// Copyright (c) 2017 The Khronos Group Inc.

// Copyright (c) 2017 Valve Corporation

// Copyright (c) 2017 LunarG Inc.

//

// 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.

#ifndef SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_

#define SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_

#include <algorithm>

#include <map>

#include <queue>

#include <unordered_map>

#include <unordered_set>

#include <utility>

#include <vector>

#include "source/opt/basic_block.h"

#include "source/opt/def_use_manager.h"

#include "source/opt/mem_pass.h"

#include "source/opt/module.h"

namespace spvtools {

namespace opt {

// See optimizer.hpp for documentation.

class DeadBranchElimPass : public MemPass {

  using cbb_ptr = const BasicBlock*;

 public:

  DeadBranchElimPass() = default;

  const char* name() const override { return "eliminate-dead-branches"; }

  Status Process() override;

  IRContext::Analysis GetPreservedAnalyses() override {

    return IRContext::kAnalysisDefUse |

           IRContext::kAnalysisInstrToBlockMapping |

           IRContext::kAnalysisConstants | IRContext::kAnalysisTypes;

  }

 private:

  // If |condId| is boolean constant, return conditional value in |condVal| and

  // return true, otherwise return false.

  bool GetConstCondition(uint32_t condId, bool* condVal);

  // If |valId| is a 32-bit integer constant, return value via |value| and

  // return true, otherwise return false.

  bool GetConstInteger(uint32_t valId, uint32_t* value);

  // Add branch to |labelId| to end of block |bp|.

  void AddBranch(uint32_t labelId, BasicBlock* bp);

  // For function |func|, look for BranchConditionals with constant condition

  // and convert to a Branch to the indicated label. Delete resulting dead

  // blocks. Note some such branches and blocks may be left to avoid creating

  // invalid control flow.

  // TODO(greg-lunarg): Remove remaining constant conditional branches and dead

  // blocks.

  bool EliminateDeadBranches(Function* func);

  // Returns the basic block containing |id|.

  // Note: this pass only requires correct instruction block mappings for the

  // input. This pass does not preserve the block mapping, so it is not kept

  // up-to-date during processing.

  BasicBlock* GetParentBlock(uint32_t id);

  // Marks live blocks reachable from the entry of |func|. Simplifies constant

  // branches and switches as it proceeds, to limit the number of live blocks.

  // It is careful not to eliminate backedges even if they are dead, but the

  // header is live. Likewise, unreachable merge blocks named in live merge

  // instruction must be retained (though they may be clobbered).

  bool MarkLiveBlocks(Function* func,

                      std::unordered_set<BasicBlock*>* live_blocks);

  // Checks for unreachable merge and continue blocks with live headers; those

  // blocks must be retained. Continues are tracked separately so that a live

  // phi can be updated to take an undef value from any of its predecessors

  // that are unreachable continues.

  //

  // |unreachable_continues| maps the id of an unreachable continue target to

  // the header block that declares it.

  void MarkUnreachableStructuredTargets(

      const std::unordered_set<BasicBlock*>& live_blocks,

      std::unordered_set<BasicBlock*>* unreachable_merges,

      std::unordered_map<BasicBlock*, BasicBlock*>* unreachable_continues);

  // Fix phis in reachable blocks so that only live (or unremovable) incoming

  // edges are present. If the block now only has a single live incoming edge,

  // remove the phi and replace its uses with its data input. If the single

  // remaining incoming edge is from the phi itself, the phi is in an

  // unreachable single block loop. Either the block is dead and will be

  // removed, or it's reachable from an unreachable continue target. In the

  // latter case that continue target block will be collapsed into a block that

  // only branches back to its header and we'll eliminate the block with the

  // phi.

  //

  // |unreachable_continues| maps continue targets that cannot be reached to

  // merge instruction that declares them.

  bool FixPhiNodesInLiveBlocks(

      Function* func, const std::unordered_set<BasicBlock*>& live_blocks,

      const std::unordered_map<BasicBlock*, BasicBlock*>&

          unreachable_continues);

  // Erases dead blocks. Any block captured in |unreachable_merges| or

  // |unreachable_continues| is a dead block that is required to remain due to

  // a live merge instruction in the corresponding header. These blocks will

  // have their instructions clobbered and will become a label and terminator.

  // Unreachable merge blocks are terminated by OpUnreachable, while

  // unreachable continue blocks are terminated by an unconditional branch to

  // the header. Otherwise, blocks are dead if not explicitly captured in

  // |live_blocks| and are totally removed.

  //

  // |unreachable_continues| maps continue targets that cannot be reached to

  // corresponding header block that declares them.

  bool EraseDeadBlocks(

      Function* func, const std::unordered_set<BasicBlock*>& live_blocks,

      const std::unordered_set<BasicBlock*>& unreachable_merges,

      const std::unordered_map<BasicBlock*, BasicBlock*>&

          unreachable_continues);

  // Reorders blocks in reachable functions so that they satisfy dominator

  // block ordering rules.

  void FixBlockOrder();

  // Return the first branch instruction that is a conditional branch to

  // |merge_block_id|. Returns |nullptr| if no such branch exists. If there are

  // multiple such branches, the first one is the one that would be executed

  // first when running the code.  That is, the one that dominates all of the

  // others.

  //

  // |start_block_id| must be a block whose innermost containing merge construct

  // has |merge_block_id| as the merge block.

  //

  // |loop_merge_id| and |loop_continue_id| are the merge and continue block ids

  // of the innermost loop containing |start_block_id|.

  Instruction* FindFirstExitFromSelectionMerge(uint32_t start_block_id,

                                               uint32_t merge_block_id,

                                               uint32_t loop_merge_id,

                                               uint32_t loop_continue_id,

                                               uint32_t switch_merge_id);

  // Adds to |blocks_with_back_edges| all of the blocks on the path from the

  // basic block |cont_id| to |header_id| and |merge_id|.  The intention is that

  // |cond_id| is a the continue target of a loop, |header_id| is the header of

  // the loop, and |merge_id| is the merge block of the loop.

  void AddBlocksWithBackEdge(

      uint32_t cont_id, uint32_t header_id, uint32_t merge_id,

      std::unordered_set<BasicBlock*>* blocks_with_back_edges);

  // Returns true if there is a branch to the merge node of the selection

  // construct |switch_header_id| that is inside a nested selection construct or

  // in the header of the nested selection construct.

  bool SwitchHasNestedBreak(uint32_t switch_header_id);

  // Return true of the terminator of |block| is successfully replaced with a

  // branch to |live_lab_id|.  The merge instruction is deleted or moved as

  // needed to maintain structured control flow.  Assumes that the

  // StructuredCFGAnalysis is valid for the constructs containing |block|.

  bool SimplifyBranch(BasicBlock* block, uint32_t live_lab_id);

};

}  // namespace opt

}  // namespace spvtools

#endif  // SOURCE_OPT_DEAD_BRANCH_ELIM_PASS_H_