source/reduce/remove_block_reduction_opportunity_finder.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/reduce/remove_block_reduction_opportunity_finder.h"

#include "source/reduce/remove_block_reduction_opportunity.h"

namespace spvtools {
namespace reduce {

using opt::Function;
using opt::IRContext;
using opt::Instruction;

std::string RemoveBlockReductionOpportunityFinder::GetName() const {
  return "RemoveBlockReductionOpportunityFinder";
}

std::vector<std::unique_ptr<ReductionOpportunity>>
RemoveBlockReductionOpportunityFinder::GetAvailableOpportunities(
    IRContext* context) const {
  std::vector<std::unique_ptr<ReductionOpportunity>> result;

  // Consider every block in every function.
  for (auto& function : *context->module()) {
    for (auto bi = function.begin(); bi != function.end(); ++bi) {
      if (IsBlockValidOpportunity(context, function, bi)) {
        result.push_back(spvtools::MakeUnique<RemoveBlockReductionOpportunity>(
            &function, &*bi));
      }
    }
  }
  return result;
}

bool RemoveBlockReductionOpportunityFinder::IsBlockValidOpportunity(
    IRContext* context, Function& function, Function::iterator& bi) {
  assert(bi != function.end() && "Block iterator was out of bounds");

  // Don't remove first block; we don't want to end up with no blocks.
  if (bi == function.begin()) {
    return false;
  }

  // Don't remove blocks with references.
  if (context->get_def_use_mgr()->NumUsers(bi->id()) > 0) {
    return false;
  }

  // Don't remove blocks whose instructions have outside references.
  if (!BlockInstructionsHaveNoOutsideReferences(context, bi)) {
    return false;
  }

  return true;
}

bool RemoveBlockReductionOpportunityFinder::
    BlockInstructionsHaveNoOutsideReferences(IRContext* context,
                                             const Function::iterator& bi) {
  // Get all instructions in block.
  std::unordered_set<uint32_t> instructions_in_block;
  for (const Instruction& instruction : *bi) {
    instructions_in_block.insert(instruction.unique_id());
  }

  // For each instruction...
  for (const Instruction& instruction : *bi) {
    // For each use of the instruction...
    bool no_uses_outside_block = context->get_def_use_mgr()->WhileEachUser(
        &instruction, [&instructions_in_block](Instruction* user) -> bool {
          // If the use is in this block, continue (return true). Otherwise, we
          // found an outside use; return false (and stop).
          return instructions_in_block.find(user->unique_id()) !=
                 instructions_in_block.end();
        });

    if (!no_uses_outside_block) {
      return false;
    }
  }

  return true;
}

}  // namespace reduce
}  // namespace spvtools