lib/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;
  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.

  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
  ArgumentConstants.resize(F.arg_size());

  unsigned NumNonconstant = 0;

  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
    if (!isa<Instruction>(*I))
      return false;  // Used by a non-instruction, do not transform
    else {
      CallSite CS = CallSite::get(cast<Instruction>(*I));
      if (CS.getInstruction() == 0 ||
          CS.getCalledFunction() != &F)
        return false;  // Not a direct call site?

      // Check out all of the potentially constant arguments
      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 (*AI == &F) return false;  // Passes the function into itself

        if (!ArgumentConstants[i].second) {
          if (Constant *C = dyn_cast<Constant>(*AI)) {
            if (!ArgumentConstants[i].first)
              ArgumentConstants[i].first = C;
            else if (ArgumentConstants[i].first != C) {
              // Became non-constant
              ArgumentConstants[i].second = true;
              ++NumNonconstant;
              if (NumNonconstant == ArgumentConstants.size()) return false;
            }
          } else if (*AI != &*Arg) {    // Ignore recursive calls with same arg
            // This is not a constant argument.  Mark the argument as
            // non-constant.
            ArgumentConstants[i].second = true;
            ++NumNonconstant;
            if (NumNonconstant == ArgumentConstants.size()) return false;
          }
        }
      }
    }

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

  // Check to see if this function returns a constant.
  SmallVector<Value *,4> RetVals;
  unsigned N = 0;
  const StructType *STy = dyn_cast<StructType>(F.getReturnType());
  if (STy)
    N = STy->getNumElements();
  else
    N = 1;
  for (unsigned i = 0; i < N; ++i)
    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 (N == RI->getNumOperands() && "Invalid ReturnInst operands!");
      for (unsigned i = 0; i < N; ++i) {
        if (isa<UndefValue>(RI->getOperand(i))) {
          // Ignore
        } else if (Constant *C = dyn_cast<Constant>(RI->getOperand(i))) { 
          Value *RV = RetVals[i];
          if (RV == 0)
            RetVals[i] = C;
          else if (RV != C)
            return false; // Does not return the same constant.
        } else {
          return false; // Does not return a constant.
        }
      }
    }

  if (N == 1) {
    if (RetVals[0] == 0)
      RetVals[0] = UndefValue::get(F.getReturnType());
  } else {
    for (unsigned i = 0; i < N; ++i) {
      Value *RetVal = RetVals[i];
      if (RetVal == 0) 
        RetVals[i] = UndefValue::get(STy->getElementType(i));
    }
  }

  // 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 I = F.use_begin(), E = F.use_end(); I != E; ++I)
    if (!isa<Instruction>(*I))
      ReplacedAllUsers = false;
    else {
      CallSite CS = CallSite::get(cast<Instruction>(*I));
      if (CS.getInstruction() == 0 ||
          CS.getCalledFunction() != &F) {
        ReplacedAllUsers = false;
      } else {
        Instruction *Call = CS.getInstruction();
        if (!Call->use_empty()) {
          if (N == 1)
            Call->replaceAllUsesWith(RetVals[0]);
          else {
            for(Value::use_iterator CUI = Call->use_begin(), CUE = Call->use_end();
                CUI != CUE; ++CUI) {
              GetResultInst *GR = cast<GetResultInst>(CUI);
              GR->replaceAllUsesWith(RetVals[GR->getIndex()]);
              GR->eraseFromParent();
            }
          }
          MadeChange = true;
        }
      }
    }

  // 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 (unsigned i = 0; i < N; ++i) {
      Value *RetVal = RetVals[i];
      if (isa<UndefValue>(RetVal))
        continue;
      Value *RV = UndefValue::get(RetVal->getType());
      for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
        if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
          if (RI->getOperand(i) != RV) {
            RI->setOperand(i, RV);
            MadeChange = true;
          }
        }
    }
  }

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