source/opt/code_sink.h

// 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.

#ifndef SOURCE_OPT_CODE_SINK_H_
#define SOURCE_OPT_CODE_SINK_H_

#include <unordered_map>

#include "source/opt/ir_context.h"
#include "source/opt/module.h"
#include "source/opt/pass.h"

namespace spvtools {
namespace opt {

// This pass does code sinking for OpAccessChain and OpLoad on variables in
// uniform storage or in read only memory.  Code sinking is a transformation
// where an instruction is moved into a more deeply nested construct.
//
// The goal is to move these instructions as close as possible to their uses
// without having to execute them more often or to replicate the instruction.
// Moving the instruction in this way can lead to shorter live ranges, which can
// lead to less register pressure.  It can also cause instructions to be
// executed less often because they could be moved into one path of a selection
// construct.
//
// This optimization can cause register pressure to rise if the operands of the
// instructions go dead after the instructions being moved. That is why we only
// move certain OpLoad and OpAccessChain instructions.  They generally have
// constants, loop induction variables, and global pointers as operands.  The
// operands are live for a longer time in most cases.
class CodeSinkingPass : public Pass {
 public:
  const char* name() const override { return "code-sink"; }
  Status Process() override;

  // Return the mask of preserved Analyses.
  IRContext::Analysis GetPreservedAnalyses() override {
    return IRContext::kAnalysisDefUse |
           IRContext::kAnalysisInstrToBlockMapping |
           IRContext::kAnalysisCombinators | IRContext::kAnalysisCFG |
           IRContext::kAnalysisDominatorAnalysis |
           IRContext::kAnalysisLoopAnalysis | IRContext::kAnalysisNameMap |
           IRContext::kAnalysisConstants | IRContext::kAnalysisTypes;
  }

 private:
  // Sinks the instructions in |bb| as much as possible.  Returns true if
  // something changes.
  bool SinkInstructionsInBB(BasicBlock* bb);

  // Tries the sink |inst| as much as possible.  Returns true if the instruction
  // is moved.
  bool SinkInstruction(Instruction* inst);

  // Returns the basic block in which to move |inst| to move is as close as
  // possible to the uses of |inst| without increasing the number of times
  // |inst| will be executed.  Return |nullptr| if there is no need to move
  // |inst|.
  BasicBlock* FindNewBasicBlockFor(Instruction* inst);

  // Return true if |inst| reference memory and it is possible that the data in
  // the memory changes at some point.
  bool ReferencesMutableMemory(Instruction* inst);

  // Returns true if the module contains an instruction that has a memory
  // semantics id as an operand, and the memory semantics enforces a
  // synchronization of uniform memory.  See section 3.25 of the SPIR-V
  // specification.
  bool HasUniformMemorySync();

  // Returns true if there may be a store to the variable |var_inst|.
  bool HasPossibleStore(Instruction* var_inst);

  // Returns true if one of the basic blocks in |set| exists on a path from the
  // basic block |start| to |end|.
  bool IntersectsPath(uint32_t start, uint32_t end,
                      const std::unordered_set<uint32_t>& set);

  // Returns true if |mem_semantics_id| is the id of a constant that, when
  // interpreted as a memory semantics mask enforces synchronization of uniform
  // memory.  See section 3.25 of the SPIR-V specification.
  bool IsSyncOnUniform(uint32_t mem_semantics_id) const;

  // True if a check has for uniform storage has taken place.
  bool checked_for_uniform_sync_;

  // Cache of whether or not the module has a memory sync on uniform storage.
  // only valid if |check_for_uniform_sync_| is true.
  bool has_uniform_sync_;
};

}  // namespace opt
}  // namespace spvtools

#endif  // SOURCE_OPT_CODE_SINK_H_