Transforms/IPO/IPConstantPropagation.cpp

//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements an _extremely_ simple interprocedural constant
// propagation pass.  It could certainly be improved in many different ways,
// like using a worklist.  This pass makes arguments dead, but does not remove
// them.  The existing dead argument elimination pass should be run after this
// to clean up the mess.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "ipconstprop"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallVector.h"
using namespace llvm;

STATISTIC(NumArgumentsProped, "Number of args turned into constants");
STATISTIC(NumReturnValProped, "Number of return values turned into constants");

namespace {
  /// IPCP - The interprocedural constant propagation pass
  ///
  struct VISIBILITY_HIDDEN IPCP : public ModulePass {
    static char ID; // Pass identification, replacement for typeid
    IPCP() : ModulePass((intptr_t)&ID) {}

    bool runOnModule(Module &M);
  private:
    bool PropagateConstantsIntoArguments(Function &F);
    bool PropagateConstantReturn(Function &F);
  };
}

char IPCP::ID = 0;
static RegisterPass<IPCP>
X("ipconstprop", "Interprocedural constant propagation");

ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }

bool IPCP::runOnModule(Module &M) {
  bool Changed = false;
  bool LocalChange = true;

  // FIXME: instead of using smart algorithms, we just iterate until we stop
  // making changes.
  while (LocalChange) {
    LocalChange = false;
    for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
      if (!I->isDeclaration()) {
        // Delete any klingons.
        I->removeDeadConstantUsers();
        if (I->hasInternalLinkage())
          LocalChange |= PropagateConstantsIntoArguments(*I);
        Changed |= PropagateConstantReturn(*I);
      }
    Changed |= LocalChange;
  }
  return Changed;
}

/// PropagateConstantsIntoArguments - Look at all uses of the specified
/// function.  If all uses are direct call sites, and all pass a particular
/// constant in for an argument, propagate that constant in as the argument.
///
bool IPCP::PropagateConstantsIntoArguments(Function &F) {
  if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.

  // For each argument, keep track of its constant value and whether it is a
  // constant or not.  The bool is driven to true when found to be non-constant.
  SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
  ArgumentConstants.resize(F.arg_size());

  unsigned NumNonconstant = 0;
  for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
    // Used by a non-instruction, or not the callee of a function, do not
    // transform.
    if (UI.getOperandNo() != 0 ||
        (!isa<CallInst>(*UI) && !isa<InvokeInst>(*UI)))
      return false;

    CallSite CS = CallSite::get(cast<Instruction>(*UI));

    // Check out all of the potentially constant arguments.  Note that we don't
    // inspect varargs here.
    CallSite::arg_iterator AI = CS.arg_begin();
    Function::arg_iterator Arg = F.arg_begin();
    for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
         ++i, ++AI, ++Arg) {

      // If this argument is known non-constant, ignore it.
      if (ArgumentConstants[i].second)
        continue;

      Constant *C = dyn_cast<Constant>(*AI);
      if (C && ArgumentConstants[i].first == 0) {
        ArgumentConstants[i].first = C;   // First constant seen.
      } else if (C && ArgumentConstants[i].first == C) {
        // Still the constant value we think it is.
      } else if (*AI == &*Arg) {
        // Ignore recursive calls passing argument down.
      } else {
        // Argument became non-constant.  If all arguments are non-constant now,
        // give up on this function.
        if (++NumNonconstant == ArgumentConstants.size())
          return false;
        ArgumentConstants[i].second = true;
      }
    }
  }

  // If we got to this point, there is a constant argument!
  assert(NumNonconstant != ArgumentConstants.size());
  bool MadeChange = false;
  Function::arg_iterator AI = F.arg_begin();
  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
    // Do we have a constant argument?
    if (ArgumentConstants[i].second || AI->use_empty())
      continue;

    Value *V = ArgumentConstants[i].first;
    if (V == 0) V = UndefValue::get(AI->getType());
    AI->replaceAllUsesWith(V);
    ++NumArgumentsProped;
    MadeChange = true;
  }
  return MadeChange;
}


// Check to see if this function returns a constant.  If so, replace all callers
// that user the return value with the returned valued.  If we can replace ALL
// callers,
bool IPCP::PropagateConstantReturn(Function &F) {
  if (F.getReturnType() == Type::VoidTy)
    return false; // No return value.

  // If this function could be overridden later in the link stage, we can't
  // propagate information about its results into callers.
  if (F.hasLinkOnceLinkage() || F.hasWeakLinkage())
    return false;

  // Check to see if this function returns a constant.
  SmallVector<Value *,4> RetVals;
  const StructType *STy = dyn_cast<StructType>(F.getReturnType());
  if (STy)
    RetVals.assign(STy->getNumElements(), 0);
  else
    RetVals.push_back(0);

  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
      assert(RetVals.size() == RI->getNumOperands() &&
             "Invalid ReturnInst operands!");
      for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
        if (isa<UndefValue>(RI->getOperand(i)))
          continue; // Ignore
        Constant *C = dyn_cast<Constant>(RI->getOperand(i));
        if (C == 0)
          return false; // Does not return a constant.

        Value *RV = RetVals[i];
        if (RV == 0)
          RetVals[i] = C;
        else if (RV != C)
          return false; // Does not return the same constant.
      }
    }

  if (STy) {
    for (unsigned i = 0, e = RetVals.size(); i < e; ++i)
      if (RetVals[i] == 0)
        RetVals[i] = UndefValue::get(STy->getElementType(i));
  } else {
    assert(RetVals.size() == 1);
    if (RetVals[0] == 0)
      RetVals[0] = UndefValue::get(F.getReturnType());
  }

  // If we got here, the function returns a constant value.  Loop over all
  // users, replacing any uses of the return value with the returned constant.
  bool ReplacedAllUsers = true;
  bool MadeChange = false;
  for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
    // Make sure this is an invoke or call and that the use is for the callee.
    if (!(isa<InvokeInst>(*UI) || isa<CallInst>(*UI)) ||
        UI.getOperandNo() != 0) {
      ReplacedAllUsers = false;
      continue;
    }

    Instruction *Call = cast<Instruction>(*UI);
    if (Call->use_empty())
      continue;

    MadeChange = true;

    if (STy == 0) {
      Call->replaceAllUsesWith(RetVals[0]);
      continue;
    }

    while (!Call->use_empty()) {
      GetResultInst *GR = cast<GetResultInst>(Call->use_back());
      GR->replaceAllUsesWith(RetVals[GR->getIndex()]);
      GR->eraseFromParent();
    }
  }

  // If we replace all users with the returned constant, and there can be no
  // other callers of the function, replace the constant being returned in the
  // function with an undef value.
  if (ReplacedAllUsers && F.hasInternalLinkage()) {
    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
      if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
        for (unsigned i = 0, e = RetVals.size(); i < e; ++i) {
          Value *RetVal = RetVals[i];
          if (isa<UndefValue>(RetVal))
            continue;
          Value *RV = UndefValue::get(RetVal->getType());
          if (RI->getOperand(i) != RV) {
            RI->setOperand(i, RV);
            MadeChange = true;
          }
        }
      }
    }
  }

  if (MadeChange) ++NumReturnValProped;
  return MadeChange;
}