source/fuzz/fuzzer_pass_add_dead_breaks.cpp

// Copyright (c) 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "source/fuzz/fuzzer_pass_add_dead_breaks.h"

#include "source/fuzz/fuzzer_util.h"
#include "source/fuzz/transformation_add_dead_break.h"
#include "source/opt/ir_context.h"

namespace spvtools {
namespace fuzz {

FuzzerPassAddDeadBreaks::FuzzerPassAddDeadBreaks(
    opt::IRContext* ir_context, TransformationContext* transformation_context,
    FuzzerContext* fuzzer_context,
    protobufs::TransformationSequence* transformations)
    : FuzzerPass(ir_context, transformation_context, fuzzer_context,
                 transformations) {}

void FuzzerPassAddDeadBreaks::Apply() {
  // We first collect up lots of possibly-applicable transformations.
  std::vector<TransformationAddDeadBreak> candidate_transformations;
  // We consider each function separately.
  for (auto& function : *GetIRContext()->module()) {
    // For a given function, we find all the merge blocks in that function.
    std::vector<opt::BasicBlock*> merge_blocks;
    for (auto& block : function) {
      auto maybe_merge_id = block.MergeBlockIdIfAny();
      if (maybe_merge_id) {
        auto merge_block =
            fuzzerutil::MaybeFindBlock(GetIRContext(), maybe_merge_id);

        assert(merge_block && "Merge block can't be null");

        merge_blocks.push_back(merge_block);
      }
    }
    // We rather aggressively consider the possibility of adding a break from
    // every block in the function to every merge block.  Many of these will be
    // inapplicable as they would be illegal.  That's OK - we later discard the
    // ones that turn out to be no good.
    for (auto& block : function) {
      for (auto* merge_block : merge_blocks) {
        // Populate this vector with ids that are available at the branch point
        // of this basic block. We will use these ids to update OpPhi
        // instructions later.
        std::vector<uint32_t> phi_ids;

        // Determine how we need to adjust OpPhi instructions' operands
        // for this transformation to be valid.
        //
        // If |block| has a branch to |merge_block|, the latter must have all of
        // its OpPhi instructions set up correctly - we don't need to adjust
        // anything.
        if (!block.IsSuccessor(merge_block)) {
          merge_block->ForEachPhiInst([this, &phi_ids](opt::Instruction* phi) {
            // Add an additional operand for OpPhi instruction.  Use a constant
            // if possible, and an undef otherwise.
            if (fuzzerutil::CanCreateConstant(GetIRContext(), phi->type_id())) {
              // We mark the constant as irrelevant so that we can replace it
              // with a more interesting value later.
              phi_ids.push_back(FindOrCreateZeroConstant(phi->type_id(), true));
            } else {
              phi_ids.push_back(FindOrCreateGlobalUndef(phi->type_id()));
            }
          });
        }

        // Make sure the module has a required boolean constant to be used in
        // OpBranchConditional instruction.
        auto break_condition = GetFuzzerContext()->ChooseEven();
        FindOrCreateBoolConstant(break_condition, false);

        auto candidate_transformation = TransformationAddDeadBreak(
            block.id(), merge_block->id(), break_condition, std::move(phi_ids));
        if (candidate_transformation.IsApplicable(
                GetIRContext(), *GetTransformationContext())) {
          // Only consider a transformation as a candidate if it is applicable.
          candidate_transformations.push_back(
              std::move(candidate_transformation));
        }
      }
    }
  }

  // Go through the candidate transformations that were accumulated,
  // probabilistically deciding whether to consider each one further and
  // applying the still-applicable ones that are considered further.
  //
  // We iterate through the candidate transformations in a random order by
  // repeatedly removing a random candidate transformation from the sequence
  // until no candidate transformations remain.  This is done because
  // transformations can potentially disable one another, so that iterating
  // through them in order would lead to a higher probability of
  // transformations appearing early in the sequence being applied compared
  // with later transformations.
  while (!candidate_transformations.empty()) {
    // Choose a random index into the sequence of remaining candidate
    // transformations.
    auto index = GetFuzzerContext()->RandomIndex(candidate_transformations);
    // Remove the transformation at the chosen index from the sequence.
    auto transformation = std::move(candidate_transformations[index]);
    candidate_transformations.erase(candidate_transformations.begin() + index);
    // Probabilistically decide whether to try to apply it vs. ignore it, in the
    // case that it is applicable.
    if (GetFuzzerContext()->ChoosePercentage(
            GetFuzzerContext()->GetChanceOfAddingDeadBreak())) {
      MaybeApplyTransformation(transformation);
    }
  }
}

}  // namespace fuzz
}  // namespace spvtools