llvm-project/llvm/lib/Transforms/Scalar/TailDuplication.cpp

//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
// 
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// 
//===----------------------------------------------------------------------===//
//
// This pass performs a limited form of tail duplication, intended to simplify
// CFGs by removing some unconditional branches.  This pass is necessary to
// straighten out loops created by the C front-end, but also is capable of
// making other code nicer.  After this pass is run, the CFG simplify pass
// should be run to clean up the mess.
//
// This pass could be enhanced in the future to use profile information to be
// more aggressive.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/Constant.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

namespace {
  cl::opt<unsigned>
  Threshold("taildup-threshold", cl::desc("Max block size to tail duplicate"),
            cl::init(6), cl::Hidden);
  Statistic<> NumEliminated("tailduplicate",
                            "Number of unconditional branches eliminated");
  Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");

  class TailDup : public FunctionPass {
    bool runOnFunction(Function &F);
  private:
    inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
    inline void eliminateUnconditionalBranch(BranchInst *BI);
  };
  RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
}

// Public interface to the Tail Duplication pass
Pass *llvm::createTailDuplicationPass() { return new TailDup(); }

/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
/// the function, eliminating it if it looks attractive enough.
///
bool TailDup::runOnFunction(Function &F) {
  bool Changed = false;
  for (Function::iterator I = F.begin(), E = F.end(); I != E; )
    if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
      eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
      Changed = true;
    } else {
      ++I;
    }
  return Changed;
}

/// shouldEliminateUnconditionalBranch - Return true if this branch looks
/// attractive to eliminate.  We eliminate the branch if the destination basic
/// block has <= 5 instructions in it, not counting PHI nodes.  In practice,
/// since one of these is a terminator instruction, this means that we will add
/// up to 4 instructions to the new block.
///
/// We don't count PHI nodes in the count since they will be removed when the
/// contents of the block are copied over.
///
bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
  BranchInst *BI = dyn_cast<BranchInst>(TI);
  if (!BI || !BI->isUnconditional()) return false;  // Not an uncond branch!

  BasicBlock *Dest = BI->getSuccessor(0);
  if (Dest == BI->getParent()) return false;        // Do not loop infinitely!

  // Do not inline a block if we will just get another branch to the same block!
  TerminatorInst *DTI = Dest->getTerminator();
  if (BranchInst *DBI = dyn_cast<BranchInst>(DTI))
    if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
      return false;                                 // Do not loop infinitely!

  // FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
  // because doing so would require breaking critical edges.  This should be
  // fixed eventually.
  if (!DTI->use_empty())
    return false;

  // Do not bother working on dead blocks...
  pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
  if (PI == PE && Dest != Dest->getParent()->begin())
    return false;   // It's just a dead block, ignore it...

  // Also, do not bother with blocks with only a single predecessor: simplify
  // CFG will fold these two blocks together!
  ++PI;
  if (PI == PE) return false;  // Exactly one predecessor!

  BasicBlock::iterator I = Dest->begin();
  while (isa<PHINode>(*I)) ++I;

  for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
    if (Size == Threshold) return false;  // The block is too large...

  // Do not tail duplicate a block that has thousands of successors into a block
  // with a single successor if the block has many other predecessors.  This can
  // cause an N^2 explosion in CFG edges (and PHI node entries), as seen in
  // cases that have a large number of indirect gotos.
  if (DTI->getNumSuccessors() > 8)
    if (std::distance(PI, PE) * DTI->getNumSuccessors() > 128)
      return false;

  return true;  
}


/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.
/// If the destination block defines values used by successors of the dest
/// block, we may need to insert PHI nodes.
///
void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
  BasicBlock *SourceBlock = Branch->getParent();
  BasicBlock *DestBlock = Branch->getSuccessor(0);
  assert(SourceBlock != DestBlock && "Our predicate is broken!");

  DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
                  << "]: Eliminating branch: " << *Branch);

  // Tail duplication can not update SSA properties correctly if the values
  // defined in the duplicated tail are used outside of the tail itself.  For
  // this reason, we spill all values that are used outside of the tail to the
  // stack.
  for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I)
    for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
         ++UI) {
      bool ShouldDemote = false;
      if (cast<Instruction>(*UI)->getParent() != DestBlock) {
        // We must allow our successors to use tail values in their PHI nodes
        // (if the incoming value corresponds to the tail block).
        if (PHINode *PN = dyn_cast<PHINode>(*UI)) {
          for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
            if (PN->getIncomingValue(i) == I &&
                PN->getIncomingBlock(i) != DestBlock) {
              ShouldDemote = true;
              break;
            }

        } else {
          ShouldDemote = true;
        }
      } else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) {
        // If the user of this instruction is a PHI node in the current block,
        // which has an entry from another block using the value, spill it.
        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
          if (PN->getIncomingValue(i) == I &&
              PN->getIncomingBlock(i) != DestBlock) {
            ShouldDemote = true;
            break;
          }
      }

      if (ShouldDemote) {
        // We found a use outside of the tail.  Create a new stack slot to
        // break this inter-block usage pattern.
        DemoteRegToStack(*I);
        break;
      }
    }

  // We are going to have to map operands from the original block B to the new
  // copy of the block B'.  If there are PHI nodes in the DestBlock, these PHI
  // nodes also define part of this mapping.  Loop over these PHI nodes, adding
  // them to our mapping.
  //
  std::map<Value*, Value*> ValueMapping;

  BasicBlock::iterator BI = DestBlock->begin();
  bool HadPHINodes = isa<PHINode>(BI);
  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
    ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);

  // Clone the non-phi instructions of the dest block into the source block,
  // keeping track of the mapping...
  //
  for (; BI != DestBlock->end(); ++BI) {
    Instruction *New = BI->clone();
    New->setName(BI->getName());
    SourceBlock->getInstList().push_back(New);
    ValueMapping[BI] = New;
  }

  // Now that we have built the mapping information and cloned all of the
  // instructions (giving us a new terminator, among other things), walk the new
  // instructions, rewriting references of old instructions to use new
  // instructions.
  //
  BI = Branch; ++BI;  // Get an iterator to the first new instruction
  for (; BI != SourceBlock->end(); ++BI)
    for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
      if (Value *Remapped = ValueMapping[BI->getOperand(i)])
        BI->setOperand(i, Remapped);

  // Next we check to see if any of the successors of DestBlock had PHI nodes.
  // If so, we need to add entries to the PHI nodes for SourceBlock now.
  for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
       SI != SE; ++SI) {
    BasicBlock *Succ = *SI;
    for (BasicBlock::iterator PNI = Succ->begin(); isa<PHINode>(PNI); ++PNI) {
      PHINode *PN = cast<PHINode>(PNI);
      // Ok, we have a PHI node.  Figure out what the incoming value was for the
      // DestBlock.
      Value *IV = PN->getIncomingValueForBlock(DestBlock);
      
      // Remap the value if necessary...
      if (Value *MappedIV = ValueMapping[IV])
        IV = MappedIV;
      PN->addIncoming(IV, SourceBlock);
    }
  }

  // Next, remove the old branch instruction, and any PHI node entries that we
  // had.
  BI = Branch; ++BI;  // Get an iterator to the first new instruction
  DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
  SourceBlock->getInstList().erase(Branch);  // Destroy the uncond branch...

  // Final step: now that we have finished everything up, walk the cloned
  // instructions one last time, constant propagating and DCE'ing them, because
  // they may not be needed anymore.
  //
  if (HadPHINodes)
    while (BI != SourceBlock->end())
      if (!dceInstruction(BI) && !doConstantPropagation(BI))
        ++BI;

  ++NumEliminated;  // We just killed a branch!
}