SPIRV-Tools/source/opt/strength_reduction_pass.cpp
Alan Baker 616908503d Improving the usability of the type manager. The type manager hashes
types. This allows the lookup of type declaration ids from arbitrarily
constructed types. Users should be cautious when dealing with non-unique
types (structs and potentially pointers) to get the exact id if
necessary.

* Changed the spec composite constant folder to handle ambiguous composites
* Added functionality to create necessary instructions for a type
* Added ability to remove ids from the type manager
2017-12-18 08:20:56 -05:00

200 lines
6.4 KiB
C++

// 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 "strength_reduction_pass.h"
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
#include "def_use_manager.h"
#include "ir_context.h"
#include "log.h"
#include "reflect.h"
namespace {
// Count the number of trailing zeros in the binary representation of
// |constVal|.
uint32_t CountTrailingZeros(uint32_t constVal) {
// Faster if we use the hardware count trailing zeros instruction.
// If not available, we could create a table.
uint32_t shiftAmount = 0;
while ((constVal & 1) == 0) {
++shiftAmount;
constVal = (constVal >> 1);
}
return shiftAmount;
}
// Return true if |val| is a power of 2.
bool IsPowerOf2(uint32_t val) {
// The idea is that the & will clear out the least
// significant 1 bit. If it is a power of 2, then
// there is exactly 1 bit set, and the value becomes 0.
if (val == 0) return false;
return ((val - 1) & val) == 0;
}
} // namespace
namespace spvtools {
namespace opt {
Pass::Status StrengthReductionPass::Process(ir::IRContext* c) {
InitializeProcessing(c);
// Initialize the member variables on a per module basis.
bool modified = false;
int32_type_id_ = 0;
uint32_type_id_ = 0;
std::memset(constant_ids_, 0, sizeof(constant_ids_));
FindIntTypesAndConstants();
modified = ScanFunctions();
return (modified ? Status::SuccessWithChange : Status::SuccessWithoutChange);
}
bool StrengthReductionPass::ReplaceMultiplyByPowerOf2(
ir::BasicBlock::iterator* inst) {
assert((*inst)->opcode() == SpvOp::SpvOpIMul &&
"Only works for multiplication of integers.");
bool modified = false;
// Currently only works on 32-bit integers.
if ((*inst)->type_id() != int32_type_id_ &&
(*inst)->type_id() != uint32_type_id_) {
return modified;
}
// Check the operands for a constant that is a power of 2.
for (int i = 0; i < 2; i++) {
uint32_t opId = (*inst)->GetSingleWordInOperand(i);
ir::Instruction* opInst = get_def_use_mgr()->GetDef(opId);
if (opInst->opcode() == SpvOp::SpvOpConstant) {
// We found a constant operand.
uint32_t constVal = opInst->GetSingleWordOperand(2);
if (IsPowerOf2(constVal)) {
modified = true;
uint32_t shiftAmount = CountTrailingZeros(constVal);
uint32_t shiftConstResultId = GetConstantId(shiftAmount);
// Create the new instruction.
uint32_t newResultId = TakeNextId();
std::vector<ir::Operand> newOperands;
newOperands.push_back((*inst)->GetInOperand(1 - i));
ir::Operand shiftOperand(spv_operand_type_t::SPV_OPERAND_TYPE_ID,
{shiftConstResultId});
newOperands.push_back(shiftOperand);
std::unique_ptr<ir::Instruction> newInstruction(
new ir::Instruction(context(), SpvOp::SpvOpShiftLeftLogical,
(*inst)->type_id(), newResultId, newOperands));
// Insert the new instruction and update the data structures.
(*inst) = (*inst).InsertBefore(std::move(newInstruction));
get_def_use_mgr()->AnalyzeInstDefUse(&*(*inst));
++(*inst);
context()->ReplaceAllUsesWith((*inst)->result_id(), newResultId);
// Remove the old instruction.
ir::Instruction* inst_to_delete = &*(*inst);
--(*inst);
context()->KillInst(inst_to_delete);
// We do not want to replace the instruction twice if both operands
// are constants that are a power of 2. So we break here.
break;
}
}
}
return modified;
}
void StrengthReductionPass::FindIntTypesAndConstants() {
analysis::Integer int32(32, true);
int32_type_id_ = context()->get_type_mgr()->GetId(&int32);
analysis::Integer uint32(32, false);
uint32_type_id_ = context()->get_type_mgr()->GetId(&uint32);
for (auto iter = get_module()->types_values_begin();
iter != get_module()->types_values_end(); ++iter) {
switch (iter->opcode()) {
case SpvOp::SpvOpConstant:
if (iter->type_id() == uint32_type_id_) {
uint32_t value = iter->GetSingleWordOperand(2);
if (value <= 32) constant_ids_[value] = iter->result_id();
}
break;
default:
break;
}
}
}
uint32_t StrengthReductionPass::GetConstantId(uint32_t val) {
assert(val <= 32 &&
"This function does not handle constants larger than 32.");
if (constant_ids_[val] == 0) {
if (uint32_type_id_ == 0) {
analysis::Integer uint(32, false);
uint32_type_id_ = context()->get_type_mgr()->GetTypeInstruction(&uint);
}
// Construct the constant.
uint32_t resultId = TakeNextId();
ir::Operand constant(spv_operand_type_t::SPV_OPERAND_TYPE_LITERAL_INTEGER,
{val});
std::unique_ptr<ir::Instruction> newConstant(
new ir::Instruction(context(), SpvOp::SpvOpConstant, uint32_type_id_,
resultId, {constant}));
get_module()->AddGlobalValue(std::move(newConstant));
// Notify the DefUseManager about this constant.
auto constantIter = --get_module()->types_values_end();
get_def_use_mgr()->AnalyzeInstDef(&*constantIter);
// Store the result id for next time.
constant_ids_[val] = resultId;
}
return constant_ids_[val];
}
bool StrengthReductionPass::ScanFunctions() {
// I did not use |ForEachInst| in the module because the function that acts on
// the instruction gets a pointer to the instruction. We cannot use that to
// insert a new instruction. I want an iterator.
bool modified = false;
for (auto& func : *get_module()) {
for (auto& bb : func) {
for (auto inst = bb.begin(); inst != bb.end(); ++inst) {
switch (inst->opcode()) {
case SpvOp::SpvOpIMul:
if (ReplaceMultiplyByPowerOf2(&inst)) modified = true;
break;
default:
break;
}
}
}
}
return modified;
}
} // namespace opt
} // namespace spvtools