C++ GlobalVariable::uses方法代码示例

bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                          Module &M, bool isConst, unsigned AddrSpace) const {
  auto &DL = M.getDataLayout();
  // FIXME: Find better heuristics
  std::stable_sort(Globals.begin(), Globals.end(),
                   [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
                     return DL.getTypeAllocSize(GV1->getValueType()) <
                            DL.getTypeAllocSize(GV2->getValueType());
                   });

  // If we want to just blindly group all globals together, do so.
  if (!GlobalMergeGroupByUse) {
    BitVector AllGlobals(Globals.size());
    AllGlobals.set();
    return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
  }

  // If we want to be smarter, look at all uses of each global, to try to
  // discover all sets of globals used together, and how many times each of
  // these sets occurred.
  //
  // Keep this reasonably efficient, by having an append-only list of all sets
  // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
  // code (currently, a Function) to the set of globals seen so far that are
  // used together in that unit (GlobalUsesByFunction).
  //
  // When we look at the Nth global, we know that any new set is either:
  // - the singleton set {N}, containing this global only, or
  // - the union of {N} and a previously-discovered set, containing some
  //   combination of the previous N-1 globals.
  // Using that knowledge, when looking at the Nth global, we can keep:
  // - a reference to the singleton set {N} (CurGVOnlySetIdx)
  // - a list mapping each previous set to its union with {N} (EncounteredUGS),
  //   if it actually occurs.

  // We keep track of the sets of globals used together "close enough".
  struct UsedGlobalSet {
    BitVector Globals;
    unsigned UsageCount = 1;

    UsedGlobalSet(size_t Size) : Globals(Size) {}
  };

  // Each set is unique in UsedGlobalSets.
  std::vector<UsedGlobalSet> UsedGlobalSets;

  // Avoid repeating the create-global-set pattern.
  auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
    UsedGlobalSets.emplace_back(Globals.size());
    return UsedGlobalSets.back();
  };

  // The first set is the empty set.
  CreateGlobalSet().UsageCount = 0;

  // We define "close enough" to be "in the same function".
  // FIXME: Grouping uses by function is way too aggressive, so we should have
  // a better metric for distance between uses.
  // The obvious alternative would be to group by BasicBlock, but that's in
  // turn too conservative..
  // Anything in between wouldn't be trivial to compute, so just stick with
  // per-function grouping.

  // The value type is an index into UsedGlobalSets.
  // The default (0) conveniently points to the empty set.
  DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;

  // Now, look at each merge-eligible global in turn.

  // Keep track of the sets we already encountered to which we added the
  // current global.
  // Each element matches the same-index element in UsedGlobalSets.
  // This lets us efficiently tell whether a set has already been expanded to
  // include the current global.
  std::vector<size_t> EncounteredUGS;

  for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
    GlobalVariable *GV = Globals[GI];

    // Reset the encountered sets for this global...
    std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
    // ...and grow it in case we created new sets for the previous global.
    EncounteredUGS.resize(UsedGlobalSets.size());

    // We might need to create a set that only consists of the current global.
    // Keep track of its index into UsedGlobalSets.
    size_t CurGVOnlySetIdx = 0;

    // For each global, look at all its Uses.
    for (auto &U : GV->uses()) {
      // This Use might be a ConstantExpr.  We're interested in Instruction
      // users, so look through ConstantExpr...
      Use *UI, *UE;
      if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
        if (CE->use_empty())
          continue;
        UI = &*CE->use_begin();
        UE = nullptr;
      } else if (isa<Instruction>(U.getUser())) {
        UI = &U;
//.........这里部分代码省略.........