // We've computed IDF, now insert new phi-nodes there.
    SmallVector<PHINode *, 4> InsertedPHIsForVar;
    for (auto FrontierBB : IDFBlocks) {
      IRBuilder<> B(FrontierBB, FrontierBB->begin());
      PHINode *PN = B.CreatePHI(R.Ty, 0, R.Name);
      R.Defines[FrontierBB] = PN;
      InsertedPHIsForVar.push_back(PN);
      if (InsertedPHIs)
        InsertedPHIs->push_back(PN);
    }

    // Fill in arguments of the inserted PHIs.
    for (auto PN : InsertedPHIsForVar) {
      BasicBlock *PBB = PN->getParent();
      for (BasicBlock *Pred : PredCache.get(PBB))
        PN->addIncoming(computeValueAt(Pred, R, DT), Pred);
    }

    // Rewrite actual uses with the inserted definitions.
    for (auto U : R.Uses) {
      Value *V = computeValueAt(getUserBB(U), R, DT);
      Value *OldVal = U->get();
      // Notify that users of the existing value that it is being replaced.
      if (OldVal != V && OldVal->hasValueHandle())
        ValueHandleBase::ValueIsRAUWd(OldVal, V);
      U->set(V);
    }
  }
}