SPIRV-Tools/source/opt/basic_block.cpp
Diego Novillo 735d8a579e SSA rewrite pass.
This pass replaces the load/store elimination passes.  It implements the
SSA re-writing algorithm proposed in

     Simple and Efficient Construction of Static Single Assignment Form.
     Braun M., Buchwald S., Hack S., Leißa R., Mallon C., Zwinkau A. (2013)
     In: Jhala R., De Bosschere K. (eds)
     Compiler Construction. CC 2013.
     Lecture Notes in Computer Science, vol 7791.
     Springer, Berlin, Heidelberg

     https://link.springer.com/chapter/10.1007/978-3-642-37051-9_6

In contrast to common eager algorithms based on dominance and dominance
frontier information, this algorithm works backwards from load operations.

When a target variable is loaded, it queries the variable's reaching
definition.  If the reaching definition is unknown at the current location,
it searches backwards in the CFG, inserting Phi instructions at join points
in the CFG along the way until it finds the desired store instruction.

The algorithm avoids repeated lookups using memoization.

For reducible CFGs, which are a superset of the structured CFGs in SPIRV,
this algorithm is proven to produce minimal SSA.  That is, it inserts the
minimal number of Phi instructions required to ensure the SSA property, but
some Phi instructions may be dead
(https://en.wikipedia.org/wiki/Static_single_assignment_form).
2018-03-20 20:56:55 -04:00

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// Copyright (c) 2016 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 "basic_block.h"
#include "function.h"
#include "ir_context.h"
#include "module.h"
#include "reflect.h"
#include "make_unique.h"
#include <ostream>
namespace spvtools {
namespace ir {
namespace {
const uint32_t kLoopMergeContinueBlockIdInIdx = 1;
const uint32_t kLoopMergeMergeBlockIdInIdx = 0;
const uint32_t kSelectionMergeMergeBlockIdInIdx = 0;
} // namespace
BasicBlock* BasicBlock::Clone(IRContext* context) const {
BasicBlock* clone = new BasicBlock(
std::unique_ptr<Instruction>(GetLabelInst()->Clone(context)));
for (const auto& inst : insts_)
// Use the incoming context
clone->AddInstruction(std::unique_ptr<Instruction>(inst.Clone(context)));
return clone;
}
const Instruction* BasicBlock::GetMergeInst() const {
const Instruction* result = nullptr;
// If it exists, the merge instruction immediately precedes the
// terminator.
auto iter = ctail();
if (iter != cbegin()) {
--iter;
const auto opcode = iter->opcode();
if (opcode == SpvOpLoopMerge || opcode == SpvOpSelectionMerge) {
result = &*iter;
}
}
return result;
}
Instruction* BasicBlock::GetMergeInst() {
Instruction* result = nullptr;
// If it exists, the merge instruction immediately precedes the
// terminator.
auto iter = tail();
if (iter != begin()) {
--iter;
const auto opcode = iter->opcode();
if (opcode == SpvOpLoopMerge || opcode == SpvOpSelectionMerge) {
result = &*iter;
}
}
return result;
}
const Instruction* BasicBlock::GetLoopMergeInst() const {
if (auto* merge = GetMergeInst()) {
if (merge->opcode() == SpvOpLoopMerge) {
return merge;
}
}
return nullptr;
}
Instruction* BasicBlock::GetLoopMergeInst() {
if (auto* merge = GetMergeInst()) {
if (merge->opcode() == SpvOpLoopMerge) {
return merge;
}
}
return nullptr;
}
void BasicBlock::KillAllInsts(bool killLabel) {
ForEachInst([killLabel](ir::Instruction* ip) {
if (killLabel || ip->opcode() != SpvOpLabel) {
ip->context()->KillInst(ip);
}
});
}
void BasicBlock::ForEachSuccessorLabel(
const std::function<void(const uint32_t)>& f) const {
const auto br = &insts_.back();
switch (br->opcode()) {
case SpvOpBranch: {
f(br->GetOperand(0).words[0]);
} break;
case SpvOpBranchConditional:
case SpvOpSwitch: {
bool is_first = true;
br->ForEachInId([&is_first, &f](const uint32_t* idp) {
if (!is_first) f(*idp);
is_first = false;
});
} break;
default:
break;
}
}
void BasicBlock::ForEachSuccessorLabel(
const std::function<void(uint32_t*)>& f) {
auto br = &insts_.back();
switch (br->opcode()) {
case SpvOpBranch: {
uint32_t tmp_id = br->GetOperand(0).words[0];
f(&tmp_id);
if (tmp_id != br->GetOperand(0).words[0]) br->SetOperand(0, {tmp_id});
} break;
case SpvOpBranchConditional:
case SpvOpSwitch: {
bool is_first = true;
br->ForEachInId([&is_first, &f](uint32_t* idp) {
if (!is_first) f(idp);
is_first = false;
});
} break;
default:
break;
}
}
bool BasicBlock::IsSuccessor(const ir::BasicBlock* block) const {
uint32_t succId = block->id();
bool isSuccessor = false;
ForEachSuccessorLabel([&isSuccessor, succId](const uint32_t label) {
if (label == succId) isSuccessor = true;
});
return isSuccessor;
}
void BasicBlock::ForMergeAndContinueLabel(
const std::function<void(const uint32_t)>& f) {
auto ii = insts_.end();
--ii;
if (ii == insts_.begin()) return;
--ii;
if (ii->opcode() == SpvOpSelectionMerge || ii->opcode() == SpvOpLoopMerge) {
ii->ForEachInId([&f](const uint32_t* idp) { f(*idp); });
}
}
uint32_t BasicBlock::MergeBlockIdIfAny() const {
auto merge_ii = cend();
--merge_ii;
uint32_t mbid = 0;
if (merge_ii != cbegin()) {
--merge_ii;
if (merge_ii->opcode() == SpvOpLoopMerge) {
mbid = merge_ii->GetSingleWordInOperand(kLoopMergeMergeBlockIdInIdx);
} else if (merge_ii->opcode() == SpvOpSelectionMerge) {
mbid = merge_ii->GetSingleWordInOperand(kSelectionMergeMergeBlockIdInIdx);
}
}
return mbid;
}
uint32_t BasicBlock::ContinueBlockIdIfAny() const {
auto merge_ii = cend();
--merge_ii;
uint32_t cbid = 0;
if (merge_ii != cbegin()) {
--merge_ii;
if (merge_ii->opcode() == SpvOpLoopMerge) {
cbid = merge_ii->GetSingleWordInOperand(kLoopMergeContinueBlockIdInIdx);
}
}
return cbid;
}
std::ostream& operator<<(std::ostream& str, const BasicBlock& block) {
str << block.PrettyPrint();
return str;
}
std::string BasicBlock::PrettyPrint(uint32_t options) const {
std::ostringstream str;
ForEachInst([&str, options](const ir::Instruction* inst) {
str << inst->PrettyPrint(options);
if (!IsTerminatorInst(inst->opcode())) {
str << std::endl;
}
});
return str.str();
}
BasicBlock* BasicBlock::SplitBasicBlock(IRContext* context, uint32_t label_id,
iterator iter) {
assert(!insts_.empty());
BasicBlock* new_block = new BasicBlock(MakeUnique<Instruction>(
context, SpvOpLabel, 0, label_id, std::initializer_list<ir::Operand>{}));
new_block->insts_.Splice(new_block->end(), &insts_, iter, end());
new_block->SetParent(GetParent());
if (context->AreAnalysesValid(ir::IRContext::kAnalysisInstrToBlockMapping)) {
new_block->ForEachInst([new_block, context](ir::Instruction* inst) {
context->set_instr_block(inst, new_block);
});
}
return new_block;
}
} // namespace ir
} // namespace spvtools