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      1 // Copyright (c) 2017 Google Inc.
      2 //
      3 // Licensed under the Apache License, Version 2.0 (the "License");
      4 // you may not use this file except in compliance with the License.
      5 // You may obtain a copy of the License at
      6 //
      7 //     http://www.apache.org/licenses/LICENSE-2.0
      8 //
      9 // Unless required by applicable law or agreed to in writing, software
     10 // distributed under the License is distributed on an "AS IS" BASIS,
     11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     12 // See the License for the specific language governing permissions and
     13 // limitations under the License.
     14 
     15 #include "source/opt/dead_variable_elimination.h"
     16 
     17 #include <vector>
     18 
     19 #include "source/opt/ir_context.h"
     20 #include "source/opt/reflect.h"
     21 
     22 namespace spvtools {
     23 namespace opt {
     24 
     25 // This optimization removes global variables that are not needed because they
     26 // are definitely not accessed.
     27 Pass::Status DeadVariableElimination::Process() {
     28   // The algorithm will compute the reference count for every global variable.
     29   // Anything with a reference count of 0 will then be deleted.  For variables
     30   // that might have references that are not explicit in this context, we use
     31   // the value kMustKeep as the reference count.
     32   std::vector<uint32_t> ids_to_remove;
     33 
     34   // Get the reference count for all of the global OpVariable instructions.
     35   for (auto& inst : context()->types_values()) {
     36     if (inst.opcode() != SpvOp::SpvOpVariable) {
     37       continue;
     38     }
     39 
     40     size_t count = 0;
     41     uint32_t result_id = inst.result_id();
     42 
     43     // Check the linkage.  If it is exported, it could be reference somewhere
     44     // else, so we must keep the variable around.
     45     get_decoration_mgr()->ForEachDecoration(
     46         result_id, SpvDecorationLinkageAttributes,
     47         [&count](const Instruction& linkage_instruction) {
     48           uint32_t last_operand = linkage_instruction.NumOperands() - 1;
     49           if (linkage_instruction.GetSingleWordOperand(last_operand) ==
     50               SpvLinkageTypeExport) {
     51             count = kMustKeep;
     52           }
     53         });
     54 
     55     if (count != kMustKeep) {
     56       // If we don't have to keep the instruction for other reasons, then look
     57       // at the uses and count the number of real references.
     58       count = 0;
     59       get_def_use_mgr()->ForEachUser(result_id, [&count](Instruction* user) {
     60         if (!IsAnnotationInst(user->opcode()) && user->opcode() != SpvOpName) {
     61           ++count;
     62         }
     63       });
     64     }
     65     reference_count_[result_id] = count;
     66     if (count == 0) {
     67       ids_to_remove.push_back(result_id);
     68     }
     69   }
     70 
     71   // Remove all of the variables that have a reference count of 0.
     72   bool modified = false;
     73   if (!ids_to_remove.empty()) {
     74     modified = true;
     75     for (auto result_id : ids_to_remove) {
     76       DeleteVariable(result_id);
     77     }
     78   }
     79   return (modified ? Status::SuccessWithChange : Status::SuccessWithoutChange);
     80 }
     81 
     82 void DeadVariableElimination::DeleteVariable(uint32_t result_id) {
     83   Instruction* inst = get_def_use_mgr()->GetDef(result_id);
     84   assert(inst->opcode() == SpvOpVariable &&
     85          "Should not be trying to delete anything other than an OpVariable.");
     86 
     87   // Look for an initializer that references another variable.  We need to know
     88   // if that variable can be deleted after the reference is removed.
     89   if (inst->NumOperands() == 4) {
     90     Instruction* initializer =
     91         get_def_use_mgr()->GetDef(inst->GetSingleWordOperand(3));
     92 
     93     // TODO: Handle OpSpecConstantOP which might be defined in terms of other
     94     // variables.  Will probably require a unified dead code pass that does all
     95     // instruction types.  (Issue 906)
     96     if (initializer->opcode() == SpvOpVariable) {
     97       uint32_t initializer_id = initializer->result_id();
     98       size_t& count = reference_count_[initializer_id];
     99       if (count != kMustKeep) {
    100         --count;
    101       }
    102 
    103       if (count == 0) {
    104         DeleteVariable(initializer_id);
    105       }
    106     }
    107   }
    108   context()->KillDef(result_id);
    109 }
    110 }  // namespace opt
    111 }  // namespace spvtools
    112