source/opt/eliminate_dead_constant_pass.cpp

// Copyright (c) 2016 Google Inc.
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// MODIFICATIONS TO THIS FILE MAY MEAN IT NO LONGER ACCURATELY REFLECTS
// KHRONOS STANDARDS. THE UNMODIFIED, NORMATIVE VERSIONS OF KHRONOS
// SPECIFICATIONS AND HEADER INFORMATION ARE LOCATED AT
//    https://www.khronos.org/registry/
//
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#include "eliminate_dead_constant_pass.h"

#include <algorithm>
#include <unordered_map>
#include <unordered_set>

#include "def_use_manager.h"
#include "reflect.h"

namespace spvtools {
namespace opt {

bool EliminateDeadConstantPass::Process(ir::Module* module) {
  analysis::DefUseManager def_use(module);
  std::unordered_set<ir::Instruction*> working_list;
  // Traverse all the instructions to get the initial set of dead constants as
  // working list and count number of real uses for constants. Uses in
  // annotation instructions do not count.
  std::unordered_map<ir::Instruction*, size_t> use_counts;
  std::vector<ir::Instruction*> constants = module->GetConstants();
  for (auto* c : constants) {
    uint32_t const_id = c->result_id();
    size_t count = 0;
    if (analysis::UseList* uses = def_use.GetUses(const_id)) {
      count =
          std::count_if(uses->begin(), uses->end(), [](const analysis::Use& u) {
            return !(ir::IsAnnotationInst(u.inst->opcode()) ||
                     ir::IsDebugInst(u.inst->opcode()));
          });
    }
    use_counts[c] = count;
    if (!count) {
      working_list.insert(c);
    }
  }

  // Start from the constants with 0 uses, back trace through the def-use chain
  // to find all dead constants.
  std::unordered_set<ir::Instruction*> dead_consts;
  while (!working_list.empty()) {
    ir::Instruction* inst = *working_list.begin();
    // Back propagate if the instruction contains IDs in its operands.
    switch (inst->opcode()) {
      case SpvOp::SpvOpConstantComposite:
      case SpvOp::SpvOpSpecConstantComposite:
      case SpvOp::SpvOpSpecConstantOp:
        for (uint32_t i = 0; i < inst->NumInOperands(); i++) {
          // SpecConstantOp instruction contains 'opcode' as its operand. Need
          // to exclude such operands when decreasing uses.
          if (inst->GetInOperand(i).type != SPV_OPERAND_TYPE_ID) {
            continue;
          }
          uint32_t operand_id = inst->GetSingleWordInOperand(i);
          ir::Instruction* def_inst = def_use.GetDef(operand_id);
          // If the use_count does not have any count for the def_inst,
          // def_inst must not be a constant, and should be ignored here.
          if (!use_counts.count(def_inst)) {
            continue;
          }
          // The number of uses should never be less then 0, so it can not be
          // less than 1 before it decreases.
          assert(use_counts[def_inst] > 0);
          --use_counts[def_inst];
          if (!use_counts[def_inst]) {
            working_list.insert(def_inst);
          }
        }
        break;
      default:
        break;
    }
    dead_consts.insert(inst);
    working_list.erase(inst);
  }

  // Find all annotation and debug instructions that are referencing dead
  // constants.
  std::unordered_set<ir::Instruction*> dead_others;
  for (auto* dc : dead_consts) {
    if (analysis::UseList* uses = def_use.GetUses(dc->result_id())) {
      for (const auto& u : *uses) {
        if (ir::IsAnnotationInst(u.inst->opcode()) ||
            ir::IsDebugInst(u.inst->opcode())) {
          dead_others.insert(u.inst);
        }
      }
    }
  }

  // Turn all dead instructions and uses of them to nop
  for (auto* dc : dead_consts) {
    def_use.KillDef(dc->result_id());
  }
  for (auto* da : dead_others) {
    da->ToNop();
  }
  return !dead_consts.empty();
}

}  // namespace opt
}  // namespace spvtools