lib/Analysis/LoadValueNumbering.cpp

//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
//
// This file implements a value numbering pass that value #'s load instructions.
// To do this, it finds lexically identical load instructions, and uses alias
// analysis to determine which loads are guaranteed to produce the same value.
//
// This pass builds off of another value numbering pass to implement value
// numbering for non-load instructions.  It uses Alias Analysis so that it can
// disambiguate the load instructions.  The more powerful these base analyses
// are, the more powerful the resultant analysis will be.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/LoadValueNumbering.h"
#include "llvm/Analysis/ValueNumbering.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Pass.h"
#include "llvm/iMemory.h"
#include "llvm/BasicBlock.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <set>

namespace {
  // FIXME: This should not be a functionpass.
  struct LoadVN : public FunctionPass, public ValueNumbering {
    
    /// Pass Implementation stuff.  This doesn't do any analysis.
    ///
    bool runOnFunction(Function &) { return false; }
    
    /// getAnalysisUsage - Does not modify anything.  It uses Value Numbering
    /// and Alias Analysis.
    ///
    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
    
    /// getEqualNumberNodes - Return nodes with the same value number as the
    /// specified Value.  This fills in the argument vector with any equal
    /// values.
    ///
    virtual void getEqualNumberNodes(Value *V1,
                                     std::vector<Value*> &RetVals) const;
  private:
    /// haveEqualValueNumber - Given two load instructions, determine if they
    /// both produce the same value on every execution of the program, assuming
    /// that their source operands always give the same value.  This uses the
    /// AliasAnalysis implementation to invalidate loads when stores or function
    /// calls occur that could modify the value produced by the load.
    ///
    bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
                              DominatorSet &DomSetInfo) const;
  };

  // Register this pass...
  RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");

  // Declare that we implement the ValueNumbering interface
  RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
}



Pass *createLoadValueNumberingPass() { return new LoadVN(); }


/// getAnalysisUsage - Does not modify anything.  It uses Value Numbering and
/// Alias Analysis.
///
void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  AU.addRequired<AliasAnalysis>();
  AU.addRequired<ValueNumbering>();
  AU.addRequired<DominatorSet>();
}

// getEqualNumberNodes - Return nodes with the same value number as the
// specified Value.  This fills in the argument vector with any equal values.
//
void LoadVN::getEqualNumberNodes(Value *V,
                                 std::vector<Value*> &RetVals) const {

  if (LoadInst *LI = dyn_cast<LoadInst>(V)) {
    // If we have a load instruction find all of the load instructions that use
    // the same source operand.  We implement this recursively, because there
    // could be a load of a load of a load that are all identical.  We are
    // guaranteed that this cannot be an infinite recursion because load
    // instructions would have to pass through a PHI node in order for there to
    // be a cycle.  The PHI node would be handled by the else case here,
    // breaking the infinite recursion.
    //
    std::vector<Value*> PointerSources;
    getEqualNumberNodes(LI->getOperand(0), PointerSources);
    PointerSources.push_back(LI->getOperand(0));

    Function *F = LI->getParent()->getParent();

    // Now that we know the set of equivalent source pointers for the load
    // instruction, look to see if there are any load candiates that are
    // identical.
    //
    std::vector<LoadInst*> CandidateLoads;
    while (!PointerSources.empty()) {
      Value *Source = PointerSources.back();
      PointerSources.pop_back();                // Get a source pointer...

      for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
           UI != UE; ++UI)
        if (LoadInst *Cand = dyn_cast<LoadInst>(*UI))  // Is a load of source?
          if (Cand->getParent()->getParent() == F &&   // In the same function?
              Cand != LI)                              // Not LI itself?
            CandidateLoads.push_back(Cand);     // Got one...
    }

    // Remove duplicates from the CandidateLoads list because alias analysis
    // processing may be somewhat expensive and we don't want to do more work
    // than neccesary.
    //
    std::sort(CandidateLoads.begin(), CandidateLoads.end());
    CandidateLoads.erase(std::unique(CandidateLoads.begin(),
                                     CandidateLoads.end()),
                         CandidateLoads.end());

    // Get Alias Analysis...
    AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
    DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
    
    // Loop over all of the candindate loads.  If they are not invalidated by
    // stores or calls between execution of them and LI, then add them to
    // RetVals.
    for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
      if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
        RetVals.push_back(CandidateLoads[i]);
    
  } else {
    // Make sure passmanager doesn't try to fulfill our request with ourself!
    assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
           "getAnalysis() returned this!");

    // Not a load instruction?  Just chain to the base value numbering
    // implementation to satisfy the request...
    return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
  }
}

// CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
// (until DestBB) contain an instruction that might invalidate Ptr.
//
static bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
                                     Value *Ptr, AliasAnalysis &AA,
                                     std::set<BasicBlock*> &VisitedSet) {
  // Found the termination point!
  if (BB == DestBB || VisitedSet.count(BB)) return false;

  // Avoid infinite recursion!
  VisitedSet.insert(BB);

  // Can this basic block modify Ptr?
  if (AA.canBasicBlockModify(*BB, Ptr))
    return true;

  // Check all of our predecessor blocks...
  for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
    if (CheckForInvalidatingInst(*PI, DestBB, Ptr, AA, VisitedSet))
      return true;

  // None of our predecessor blocks contain an invalidating instruction, and we
  // don't either!
  return false;
}


/// haveEqualValueNumber - Given two load instructions, determine if they both
/// produce the same value on every execution of the program, assuming that
/// their source operands always give the same value.  This uses the
/// AliasAnalysis implementation to invalidate loads when stores or function
/// calls occur that could modify the value produced by the load.
///
bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
                                  AliasAnalysis &AA,
                                  DominatorSet &DomSetInfo) const {
  // Figure out which load dominates the other one.  If neither dominates the
  // other we cannot eliminate them.
  //
  // FIXME: This could be enhanced to some cases with a shared dominator!
  //
  if (DomSetInfo.dominates(L2, L1)) 
    std::swap(L1, L2);   // Make L1 dominate L2
  else if (!DomSetInfo.dominates(L1, L2))
    return false;  // Neither instruction dominates the other one...

  BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
  Value *LoadAddress = L1->getOperand(0);

  // L1 now dominates L2.  Check to see if the intervening instructions between
  // the two loads include a store or call...
  //
  if (BB1 == BB2) {  // In same basic block?
    // In this degenerate case, no checking of global basic blocks has to occur
    // just check the instructions BETWEEN L1 & L2...
    //
    if (AA.canInstructionRangeModify(*L1, *L2, LoadAddress))
      return false;   // Cannot eliminate load

    // No instructions invalidate the loads, they produce the same value!
    return true;
  } else {
    // Make sure that there are no store instructions between L1 and the end of
    // it's basic block...
    //
    if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress))
      return false;   // Cannot eliminate load

    // Make sure that there are no store instructions between the start of BB2
    // and the second load instruction...
    //
    if (AA.canInstructionRangeModify(BB2->front(), *L2, LoadAddress))
      return false;   // Cannot eliminate load

    // Do a depth first traversal of the inverse CFG starting at L2's block,
    // looking for L1's block.  The inverse CFG is made up of the predecessor
    // nodes of a block... so all of the edges in the graph are "backward".
    //
    std::set<BasicBlock*> VisitedSet;
    for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
      if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, AA, VisitedSet))
        return false;

    // If we passed all of these checks then we are sure that the two loads
    // produce the same value.
    return true;
  }
}