lib/Transforms/Utils/Local.cpp

//===-- Local.cpp - Functions to perform local transformations ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This family of functions perform various local transformations to the
// program.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
using namespace llvm;

//===----------------------------------------------------------------------===//
//  Local constant propagation.
//

// ConstantFoldTerminator - If a terminator instruction is predicated on a
// constant value, convert it into an unconditional branch to the constant
// destination.
//
bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
  TerminatorInst *T = BB->getTerminator();

  // Branch - See if we are conditional jumping on constant
  if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
    if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
    BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
    BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));

    if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
      // Are we branching on constant?
      // YES.  Change to unconditional branch...
      BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
      BasicBlock *OldDest     = Cond->getZExtValue() ? Dest2 : Dest1;

      //cerr << "Function: " << T->getParent()->getParent()
      //     << "\nRemoving branch from " << T->getParent()
      //     << "\n\nTo: " << OldDest << endl;

      // Let the basic block know that we are letting go of it.  Based on this,
      // it will adjust it's PHI nodes.
      assert(BI->getParent() && "Terminator not inserted in block!");
      OldDest->removePredecessor(BI->getParent());

      // Set the unconditional destination, and change the insn to be an
      // unconditional branch.
      BI->setUnconditionalDest(Destination);
      return true;
    } else if (Dest2 == Dest1) {       // Conditional branch to same location?
      // This branch matches something like this:
      //     br bool %cond, label %Dest, label %Dest
      // and changes it into:  br label %Dest

      // Let the basic block know that we are letting go of one copy of it.
      assert(BI->getParent() && "Terminator not inserted in block!");
      Dest1->removePredecessor(BI->getParent());

      // Change a conditional branch to unconditional.
      BI->setUnconditionalDest(Dest1);
      return true;
    }
  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
    // If we are switching on a constant, we can convert the switch into a
    // single branch instruction!
    ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
    BasicBlock *TheOnlyDest = SI->getSuccessor(0);  // The default dest
    BasicBlock *DefaultDest = TheOnlyDest;
    assert(TheOnlyDest == SI->getDefaultDest() &&
           "Default destination is not successor #0?");

    // Figure out which case it goes to...
    for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
      // Found case matching a constant operand?
      if (SI->getSuccessorValue(i) == CI) {
        TheOnlyDest = SI->getSuccessor(i);
        break;
      }

      // Check to see if this branch is going to the same place as the default
      // dest.  If so, eliminate it as an explicit compare.
      if (SI->getSuccessor(i) == DefaultDest) {
        // Remove this entry...
        DefaultDest->removePredecessor(SI->getParent());
        SI->removeCase(i);
        --i; --e;  // Don't skip an entry...
        continue;
      }

      // Otherwise, check to see if the switch only branches to one destination.
      // We do this by reseting "TheOnlyDest" to null when we find two non-equal
      // destinations.
      if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
    }

    if (CI && !TheOnlyDest) {
      // Branching on a constant, but not any of the cases, go to the default
      // successor.
      TheOnlyDest = SI->getDefaultDest();
    }

    // If we found a single destination that we can fold the switch into, do so
    // now.
    if (TheOnlyDest) {
      // Insert the new branch..
      BranchInst::Create(TheOnlyDest, SI);
      BasicBlock *BB = SI->getParent();

      // Remove entries from PHI nodes which we no longer branch to...
      for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
        // Found case matching a constant operand?
        BasicBlock *Succ = SI->getSuccessor(i);
        if (Succ == TheOnlyDest)
          TheOnlyDest = 0;  // Don't modify the first branch to TheOnlyDest
        else
          Succ->removePredecessor(BB);
      }

      // Delete the old switch...
      BB->getInstList().erase(SI);
      return true;
    } else if (SI->getNumSuccessors() == 2) {
      // Otherwise, we can fold this switch into a conditional branch
      // instruction if it has only one non-default destination.
      Value *Cond = new ICmpInst(ICmpInst::ICMP_EQ, SI->getCondition(),
                                 SI->getSuccessorValue(1), "cond", SI);
      // Insert the new branch...
      BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);

      // Delete the old switch...
      SI->eraseFromParent();
      return true;
    }
  }
  return false;
}


//===----------------------------------------------------------------------===//
//  Local dead code elimination...
//

/// isInstructionTriviallyDead - Return true if the result produced by the
/// instruction is not used, and the instruction has no side effects.
///
bool llvm::isInstructionTriviallyDead(Instruction *I) {
  if (!I->use_empty() || isa<TerminatorInst>(I)) return false;

  if (!I->mayWriteToMemory())
    return true;

  // Special case intrinsics that "may write to memory" but can be deleted when
  // dead.
  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
    // Safe to delete llvm.stacksave if dead.
    if (II->getIntrinsicID() == Intrinsic::stacksave)
      return true;
  
  return false;
}

/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
/// trivially dead instruction, delete it.  If that makes any of its operands
/// trivially dead, delete them too, recursively.
///
/// If DeadInst is specified, the vector is filled with the instructions that
/// are actually deleted.
void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
                                      SmallVectorImpl<Instruction*> *DeadInst) {
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
    return;
  
  SmallVector<Instruction*, 16> DeadInsts;
  DeadInsts.push_back(I);
  
  while (!DeadInsts.empty()) {
    I = DeadInsts.back();
    DeadInsts.pop_back();

    // If the client wanted to know, tell it about deleted instructions.
    if (DeadInst)
      DeadInst->push_back(I);
    
    // Null out all of the instruction's operands to see if any operand becomes
    // dead as we go.
    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
      Value *OpV = I->getOperand(i);
      I->setOperand(i, 0);
      
      if (!OpV->use_empty()) continue;
    
      // If the operand is an instruction that became dead as we nulled out the
      // operand, and if it is 'trivially' dead, delete it in a future loop
      // iteration.
      if (Instruction *OpI = dyn_cast<Instruction>(OpV))
        if (isInstructionTriviallyDead(OpI))
          DeadInsts.push_back(OpI);
    }
    
    I->eraseFromParent();
  }
}


//===----------------------------------------------------------------------===//
//  Control Flow Graph Restructuring...
//

/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
/// predecessor is known to have one successor (DestBB!).  Eliminate the edge
/// between them, moving the instructions in the predecessor into DestBB and
/// deleting the predecessor block.
///
void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) {
  // If BB has single-entry PHI nodes, fold them.
  while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
    Value *NewVal = PN->getIncomingValue(0);
    // Replace self referencing PHI with undef, it must be dead.
    if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
    PN->replaceAllUsesWith(NewVal);
    PN->eraseFromParent();
  }
  
  BasicBlock *PredBB = DestBB->getSinglePredecessor();
  assert(PredBB && "Block doesn't have a single predecessor!");
  
  // Splice all the instructions from PredBB to DestBB.
  PredBB->getTerminator()->eraseFromParent();
  DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
  
  // Anything that branched to PredBB now branches to DestBB.
  PredBB->replaceAllUsesWith(DestBB);
  
  // Nuke BB.
  PredBB->eraseFromParent();
}