CodeGenPrepare.cpp

//===- CodeGenPrepare.cpp - Prepare a function for code generation --------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Chris Lattner and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass munges the code in the input function to better prepare it for
// SelectionDAG-based code generation.  This works around limitations in it's
// basic-block-at-a-time approach.  It should eventually be removed.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "codegenprepare"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetAsmInfo.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
using namespace llvm;

namespace {
  cl::opt<bool> OptExtUses("optimize-ext-uses",
                           cl::init(true), cl::Hidden);
}

namespace {  
  class VISIBILITY_HIDDEN CodeGenPrepare : public FunctionPass {
    /// TLI - Keep a pointer of a TargetLowering to consult for determining
    /// transformation profitability.
    const TargetLowering *TLI;
  public:
    static char ID; // Pass identification, replacement for typeid
    explicit CodeGenPrepare(const TargetLowering *tli = 0)
      : FunctionPass((intptr_t)&ID), TLI(tli) {}
    bool runOnFunction(Function &F);
    
  private:
    bool EliminateMostlyEmptyBlocks(Function &F);
    bool CanMergeBlocks(const BasicBlock *BB, const BasicBlock *DestBB) const;
    void EliminateMostlyEmptyBlock(BasicBlock *BB);
    bool OptimizeBlock(BasicBlock &BB);
    bool OptimizeLoadStoreInst(Instruction *I, Value *Addr,
                               const Type *AccessTy,
                               DenseMap<Value*,Value*> &SunkAddrs);
    bool OptimizeExtUses(Instruction *I);
  };
}

char CodeGenPrepare::ID = 0;
static RegisterPass<CodeGenPrepare> X("codegenprepare",
                                      "Optimize for code generation");

FunctionPass *llvm::createCodeGenPreparePass(const TargetLowering *TLI) {
  return new CodeGenPrepare(TLI);
}


bool CodeGenPrepare::runOnFunction(Function &F) {
  bool EverMadeChange = false;
  
  // First pass, eliminate blocks that contain only PHI nodes and an
  // unconditional branch.
  EverMadeChange |= EliminateMostlyEmptyBlocks(F);
  
  bool MadeChange = true;
  while (MadeChange) {
    MadeChange = false;
    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
      MadeChange |= OptimizeBlock(*BB);
    EverMadeChange |= MadeChange;
  }
  return EverMadeChange;
}

/// EliminateMostlyEmptyBlocks - eliminate blocks that contain only PHI nodes
/// and an unconditional branch.  Passes before isel (e.g. LSR/loopsimplify) 
/// often split edges in ways that are non-optimal for isel.  Start by
/// eliminating these blocks so we can split them the way we want them.
bool CodeGenPrepare::EliminateMostlyEmptyBlocks(Function &F) {
  bool MadeChange = false;
  // Note that this intentionally skips the entry block.
  for (Function::iterator I = ++F.begin(), E = F.end(); I != E; ) {
    BasicBlock *BB = I++;

    // If this block doesn't end with an uncond branch, ignore it.
    BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
    if (!BI || !BI->isUnconditional())
      continue;
    
    // If the instruction before the branch isn't a phi node, then other stuff
    // is happening here.
    BasicBlock::iterator BBI = BI;
    if (BBI != BB->begin()) {
      --BBI;
      if (!isa<PHINode>(BBI)) continue;
    }
    
    // Do not break infinite loops.
    BasicBlock *DestBB = BI->getSuccessor(0);
    if (DestBB == BB)
      continue;
    
    if (!CanMergeBlocks(BB, DestBB))
      continue;
    
    EliminateMostlyEmptyBlock(BB);
    MadeChange = true;
  }
  return MadeChange;
}

/// CanMergeBlocks - Return true if we can merge BB into DestBB if there is a
/// single uncond branch between them, and BB contains no other non-phi
/// instructions.
bool CodeGenPrepare::CanMergeBlocks(const BasicBlock *BB,
                                    const BasicBlock *DestBB) const {
  // We only want to eliminate blocks whose phi nodes are used by phi nodes in
  // the successor.  If there are more complex condition (e.g. preheaders),
  // don't mess around with them.
  BasicBlock::const_iterator BBI = BB->begin();
  while (const PHINode *PN = dyn_cast<PHINode>(BBI++)) {
    for (Value::use_const_iterator UI = PN->use_begin(), E = PN->use_end();
         UI != E; ++UI) {
      const Instruction *User = cast<Instruction>(*UI);
      if (User->getParent() != DestBB || !isa<PHINode>(User))
        return false;
      // If User is inside DestBB block and it is a PHINode then check 
      // incoming value. If incoming value is not from BB then this is 
      // a complex condition (e.g. preheaders) we want to avoid here.
      if (User->getParent() == DestBB) {
        if (const PHINode *UPN = dyn_cast<PHINode>(User))
          for (unsigned I = 0, E = UPN->getNumIncomingValues(); I != E; ++I) {
            Instruction *Insn = dyn_cast<Instruction>(UPN->getIncomingValue(I));
            if (Insn && Insn->getParent() == BB &&
                Insn->getParent() != UPN->getIncomingBlock(I))
              return false;
          }
      }
    }
  }
  
  // If BB and DestBB contain any common predecessors, then the phi nodes in BB
  // and DestBB may have conflicting incoming values for the block.  If so, we
  // can't merge the block.
  const PHINode *DestBBPN = dyn_cast<PHINode>(DestBB->begin());
  if (!DestBBPN) return true;  // no conflict.
  
  // Collect the preds of BB.
  SmallPtrSet<const BasicBlock*, 16> BBPreds;
  if (const PHINode *BBPN = dyn_cast<PHINode>(BB->begin())) {
    // It is faster to get preds from a PHI than with pred_iterator.
    for (unsigned i = 0, e = BBPN->getNumIncomingValues(); i != e; ++i)
      BBPreds.insert(BBPN->getIncomingBlock(i));
  } else {
    BBPreds.insert(pred_begin(BB), pred_end(BB));
  }
  
  // Walk the preds of DestBB.
  for (unsigned i = 0, e = DestBBPN->getNumIncomingValues(); i != e; ++i) {
    BasicBlock *Pred = DestBBPN->getIncomingBlock(i);
    if (BBPreds.count(Pred)) {   // Common predecessor?
      BBI = DestBB->begin();
      while (const PHINode *PN = dyn_cast<PHINode>(BBI++)) {
        const Value *V1 = PN->getIncomingValueForBlock(Pred);
        const Value *V2 = PN->getIncomingValueForBlock(BB);
        
        // If V2 is a phi node in BB, look up what the mapped value will be.
        if (const PHINode *V2PN = dyn_cast<PHINode>(V2))
          if (V2PN->getParent() == BB)
            V2 = V2PN->getIncomingValueForBlock(Pred);
        
        // If there is a conflict, bail out.
        if (V1 != V2) return false;
      }
    }
  }

  return true;
}


/// EliminateMostlyEmptyBlock - Eliminate a basic block that have only phi's and
/// an unconditional branch in it.
void CodeGenPrepare::EliminateMostlyEmptyBlock(BasicBlock *BB) {