llvm-project/llvm/lib/Transforms/Utils/Local.cpp

//===-- Local.cpp - Functions to perform local transformations ------------===//
//
// This family of functions perform various local transformations to the
// program.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/Local.h"
#include "llvm/iTerminators.h"
#include "llvm/iOperators.h"
#include "llvm/ConstantHandling.h"

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

// ConstantFoldInstruction - If an instruction references constants, try to fold
// them together...
//
bool doConstantPropagation(BasicBlock::iterator &II) {
  if (Constant *C = ConstantFoldInstruction(II)) {
    // Replaces all of the uses of a variable with uses of the constant.
    II->replaceAllUsesWith(C);
    
    // Remove the instruction from the basic block...
    II = II->getParent()->getInstList().erase(II);
    return true;
  }

  return false;
}

// ConstantFoldTerminator - If a terminator instruction is predicated on a
// constant value, convert it into an unconditional branch to the constant
// destination.
//
bool 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 (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
      // Are we branching on constant?
      // YES.  Change to unconditional branch...
      BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
      BasicBlock *OldDest     = Cond->getValue() ? 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..
      new BranchInst(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 SetCondInst(Instruction::SetEQ, SI->getCondition(),
                                    SI->getSuccessorValue(1), "cond", SI);
      // Insert the new branch...
      new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);

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



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

bool isInstructionTriviallyDead(Instruction *I) {
  return I->use_empty() && !I->mayWriteToMemory() && !isa<TerminatorInst>(I);
}

// dceInstruction - Inspect the instruction at *BBI and figure out if it's
// [trivially] dead.  If so, remove the instruction and update the iterator
// to point to the instruction that immediately succeeded the original
// instruction.
//
bool dceInstruction(BasicBlock::iterator &BBI) {
  // Look for un"used" definitions...
  if (isInstructionTriviallyDead(BBI)) {
    BBI = BBI->getParent()->getInstList().erase(BBI);   // Bye bye
    return true;
  }
  return false;
}