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// Copyright (c) 2017 Google 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.
#include "source/opt/dead_variable_elimination.h"
#include <vector>
#include "source/opt/ir_context.h"
#include "source/opt/reflect.h"
namespace spvtools {
namespace opt {
// This optimization removes global variables that are not needed because they
// are definitely not accessed.
Pass::Status DeadVariableElimination::Process() {
// The algorithm will compute the reference count for every global variable.
// Anything with a reference count of 0 will then be deleted. For variables
// that might have references that are not explicit in this context, we use
// the value kMustKeep as the reference count.
std::vector<uint32_t> ids_to_remove;
// Get the reference count for all of the global OpVariable instructions.
for (auto& inst : context()->types_values()) {
if (inst.opcode() != SpvOp::SpvOpVariable) {
continue;
}
size_t count = 0;
uint32_t result_id = inst.result_id();
// Check the linkage. If it is exported, it could be reference somewhere
// else, so we must keep the variable around.
get_decoration_mgr()->ForEachDecoration(
result_id, SpvDecorationLinkageAttributes,
[&count](const Instruction& linkage_instruction) {
uint32_t last_operand = linkage_instruction.NumOperands() - 1;
if (linkage_instruction.GetSingleWordOperand(last_operand) ==
SpvLinkageTypeExport) {
count = kMustKeep;
}
});
if (count != kMustKeep) {
// If we don't have to keep the instruction for other reasons, then look
// at the uses and count the number of real references.
count = 0;
get_def_use_mgr()->ForEachUser(result_id, [&count](Instruction* user) {
if (!IsAnnotationInst(user->opcode()) && user->opcode() != SpvOpName) {
++count;
}
});
}
reference_count_[result_id] = count;
if (count == 0) {
ids_to_remove.push_back(result_id);
}
}
// Remove all of the variables that have a reference count of 0.
bool modified = false;
if (!ids_to_remove.empty()) {
modified = true;
for (auto result_id : ids_to_remove) {
DeleteVariable(result_id);
}
}
return (modified ? Status::SuccessWithChange : Status::SuccessWithoutChange);
}
void DeadVariableElimination::DeleteVariable(uint32_t result_id) {
Instruction* inst = get_def_use_mgr()->GetDef(result_id);
assert(inst->opcode() == SpvOpVariable &&
"Should not be trying to delete anything other than an OpVariable.");
// Look for an initializer that references another variable. We need to know
// if that variable can be deleted after the reference is removed.
if (inst->NumOperands() == 4) {
Instruction* initializer =
get_def_use_mgr()->GetDef(inst->GetSingleWordOperand(3));
// TODO: Handle OpSpecConstantOP which might be defined in terms of other
// variables. Will probably require a unified dead code pass that does all
// instruction types. (Issue 906)
if (initializer->opcode() == SpvOpVariable) {
uint32_t initializer_id = initializer->result_id();
size_t& count = reference_count_[initializer_id];
if (count != kMustKeep) {
--count;
}
if (count == 0) {
DeleteVariable(initializer_id);
}
}
}
context()->KillDef(result_id);
}
} // namespace opt
} // namespace spvtools
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