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/*
* Copyright (C) 2014 The Android Open Source Project
*
* 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 "nodes.h"
#include "ssa_builder.h"
#include "utils/growable_array.h"
namespace art {
void HGraph::AddBlock(HBasicBlock* block) {
block->SetBlockId(blocks_.Size());
blocks_.Add(block);
}
void HGraph::FindBackEdges(ArenaBitVector* visited) {
ArenaBitVector visiting(arena_, blocks_.Size(), false);
VisitBlockForBackEdges(entry_block_, visited, &visiting);
}
void HGraph::RemoveDeadBlocks(const ArenaBitVector& visited) const {
for (size_t i = 0; i < blocks_.Size(); ++i) {
if (!visited.IsBitSet(i)) {
HBasicBlock* block = blocks_.Get(i);
for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
block->GetSuccessors().Get(j)->RemovePredecessor(block);
}
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
block->RemovePhi(it.Current()->AsPhi());
}
for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
block->RemoveInstruction(it.Current());
}
}
}
}
void HGraph::VisitBlockForBackEdges(HBasicBlock* block,
ArenaBitVector* visited,
ArenaBitVector* visiting) {
int id = block->GetBlockId();
if (visited->IsBitSet(id)) return;
visited->SetBit(id);
visiting->SetBit(id);
for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
HBasicBlock* successor = block->GetSuccessors().Get(i);
if (visiting->IsBitSet(successor->GetBlockId())) {
successor->AddBackEdge(block);
} else {
VisitBlockForBackEdges(successor, visited, visiting);
}
}
visiting->ClearBit(id);
}
void HGraph::BuildDominatorTree() {
ArenaBitVector visited(arena_, blocks_.Size(), false);
// (1) Find the back edges in the graph doing a DFS traversal.
FindBackEdges(&visited);
// (2) Remove blocks not visited during the initial DFS.
// Step (3) requires dead blocks to be removed from the
// predecessors list of live blocks.
RemoveDeadBlocks(visited);
// (3) Simplify the CFG now, so that we don't need to recompute
// dominators and the reverse post order.
SimplifyCFG();
// (4) Compute the immediate dominator of each block. We visit
// the successors of a block only when all its forward branches
// have been processed.
GrowableArray<size_t> visits(arena_, blocks_.Size());
visits.SetSize(blocks_.Size());
reverse_post_order_.Add(entry_block_);
for (size_t i = 0; i < entry_block_->GetSuccessors().Size(); i++) {
VisitBlockForDominatorTree(entry_block_->GetSuccessors().Get(i), entry_block_, &visits);
}
}
HBasicBlock* HGraph::FindCommonDominator(HBasicBlock* first, HBasicBlock* second) const {
ArenaBitVector visited(arena_, blocks_.Size(), false);
// Walk the dominator tree of the first block and mark the visited blocks.
while (first != nullptr) {
visited.SetBit(first->GetBlockId());
first = first->GetDominator();
}
// Walk the dominator tree of the second block until a marked block is found.
while (second != nullptr) {
if (visited.IsBitSet(second->GetBlockId())) {
return second;
}
second = second->GetDominator();
}
LOG(ERROR) << "Could not find common dominator";
return nullptr;
}
void HGraph::VisitBlockForDominatorTree(HBasicBlock* block,
HBasicBlock* predecessor,
GrowableArray<size_t>* visits) {
if (block->GetDominator() == nullptr) {
block->SetDominator(predecessor);
} else {
block->SetDominator(FindCommonDominator(block->GetDominator(), predecessor));
}
visits->Increment(block->GetBlockId());
// Once all the forward edges have been visited, we know the immediate
// dominator of the block. We can then start visiting its successors.
if (visits->Get(block->GetBlockId()) ==
block->GetPredecessors().Size() - block->NumberOfBackEdges()) {
reverse_post_order_.Add(block);
for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
VisitBlockForDominatorTree(block->GetSuccessors().Get(i), block, visits);
}
}
}
void HGraph::TransformToSSA() {
DCHECK(!reverse_post_order_.IsEmpty());
SsaBuilder ssa_builder(this);
ssa_builder.BuildSsa();
}
void HGraph::SplitCriticalEdge(HBasicBlock* block, HBasicBlock* successor) {
// Insert a new node between `block` and `successor` to split the
// critical edge.
HBasicBlock* new_block = new (arena_) HBasicBlock(this);
AddBlock(new_block);
new_block->AddInstruction(new (arena_) HGoto());
block->ReplaceSuccessor(successor, new_block);
new_block->AddSuccessor(successor);
if (successor->IsLoopHeader()) {
// If we split at a back edge boundary, make the new block the back edge.
HLoopInformation* info = successor->GetLoopInformation();
if (info->IsBackEdge(block)) {
info->RemoveBackEdge(block);
info->AddBackEdge(new_block);
}
}
}
void HGraph::SimplifyLoop(HBasicBlock* header) {
HLoopInformation* info = header->GetLoopInformation();
// If there are more than one back edge, make them branch to the same block that
// will become the only back edge. This simplifies finding natural loops in the
// graph.
if (info->NumberOfBackEdges() > 1) {
HBasicBlock* new_back_edge = new (arena_) HBasicBlock(this);
AddBlock(new_back_edge);
new_back_edge->AddInstruction(new (arena_) HGoto());
for (size_t pred = 0, e = info->GetBackEdges().Size(); pred < e; ++pred) {
HBasicBlock* back_edge = info->GetBackEdges().Get(pred);
back_edge->ReplaceSuccessor(header, new_back_edge);
}
info->ClearBackEdges();
info->AddBackEdge(new_back_edge);
new_back_edge->AddSuccessor(header);
}
// Make sure the loop has only one pre header. This simplifies SSA building by having
// to just look at the pre header to know which locals are initialized at entry of the
// loop.
size_t number_of_incomings = header->GetPredecessors().Size() - info->NumberOfBackEdges();
if (number_of_incomings != 1) {
HBasicBlock* pre_header = new (arena_) HBasicBlock(this);
AddBlock(pre_header);
pre_header->AddInstruction(new (arena_) HGoto());
ArenaBitVector back_edges(arena_, GetBlocks().Size(), false);
HBasicBlock* back_edge = info->GetBackEdges().Get(0);
for (size_t pred = 0; pred < header->GetPredecessors().Size(); ++pred) {
HBasicBlock* predecessor = header->GetPredecessors().Get(pred);
if (predecessor != back_edge) {
predecessor->ReplaceSuccessor(header, pre_header);
pred--;
}
}
pre_header->AddSuccessor(header);
}
}
void HGraph::SimplifyCFG() {
// Simplify the CFG for future analysis, and code generation:
// (1): Split critical edges.
// (2): Simplify loops by having only one back edge, and one preheader.
for (size_t i = 0; i < blocks_.Size(); ++i) {
HBasicBlock* block = blocks_.Get(i);
if (block->GetSuccessors().Size() > 1) {
for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
HBasicBlock* successor = block->GetSuccessors().Get(j);
if (successor->GetPredecessors().Size() > 1) {
SplitCriticalEdge(block, successor);
--j;
}
}
}
if (block->IsLoopHeader()) {
SimplifyLoop(block);
}
}
}
bool HGraph::FindNaturalLoops() const {
for (size_t i = 0; i < blocks_.Size(); ++i) {
HBasicBlock* block = blocks_.Get(i);
if (block->IsLoopHeader()) {
HLoopInformation* info = block->GetLoopInformation();
if (!info->Populate()) {
// Abort if the loop is non natural. We currently bailout in such cases.
return false;
}
}
}
return true;
}
void HLoopInformation::PopulateRecursive(HBasicBlock* block) {
if (blocks_.IsBitSet(block->GetBlockId())) {
return;
}
blocks_.SetBit(block->GetBlockId());
block->SetInLoop(this);
for (size_t i = 0, e = block->GetPredecessors().Size(); i < e; ++i) {
PopulateRecursive(block->GetPredecessors().Get(i));
}
}
bool HLoopInformation::Populate() {
DCHECK_EQ(GetBackEdges().Size(), 1u);
HBasicBlock* back_edge = GetBackEdges().Get(0);
DCHECK(back_edge->GetDominator() != nullptr);
if (!header_->Dominates(back_edge)) {
// This loop is not natural. Do not bother going further.
return false;
}
// Populate this loop: starting with the back edge, recursively add predecessors
// that are not already part of that loop. Set the header as part of the loop
// to end the recursion.
// This is a recursive implementation of the algorithm described in
// "Advanced Compiler Design & Implementation" (Muchnick) p192.
blocks_.SetBit(header_->GetBlockId());
PopulateRecursive(back_edge);
return true;
}
HBasicBlock* HLoopInformation::GetPreHeader() const {
DCHECK_EQ(header_->GetPredecessors().Size(), 2u);
return header_->GetDominator();
}
bool HLoopInformation::Contains(const HBasicBlock& block) const {
return blocks_.IsBitSet(block.GetBlockId());
}
bool HLoopInformation::IsIn(const HLoopInformation& other) const {
return other.blocks_.IsBitSet(header_->GetBlockId());
}
bool HBasicBlock::Dominates(HBasicBlock* other) const {
// Walk up the dominator tree from `other`, to find out if `this`
// is an ancestor.
HBasicBlock* current = other;
while (current != nullptr) {
if (current == this) {
return true;
}
current = current->GetDominator();
}
return false;
}
void HBasicBlock::InsertInstructionBefore(HInstruction* instruction, HInstruction* cursor) {
DCHECK(cursor->AsPhi() == nullptr);
DCHECK(instruction->AsPhi() == nullptr);
DCHECK_EQ(instruction->GetId(), -1);
DCHECK_NE(cursor->GetId(), -1);
DCHECK_EQ(cursor->GetBlock(), this);
DCHECK(!instruction->IsControlFlow());
instruction->next_ = cursor;
instruction->previous_ = cursor->previous_;
cursor->previous_ = instruction;
if (GetFirstInstruction() == cursor) {
instructions_.first_instruction_ = instruction;
} else {
instruction->previous_->next_ = instruction;
}
instruction->SetBlock(this);
instruction->SetId(GetGraph()->GetNextInstructionId());
}
static void Add(HInstructionList* instruction_list,
HBasicBlock* block,
HInstruction* instruction) {
DCHECK(instruction->GetBlock() == nullptr);
DCHECK_EQ(instruction->GetId(), -1);
instruction->SetBlock(block);
instruction->SetId(block->GetGraph()->GetNextInstructionId());
instruction_list->AddInstruction(instruction);
}
void HBasicBlock::AddInstruction(HInstruction* instruction) {
Add(&instructions_, this, instruction);
}
void HBasicBlock::AddPhi(HPhi* phi) {
Add(&phis_, this, phi);
}
static void Remove(HInstructionList* instruction_list,
HBasicBlock* block,
HInstruction* instruction) {
DCHECK_EQ(block, instruction->GetBlock());
DCHECK(instruction->GetUses() == nullptr);
DCHECK(instruction->GetEnvUses() == nullptr);
instruction->SetBlock(nullptr);
instruction_list->RemoveInstruction(instruction);
for (size_t i = 0; i < instruction->InputCount(); i++) {
instruction->InputAt(i)->RemoveUser(instruction, i);
}
}
void HBasicBlock::RemoveInstruction(HInstruction* instruction) {
Remove(&instructions_, this, instruction);
}
void HBasicBlock::RemovePhi(HPhi* phi) {
Remove(&phis_, this, phi);
}
void HInstruction::RemoveUser(HInstruction* user, size_t input_index) {
HUseListNode<HInstruction>* previous = nullptr;
HUseListNode<HInstruction>* current = uses_;
while (current != nullptr) {
if (current->GetUser() == user && current->GetIndex() == input_index) {
if (previous == NULL) {
uses_ = current->GetTail();
} else {
previous->SetTail(current->GetTail());
}
}
previous = current;
current = current->GetTail();
}
}
void HInstructionList::AddInstruction(HInstruction* instruction) {
if (first_instruction_ == nullptr) {
DCHECK(last_instruction_ == nullptr);
first_instruction_ = last_instruction_ = instruction;
} else {
last_instruction_->next_ = instruction;
instruction->previous_ = last_instruction_;
last_instruction_ = instruction;
}
for (size_t i = 0; i < instruction->InputCount(); i++) {
instruction->InputAt(i)->AddUseAt(instruction, i);
}
}
void HInstructionList::RemoveInstruction(HInstruction* instruction) {
if (instruction->previous_ != nullptr) {
instruction->previous_->next_ = instruction->next_;
}
if (instruction->next_ != nullptr) {
instruction->next_->previous_ = instruction->previous_;
}
if (instruction == first_instruction_) {
first_instruction_ = instruction->next_;
}
if (instruction == last_instruction_) {
last_instruction_ = instruction->previous_;
}
}
void HInstruction::ReplaceWith(HInstruction* other) {
DCHECK(other != nullptr);
for (HUseIterator<HInstruction> it(GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* current = it.Current();
HInstruction* user = current->GetUser();
size_t input_index = current->GetIndex();
user->SetRawInputAt(input_index, other);
other->AddUseAt(user, input_index);
}
for (HUseIterator<HEnvironment> it(GetEnvUses()); !it.Done(); it.Advance()) {
HUseListNode<HEnvironment>* current = it.Current();
HEnvironment* user = current->GetUser();
size_t input_index = current->GetIndex();
user->SetRawEnvAt(input_index, other);
other->AddEnvUseAt(user, input_index);
}
uses_ = nullptr;
env_uses_ = nullptr;
}
void HPhi::AddInput(HInstruction* input) {
DCHECK(input->GetBlock() != nullptr);
inputs_.Add(input);
input->AddUseAt(this, inputs_.Size() - 1);
}
#define DEFINE_ACCEPT(name) \
void H##name::Accept(HGraphVisitor* visitor) { \
visitor->Visit##name(this); \
}
FOR_EACH_INSTRUCTION(DEFINE_ACCEPT)
#undef DEFINE_ACCEPT
void HGraphVisitor::VisitInsertionOrder() {
const GrowableArray<HBasicBlock*>& blocks = graph_->GetBlocks();
for (size_t i = 0 ; i < blocks.Size(); i++) {
VisitBasicBlock(blocks.Get(i));
}
}
void HGraphVisitor::VisitBasicBlock(HBasicBlock* block) {
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
it.Current()->Accept(this);
}
for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
it.Current()->Accept(this);
}
}
} // namespace art