Method GetOrCreateSimpleLoopPreheader

source/fuzz/fuzzer_pass.cpp:619–678

}

opt::BasicBlock* FuzzerPass::GetOrCreateSimpleLoopPreheader(
    uint32_t header_id) {
  auto header_block = fuzzerutil::MaybeFindBlock(GetIRContext(), header_id);

  assert(header_block && header_block->IsLoopHeader() &&
         "|header_id| should be the label id of a loop header");

  auto predecessors = GetIRContext()->cfg()->preds(header_id);

  assert(predecessors.size() >= 2 &&
         "The block |header_id| should be reachable.");

  auto function = header_block->GetParent();

  if (predecessors.size() == 2) {
    // The header has a single out-of-loop predecessor, which could be a
    // preheader.

    opt::BasicBlock* maybe_preheader;

    if (GetIRContext()->GetDominatorAnalysis(function)->Dominates(
            header_id, predecessors[0])) {
      // The first predecessor is the back-edge block, because the header
      // dominates it, so the second one is out of the loop.
      maybe_preheader = &*function->FindBlock(predecessors[1]);
    } else {
      // The first predecessor is out of the loop.
      maybe_preheader = &*function->FindBlock(predecessors[0]);
    }

    // |maybe_preheader| is a preheader if it branches unconditionally to
    // the header. We also require it not to be a loop header.
    if (maybe_preheader->terminator()->opcode() == spv::Op::OpBranch &&
        !maybe_preheader->IsLoopHeader()) {
      return maybe_preheader;
    }
  }

  // We need to add a preheader.

  // Get a fresh id for the preheader.
  uint32_t preheader_id = GetFuzzerContext()->GetFreshId();

  // Get a fresh id for each OpPhi instruction, if there is more than one
  // out-of-loop predecessor.
  std::vector<uint32_t> phi_ids;
  if (predecessors.size() > 2) {
    header_block->ForEachPhiInst(
        [this, &phi_ids](opt::Instruction* /* unused */) {
          phi_ids.push_back(GetFuzzerContext()->GetFreshId());
        });
  }

  // Add the preheader.
  ApplyTransformation(
      TransformationAddLoopPreheader(header_id, preheader_id, phi_ids));

  // Make the newly-created preheader the new entry block.