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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 "ssa_liveness_analysis.h"
#include "base/bit_vector-inl.h"
#include "code_generator.h"
#include "nodes.h"
namespace art {
void SsaLivenessAnalysis::Analyze() {
LinearizeGraph();
NumberInstructions();
ComputeLiveness();
}
static bool IsLoopExit(HLoopInformation* current, HLoopInformation* to) {
// `to` is either not part of a loop, or `current` is an inner loop of `to`.
return to == nullptr || (current != to && current->IsIn(*to));
}
static bool IsLoop(HLoopInformation* info) {
return info != nullptr;
}
static bool InSameLoop(HLoopInformation* first_loop, HLoopInformation* second_loop) {
return first_loop == second_loop;
}
static bool IsInnerLoop(HLoopInformation* outer, HLoopInformation* inner) {
return (inner != outer)
&& (inner != nullptr)
&& (outer != nullptr)
&& inner->IsIn(*outer);
}
static void VisitBlockForLinearization(HBasicBlock* block,
GrowableArray<HBasicBlock*>* order,
ArenaBitVector* visited) {
if (visited->IsBitSet(block->GetBlockId())) {
return;
}
visited->SetBit(block->GetBlockId());
size_t number_of_successors = block->GetSuccessors().Size();
if (number_of_successors == 0) {
// Nothing to do.
} else if (number_of_successors == 1) {
VisitBlockForLinearization(block->GetSuccessors().Get(0), order, visited);
} else {
DCHECK_EQ(number_of_successors, 2u);
HBasicBlock* first_successor = block->GetSuccessors().Get(0);
HBasicBlock* second_successor = block->GetSuccessors().Get(1);
HLoopInformation* my_loop = block->GetLoopInformation();
HLoopInformation* first_loop = first_successor->GetLoopInformation();
HLoopInformation* second_loop = second_successor->GetLoopInformation();
if (!IsLoop(my_loop)) {
// Nothing to do. Current order is fine.
} else if (IsLoopExit(my_loop, second_loop) && InSameLoop(my_loop, first_loop)) {
// Visit the loop exit first in post order.
std::swap(first_successor, second_successor);
} else if (IsInnerLoop(my_loop, first_loop) && !IsInnerLoop(my_loop, second_loop)) {
// Visit the inner loop last in post order.
std::swap(first_successor, second_successor);
}
VisitBlockForLinearization(first_successor, order, visited);
VisitBlockForLinearization(second_successor, order, visited);
}
order->Add(block);
}
void SsaLivenessAnalysis::LinearizeGraph() {
// For simplicity of the implementation, we create post linear order. The order for
// computing live ranges is the reverse of that order.
ArenaBitVector visited(graph_.GetArena(), graph_.GetBlocks().Size(), false);
VisitBlockForLinearization(graph_.GetEntryBlock(), &linear_post_order_, &visited);
}
void SsaLivenessAnalysis::NumberInstructions() {
int ssa_index = 0;
size_t lifetime_position = 0;
// Each instruction gets a lifetime position, and a block gets a lifetime
// start and end position. Non-phi instructions have a distinct lifetime position than
// the block they are in. Phi instructions have the lifetime start of their block as
// lifetime position.
//
// Because the register allocator will insert moves in the graph, we need
// to differentiate between the start and end of an instruction. Adding 2 to
// the lifetime position for each instruction ensures the start of an
// instruction is different than the end of the previous instruction.
HGraphVisitor* location_builder = codegen_->GetLocationBuilder();
for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
block->SetLifetimeStart(lifetime_position);
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
HInstruction* current = it.Current();
current->Accept(location_builder);
LocationSummary* locations = current->GetLocations();
if (locations != nullptr && locations->Out().IsValid()) {
instructions_from_ssa_index_.Add(current);
current->SetSsaIndex(ssa_index++);
current->SetLiveInterval(
LiveInterval::MakeInterval(graph_.GetArena(), current->GetType(), current));
}
current->SetLifetimePosition(lifetime_position);
}
lifetime_position += 2;
// Add a null marker to notify we are starting a block.
instructions_from_lifetime_position_.Add(nullptr);
for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
HInstruction* current = it.Current();
current->Accept(codegen_->GetLocationBuilder());
LocationSummary* locations = current->GetLocations();
if (locations != nullptr && locations->Out().IsValid()) {
instructions_from_ssa_index_.Add(current);
current->SetSsaIndex(ssa_index++);
current->SetLiveInterval(
LiveInterval::MakeInterval(graph_.GetArena(), current->GetType(), current));
}
instructions_from_lifetime_position_.Add(current);
current->SetLifetimePosition(lifetime_position);
lifetime_position += 2;
}
block->SetLifetimeEnd(lifetime_position);
}
number_of_ssa_values_ = ssa_index;
}
void SsaLivenessAnalysis::ComputeLiveness() {
for (HLinearOrderIterator it(*this); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
block_infos_.Put(
block->GetBlockId(),
new (graph_.GetArena()) BlockInfo(graph_.GetArena(), *block, number_of_ssa_values_));
}
// Compute the live ranges, as well as the initial live_in, live_out, and kill sets.
// This method does not handle backward branches for the sets, therefore live_in
// and live_out sets are not yet correct.
ComputeLiveRanges();
// Do a fixed point calculation to take into account backward branches,
// that will update live_in of loop headers, and therefore live_out and live_in
// of blocks in the loop.
ComputeLiveInAndLiveOutSets();
}
void SsaLivenessAnalysis::ComputeLiveRanges() {
// Do a post order visit, adding inputs of instructions live in the block where
// that instruction is defined, and killing instructions that are being visited.
for (HLinearPostOrderIterator it(*this); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
BitVector* kill = GetKillSet(*block);
BitVector* live_in = GetLiveInSet(*block);
// Set phi inputs of successors of this block corresponding to this block
// as live_in.
for (size_t i = 0, e = block->GetSuccessors().Size(); i < e; ++i) {
HBasicBlock* successor = block->GetSuccessors().Get(i);
live_in->Union(GetLiveInSet(*successor));
size_t phi_input_index = successor->GetPredecessorIndexOf(block);
for (HInstructionIterator it(successor->GetPhis()); !it.Done(); it.Advance()) {
HInstruction* phi = it.Current();
HInstruction* input = phi->InputAt(phi_input_index);
input->GetLiveInterval()->AddPhiUse(phi, phi_input_index, block);
// A phi input whose last user is the phi dies at the end of the predecessor block,
// and not at the phi's lifetime position.
live_in->SetBit(input->GetSsaIndex());
}
}
// Add a range that covers this block to all instructions live_in because of successors.
// Instructions defined in this block will have their start of the range adjusted.
for (uint32_t idx : live_in->Indexes()) {
HInstruction* current = instructions_from_ssa_index_.Get(idx);
current->GetLiveInterval()->AddRange(block->GetLifetimeStart(), block->GetLifetimeEnd());
}
for (HBackwardInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
HInstruction* current = it.Current();
if (current->HasSsaIndex()) {
// Kill the instruction and shorten its interval.
kill->SetBit(current->GetSsaIndex());
live_in->ClearBit(current->GetSsaIndex());
current->GetLiveInterval()->SetFrom(current->GetLifetimePosition());
}
// All inputs of an instruction must be live.
for (size_t i = 0, e = current->InputCount(); i < e; ++i) {
HInstruction* input = current->InputAt(i);
// Some instructions 'inline' their inputs, that is they do not need
// to be materialized.
if (input->HasSsaIndex()) {
live_in->SetBit(input->GetSsaIndex());
input->GetLiveInterval()->AddUse(current, i, false);
}
}
if (current->HasEnvironment()) {
// All instructions in the environment must be live.
GrowableArray<HInstruction*>* environment = current->GetEnvironment()->GetVRegs();
for (size_t i = 0, e = environment->Size(); i < e; ++i) {
HInstruction* instruction = environment->Get(i);
if (instruction != nullptr) {
DCHECK(instruction->HasSsaIndex());
live_in->SetBit(instruction->GetSsaIndex());
instruction->GetLiveInterval()->AddUse(current, i, true);
}
}
}
}
// Kill phis defined in this block.
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
HInstruction* current = it.Current();
if (current->HasSsaIndex()) {
kill->SetBit(current->GetSsaIndex());
live_in->ClearBit(current->GetSsaIndex());
LiveInterval* interval = current->GetLiveInterval();
DCHECK((interval->GetFirstRange() == nullptr)
|| (interval->GetStart() == current->GetLifetimePosition()));
interval->SetFrom(current->GetLifetimePosition());
}
}
if (block->IsLoopHeader()) {
HBasicBlock* back_edge = block->GetLoopInformation()->GetBackEdges().Get(0);
// For all live_in instructions at the loop header, we need to create a range
// that covers the full loop.
for (uint32_t idx : live_in->Indexes()) {
HInstruction* current = instructions_from_ssa_index_.Get(idx);
current->GetLiveInterval()->AddLoopRange(block->GetLifetimeStart(),
back_edge->GetLifetimeEnd());
}
}
}
}
void SsaLivenessAnalysis::ComputeLiveInAndLiveOutSets() {
bool changed;
do {
changed = false;
for (HPostOrderIterator it(graph_); !it.Done(); it.Advance()) {
const HBasicBlock& block = *it.Current();
// The live_in set depends on the kill set (which does not
// change in this loop), and the live_out set. If the live_out
// set does not change, there is no need to update the live_in set.
if (UpdateLiveOut(block) && UpdateLiveIn(block)) {
changed = true;
}
}
} while (changed);
}
bool SsaLivenessAnalysis::UpdateLiveOut(const HBasicBlock& block) {
BitVector* live_out = GetLiveOutSet(block);
bool changed = false;
// The live_out set of a block is the union of live_in sets of its successors.
for (size_t i = 0, e = block.GetSuccessors().Size(); i < e; ++i) {
HBasicBlock* successor = block.GetSuccessors().Get(i);
if (live_out->Union(GetLiveInSet(*successor))) {
changed = true;
}
}
return changed;
}
bool SsaLivenessAnalysis::UpdateLiveIn(const HBasicBlock& block) {
BitVector* live_out = GetLiveOutSet(block);
BitVector* kill = GetKillSet(block);
BitVector* live_in = GetLiveInSet(block);
// If live_out is updated (because of backward branches), we need to make
// sure instructions in live_out are also in live_in, unless they are killed
// by this block.
return live_in->UnionIfNotIn(live_out, kill);
}
int LiveInterval::FindFirstRegisterHint(size_t* free_until) const {
if (GetParent() == this && defined_by_ != nullptr) {
// This is the first interval for the instruction. Try to find
// a register based on its definition.
DCHECK_EQ(defined_by_->GetLiveInterval(), this);
int hint = FindHintAtDefinition();
if (hint != kNoRegister && free_until[hint] > GetStart()) {
return hint;
}
}
UsePosition* use = first_use_;
size_t start = GetStart();
size_t end = GetEnd();
while (use != nullptr && use->GetPosition() <= end) {
size_t use_position = use->GetPosition();
if (use_position >= start && !use->GetIsEnvironment()) {
HInstruction* user = use->GetUser();
size_t input_index = use->GetInputIndex();
if (user->IsPhi()) {
// If the phi has a register, try to use the same.
Location phi_location = user->GetLiveInterval()->ToLocation();
if (SameRegisterKind(phi_location) && free_until[phi_location.reg()] >= use_position) {
return phi_location.reg();
}
const GrowableArray<HBasicBlock*>& predecessors = user->GetBlock()->GetPredecessors();
// If the instruction dies at the phi assignment, we can try having the
// same register.
if (end == predecessors.Get(input_index)->GetLifetimeEnd()) {
for (size_t i = 0, e = user->InputCount(); i < e; ++i) {
if (i == input_index) {
continue;
}
HInstruction* input = user->InputAt(i);
Location location = input->GetLiveInterval()->GetLocationAt(
predecessors.Get(i)->GetLifetimeEnd() - 1);
if (location.IsRegister() && free_until[location.reg()] >= use_position) {
return location.reg();
}
}
}
} else {
// If the instruction is expected in a register, try to use it.
LocationSummary* locations = user->GetLocations();
Location expected = locations->InAt(use->GetInputIndex());
// We use the user's lifetime position - 1 (and not `use_position`) because the
// register is blocked at the beginning of the user.
size_t position = user->GetLifetimePosition() - 1;
if (SameRegisterKind(expected) && free_until[expected.reg()] >= position) {
return expected.reg();
}
}
}
use = use->GetNext();
}
return kNoRegister;
}
int LiveInterval::FindHintAtDefinition() const {
if (defined_by_->IsPhi()) {
// Try to use the same register as one of the inputs.
const GrowableArray<HBasicBlock*>& predecessors = defined_by_->GetBlock()->GetPredecessors();
for (size_t i = 0, e = defined_by_->InputCount(); i < e; ++i) {
HInstruction* input = defined_by_->InputAt(i);
size_t end = predecessors.Get(i)->GetLifetimeEnd();
const LiveInterval& input_interval = input->GetLiveInterval()->GetIntervalAt(end - 1);
if (input_interval.GetEnd() == end) {
// If the input dies at the end of the predecessor, we know its register can
// be reused.
Location input_location = input_interval.ToLocation();
if (SameRegisterKind(input_location)) {
return input_location.reg();
}
}
}
} else {
LocationSummary* locations = GetDefinedBy()->GetLocations();
Location out = locations->Out();
if (out.IsUnallocated() && out.GetPolicy() == Location::kSameAsFirstInput) {
// Try to use the same register as the first input.
const LiveInterval& input_interval =
GetDefinedBy()->InputAt(0)->GetLiveInterval()->GetIntervalAt(GetStart() - 1);
if (input_interval.GetEnd() == GetStart()) {
// If the input dies at the start of this instruction, we know its register can
// be reused.
Location location = input_interval.ToLocation();
if (SameRegisterKind(location)) {
return location.reg();
}
}
}
}
return kNoRegister;
}
bool LiveInterval::SameRegisterKind(Location other) const {
return IsFloatingPoint()
? other.IsFpuRegister()
: other.IsRegister();
}
bool LiveInterval::NeedsTwoSpillSlots() const {
return type_ == Primitive::kPrimLong || type_ == Primitive::kPrimDouble;
}
Location LiveInterval::ToLocation() const {
if (HasRegister()) {
return IsFloatingPoint()
? Location::FpuRegisterLocation(GetRegister())
: Location::RegisterLocation(GetRegister());
} else {
HInstruction* defined_by = GetParent()->GetDefinedBy();
if (defined_by->IsConstant()) {
return defined_by->GetLocations()->Out();
} else if (GetParent()->HasSpillSlot()) {
if (NeedsTwoSpillSlots()) {
return Location::DoubleStackSlot(GetParent()->GetSpillSlot());
} else {
return Location::StackSlot(GetParent()->GetSpillSlot());
}
} else {
return Location();
}
}
}
Location LiveInterval::GetLocationAt(size_t position) const {
return GetIntervalAt(position).ToLocation();
}
const LiveInterval& LiveInterval::GetIntervalAt(size_t position) const {
const LiveInterval* current = this;
while (!current->Covers(position)) {
current = current->GetNextSibling();
DCHECK(current != nullptr);
}
return *current;
}
} // namespace art