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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 "gvn.h"
#include "base/arena_bit_vector.h"
#include "base/bit_vector-inl.h"
#include "base/scoped_arena_allocator.h"
#include "base/scoped_arena_containers.h"
#include "base/utils.h"
#include "side_effects_analysis.h"
namespace art {
/**
* A ValueSet holds instructions that can replace other instructions. It is updated
* through the `Add` method, and the `Kill` method. The `Kill` method removes
* instructions that are affected by the given side effect.
*
* The `Lookup` method returns an equivalent instruction to the given instruction
* if there is one in the set. In GVN, we would say those instructions have the
* same "number".
*/
class ValueSet : public ArenaObject<kArenaAllocGvn> {
public:
// Constructs an empty ValueSet which owns all its buckets.
explicit ValueSet(ScopedArenaAllocator* allocator)
: allocator_(allocator),
num_buckets_(kMinimumNumberOfBuckets),
buckets_(allocator->AllocArray<Node*>(num_buckets_, kArenaAllocGvn)),
buckets_owned_(allocator, num_buckets_, false, kArenaAllocGvn),
num_entries_(0u) {
// ArenaAllocator returns zeroed memory, so no need to set buckets to null.
DCHECK(IsPowerOfTwo(num_buckets_));
std::fill_n(buckets_, num_buckets_, nullptr);
buckets_owned_.SetInitialBits(num_buckets_);
}
// Copy constructor. Depending on the load factor, it will either make a deep
// copy (all buckets owned) or a shallow one (buckets pointing to the parent).
ValueSet(ScopedArenaAllocator* allocator, const ValueSet& other)
: allocator_(allocator),
num_buckets_(other.IdealBucketCount()),
buckets_(allocator->AllocArray<Node*>(num_buckets_, kArenaAllocGvn)),
buckets_owned_(allocator, num_buckets_, false, kArenaAllocGvn),
num_entries_(0u) {
// ArenaAllocator returns zeroed memory, so entries of buckets_ and
// buckets_owned_ are initialized to null and false, respectively.
DCHECK(IsPowerOfTwo(num_buckets_));
PopulateFromInternal(other);
}
// Erases all values in this set and populates it with values from `other`.
void PopulateFrom(const ValueSet& other) {
if (this == &other) {
return;
}
PopulateFromInternal(other);
}
// Returns true if `this` has enough buckets so that if `other` is copied into
// it, the load factor will not cross the upper threshold.
// If `exact_match` is set, true is returned only if `this` has the ideal
// number of buckets. Larger number of buckets is allowed otherwise.
bool CanHoldCopyOf(const ValueSet& other, bool exact_match) {
if (exact_match) {
return other.IdealBucketCount() == num_buckets_;
} else {
return other.IdealBucketCount() <= num_buckets_;
}
}
// Adds an instruction in the set.
void Add(HInstruction* instruction) {
DCHECK(Lookup(instruction) == nullptr);
size_t hash_code = HashCode(instruction);
size_t index = BucketIndex(hash_code);
if (!buckets_owned_.IsBitSet(index)) {
CloneBucket(index);
}
buckets_[index] = new (allocator_) Node(instruction, hash_code, buckets_[index]);
++num_entries_;
}
// If in the set, returns an equivalent instruction to the given instruction.
// Returns null otherwise.
HInstruction* Lookup(HInstruction* instruction) const {
size_t hash_code = HashCode(instruction);
size_t index = BucketIndex(hash_code);
for (Node* node = buckets_[index]; node != nullptr; node = node->GetNext()) {
if (node->GetHashCode() == hash_code) {
HInstruction* existing = node->GetInstruction();
if (existing->Equals(instruction)) {
return existing;
}
}
}
return nullptr;
}
// Returns whether instruction is in the set.
bool Contains(HInstruction* instruction) const {
size_t hash_code = HashCode(instruction);
size_t index = BucketIndex(hash_code);
for (Node* node = buckets_[index]; node != nullptr; node = node->GetNext()) {
if (node->GetInstruction() == instruction) {
return true;
}
}
return false;
}
// Removes all instructions in the set affected by the given side effects.
void Kill(SideEffects side_effects) {
DeleteAllImpureWhich([side_effects](Node* node) {
return node->GetInstruction()->GetSideEffects().MayDependOn(side_effects);
});
}
void Clear() {
num_entries_ = 0;
for (size_t i = 0; i < num_buckets_; ++i) {
buckets_[i] = nullptr;
}
buckets_owned_.SetInitialBits(num_buckets_);
}
// Updates this set by intersecting with instructions in a predecessor's set.
void IntersectWith(ValueSet* predecessor) {
if (IsEmpty()) {
return;
} else if (predecessor->IsEmpty()) {
Clear();
} else {
// Pure instructions do not need to be tested because only impure
// instructions can be killed.
DeleteAllImpureWhich([predecessor](Node* node) {
return !predecessor->Contains(node->GetInstruction());
});
}
}
bool IsEmpty() const { return num_entries_ == 0; }
size_t GetNumberOfEntries() const { return num_entries_; }
private:
// Copies all entries from `other` to `this`.
void PopulateFromInternal(const ValueSet& other) {
DCHECK_NE(this, &other);
DCHECK_GE(num_buckets_, other.IdealBucketCount());
if (num_buckets_ == other.num_buckets_) {
// Hash table remains the same size. We copy the bucket pointers and leave
// all buckets_owned_ bits false.
buckets_owned_.ClearAllBits();
memcpy(buckets_, other.buckets_, num_buckets_ * sizeof(Node*));
} else {
// Hash table size changes. We copy and rehash all entries, and set all
// buckets_owned_ bits to true.
std::fill_n(buckets_, num_buckets_, nullptr);
for (size_t i = 0; i < other.num_buckets_; ++i) {
for (Node* node = other.buckets_[i]; node != nullptr; node = node->GetNext()) {
size_t new_index = BucketIndex(node->GetHashCode());
buckets_[new_index] = node->Dup(allocator_, buckets_[new_index]);
}
}
buckets_owned_.SetInitialBits(num_buckets_);
}
num_entries_ = other.num_entries_;
}
class Node : public ArenaObject<kArenaAllocGvn> {
public:
Node(HInstruction* instruction, size_t hash_code, Node* next)
: instruction_(instruction), hash_code_(hash_code), next_(next) {}
size_t GetHashCode() const { return hash_code_; }
HInstruction* GetInstruction() const { return instruction_; }
Node* GetNext() const { return next_; }
void SetNext(Node* node) { next_ = node; }
Node* Dup(ScopedArenaAllocator* allocator, Node* new_next = nullptr) {
return new (allocator) Node(instruction_, hash_code_, new_next);
}
private:
HInstruction* const instruction_;
const size_t hash_code_;
Node* next_;
DISALLOW_COPY_AND_ASSIGN(Node);
};
// Creates our own copy of a bucket that is currently pointing to a parent.
// This algorithm can be called while iterating over the bucket because it
// preserves the order of entries in the bucket and will return the clone of
// the given 'iterator'.
Node* CloneBucket(size_t index, Node* iterator = nullptr) {
DCHECK(!buckets_owned_.IsBitSet(index));
Node* clone_current = nullptr;
Node* clone_previous = nullptr;
Node* clone_iterator = nullptr;
for (Node* node = buckets_[index]; node != nullptr; node = node->GetNext()) {
clone_current = node->Dup(allocator_, nullptr);
if (node == iterator) {
clone_iterator = clone_current;
}
if (clone_previous == nullptr) {
buckets_[index] = clone_current;
} else {
clone_previous->SetNext(clone_current);
}
clone_previous = clone_current;
}
buckets_owned_.SetBit(index);
return clone_iterator;
}
// Iterates over buckets with impure instructions (even indices) and deletes
// the ones on which 'cond' returns true.
template<typename Functor>
void DeleteAllImpureWhich(Functor cond) {
for (size_t i = 0; i < num_buckets_; i += 2) {
Node* node = buckets_[i];
Node* previous = nullptr;
if (node == nullptr) {
continue;
}
if (!buckets_owned_.IsBitSet(i)) {
// Bucket is not owned but maybe we won't need to change it at all.
// Iterate as long as the entries don't satisfy 'cond'.
while (node != nullptr) {
if (cond(node)) {
// We do need to delete an entry but we do not own the bucket.
// Clone the bucket, make sure 'previous' and 'node' point to
// the cloned entries and break.
previous = CloneBucket(i, previous);
node = (previous == nullptr) ? buckets_[i] : previous->GetNext();
break;
}
previous = node;
node = node->GetNext();
}
}
// By this point we either own the bucket and can start deleting entries,
// or we do not own it but no entries matched 'cond'.
DCHECK(buckets_owned_.IsBitSet(i) || node == nullptr);
// We iterate over the remainder of entries and delete those that match
// the given condition.
while (node != nullptr) {
Node* next = node->GetNext();
if (cond(node)) {
if (previous == nullptr) {
buckets_[i] = next;
} else {
previous->SetNext(next);
}
} else {
previous = node;
}
node = next;
}
}
}
// Computes a bucket count such that the load factor is reasonable.
// This is estimated as (num_entries_ * 1.5) and rounded up to nearest pow2.
size_t IdealBucketCount() const {
size_t bucket_count = RoundUpToPowerOfTwo(num_entries_ + (num_entries_ >> 1));
if (bucket_count > kMinimumNumberOfBuckets) {
return bucket_count;
} else {
return kMinimumNumberOfBuckets;
}
}
// Generates a hash code for an instruction.
size_t HashCode(HInstruction* instruction) const {
size_t hash_code = instruction->ComputeHashCode();
// Pure instructions are put into odd buckets to speed up deletion. Note that in the
// case of irreducible loops, we don't put pure instructions in odd buckets, as we
// need to delete them when entering the loop.
// ClinitCheck is treated as a pure instruction since it's only executed
// once.
bool pure = !instruction->GetSideEffects().HasDependencies() ||
instruction->IsClinitCheck();
if (!pure || instruction->GetBlock()->GetGraph()->HasIrreducibleLoops()) {
return (hash_code << 1) | 0;
} else {
return (hash_code << 1) | 1;
}
}
// Converts a hash code to a bucket index.
size_t BucketIndex(size_t hash_code) const {
return hash_code & (num_buckets_ - 1);
}
ScopedArenaAllocator* const allocator_;
// The internal bucket implementation of the set.
size_t const num_buckets_;
Node** const buckets_;
// Flags specifying which buckets were copied into the set from its parent.
// If a flag is not set, the corresponding bucket points to entries in the
// parent and must be cloned prior to making changes.
ArenaBitVector buckets_owned_;
// The number of entries in the set.
size_t num_entries_;
static constexpr size_t kMinimumNumberOfBuckets = 8;
DISALLOW_COPY_AND_ASSIGN(ValueSet);
};
/**
* Optimization phase that removes redundant instruction.
*/
class GlobalValueNumberer : public ValueObject {
public:
GlobalValueNumberer(HGraph* graph,
const SideEffectsAnalysis& side_effects)
: graph_(graph),
allocator_(graph->GetArenaStack()),
side_effects_(side_effects),
sets_(graph->GetBlocks().size(), nullptr, allocator_.Adapter(kArenaAllocGvn)),
visited_blocks_(
&allocator_, graph->GetBlocks().size(), /* expandable */ false, kArenaAllocGvn) {
visited_blocks_.ClearAllBits();
}
bool Run();
private:
// Per-block GVN. Will also update the ValueSet of the dominated and
// successor blocks.
void VisitBasicBlock(HBasicBlock* block);
HGraph* graph_;
ScopedArenaAllocator allocator_;
const SideEffectsAnalysis& side_effects_;
ValueSet* FindSetFor(HBasicBlock* block) const {
ValueSet* result = sets_[block->GetBlockId()];
DCHECK(result != nullptr) << "Could not find set for block B" << block->GetBlockId();
return result;
}
void AbandonSetFor(HBasicBlock* block) {
DCHECK(sets_[block->GetBlockId()] != nullptr)
<< "Block B" << block->GetBlockId() << " expected to have a set";
sets_[block->GetBlockId()] = nullptr;
}
// Returns false if the GlobalValueNumberer has already visited all blocks
// which may reference `block`.
bool WillBeReferencedAgain(HBasicBlock* block) const;
// Iterates over visited blocks and finds one which has a ValueSet such that:
// (a) it will not be referenced in the future, and
// (b) it can hold a copy of `reference_set` with a reasonable load factor.
HBasicBlock* FindVisitedBlockWithRecyclableSet(HBasicBlock* block,
const ValueSet& reference_set) const;
// ValueSet for blocks. Initially null, but for an individual block they
// are allocated and populated by the dominator, and updated by all blocks
// in the path from the dominator to the block.
ScopedArenaVector<ValueSet*> sets_;
// BitVector which serves as a fast-access map from block id to
// visited/unvisited Boolean.
ArenaBitVector visited_blocks_;
DISALLOW_COPY_AND_ASSIGN(GlobalValueNumberer);
};
bool GlobalValueNumberer::Run() {
DCHECK(side_effects_.HasRun());
sets_[graph_->GetEntryBlock()->GetBlockId()] = new (&allocator_) ValueSet(&allocator_);
// Use the reverse post order to ensure the non back-edge predecessors of a block are
// visited before the block itself.
for (HBasicBlock* block : graph_->GetReversePostOrder()) {
VisitBasicBlock(block);
}
return true;
}
void GlobalValueNumberer::VisitBasicBlock(HBasicBlock* block) {
ValueSet* set = nullptr;
const ArenaVector<HBasicBlock*>& predecessors = block->GetPredecessors();
if (predecessors.size() == 0 || predecessors[0]->IsEntryBlock()) {
// The entry block should only accumulate constant instructions, and
// the builder puts constants only in the entry block.
// Therefore, there is no need to propagate the value set to the next block.
set = new (&allocator_) ValueSet(&allocator_);
} else {
HBasicBlock* dominator = block->GetDominator();
ValueSet* dominator_set = FindSetFor(dominator);
if (dominator->GetSuccessors().size() == 1) {
// `block` is a direct successor of its dominator. No need to clone the
// dominator's set, `block` can take over its ownership including its buckets.
DCHECK_EQ(dominator->GetSingleSuccessor(), block);
AbandonSetFor(dominator);
set = dominator_set;
} else {
// Try to find a basic block which will never be referenced again and whose
// ValueSet can therefore be recycled. We will need to copy `dominator_set`
// into the recycled set, so we pass `dominator_set` as a reference for size.
HBasicBlock* recyclable = FindVisitedBlockWithRecyclableSet(block, *dominator_set);
if (recyclable == nullptr) {
// No block with a suitable ValueSet found. Allocate a new one and
// copy `dominator_set` into it.
set = new (&allocator_) ValueSet(&allocator_, *dominator_set);
} else {
// Block with a recyclable ValueSet found. Clone `dominator_set` into it.
set = FindSetFor(recyclable);
AbandonSetFor(recyclable);
set->PopulateFrom(*dominator_set);
}
}
if (!set->IsEmpty()) {
if (block->IsLoopHeader()) {
if (block->GetLoopInformation()->ContainsIrreducibleLoop()) {
// To satisfy our linear scan algorithm, no instruction should flow in an irreducible
// loop header. We clear the set at entry of irreducible loops and any loop containing
// an irreducible loop, as in both cases, GVN can extend the liveness of an instruction
// across the irreducible loop.
// Note that, if we're not compiling OSR, we could still do GVN and introduce
// phis at irreducible loop headers. We decided it was not worth the complexity.
set->Clear();
} else {
DCHECK(!block->GetLoopInformation()->IsIrreducible());
DCHECK_EQ(block->GetDominator(), block->GetLoopInformation()->GetPreHeader());
set->Kill(side_effects_.GetLoopEffects(block));
}
} else if (predecessors.size() > 1) {
for (HBasicBlock* predecessor : predecessors) {
set->IntersectWith(FindSetFor(predecessor));
if (set->IsEmpty()) {
break;
}
}
}
}
}
sets_[block->GetBlockId()] = set;
HInstruction* current = block->GetFirstInstruction();
while (current != nullptr) {
// Save the next instruction in case `current` is removed from the graph.
HInstruction* next = current->GetNext();
// Do not kill the set with the side effects of the instruction just now: if
// the instruction is GVN'ed, we don't need to kill.
//
// BoundType is a special case example of an instruction which shouldn't be moved but can be
// GVN'ed.
if (current->CanBeMoved() || current->IsBoundType()) {
if (current->IsBinaryOperation() && current->AsBinaryOperation()->IsCommutative()) {
// For commutative ops, (x op y) will be treated the same as (y op x)
// after fixed ordering.
current->AsBinaryOperation()->OrderInputs();
}
HInstruction* existing = set->Lookup(current);
if (existing != nullptr) {
// This replacement doesn't make more OrderInputs() necessary since
// current is either used by an instruction that it dominates,
// which hasn't been visited yet due to the order we visit instructions.
// Or current is used by a phi, and we don't do OrderInputs() on a phi anyway.
current->ReplaceWith(existing);
current->GetBlock()->RemoveInstruction(current);
} else {
set->Kill(current->GetSideEffects());
set->Add(current);
}
} else {
set->Kill(current->GetSideEffects());
}
current = next;
}
visited_blocks_.SetBit(block->GetBlockId());
}
bool GlobalValueNumberer::WillBeReferencedAgain(HBasicBlock* block) const {
DCHECK(visited_blocks_.IsBitSet(block->GetBlockId()));
for (const HBasicBlock* dominated_block : block->GetDominatedBlocks()) {
if (!visited_blocks_.IsBitSet(dominated_block->GetBlockId())) {
return true;
}
}
for (const HBasicBlock* successor : block->GetSuccessors()) {
if (!visited_blocks_.IsBitSet(successor->GetBlockId())) {
return true;
}
}
return false;
}
HBasicBlock* GlobalValueNumberer::FindVisitedBlockWithRecyclableSet(
HBasicBlock* block, const ValueSet& reference_set) const {
HBasicBlock* secondary_match = nullptr;
for (size_t block_id : visited_blocks_.Indexes()) {
ValueSet* current_set = sets_[block_id];
if (current_set == nullptr) {
// Set was already recycled.
continue;
}
HBasicBlock* current_block = block->GetGraph()->GetBlocks()[block_id];
// We test if `current_set` has enough buckets to store a copy of
// `reference_set` with a reasonable load factor. If we find a set whose
// number of buckets matches perfectly, we return right away. If we find one
// that is larger, we return it if no perfectly-matching set is found.
// Note that we defer testing WillBeReferencedAgain until all other criteria
// have been satisfied because it might be expensive.
if (current_set->CanHoldCopyOf(reference_set, /* exact_match */ true)) {
if (!WillBeReferencedAgain(current_block)) {
return current_block;
}
} else if (secondary_match == nullptr &&
current_set->CanHoldCopyOf(reference_set, /* exact_match */ false)) {
if (!WillBeReferencedAgain(current_block)) {
secondary_match = current_block;
}
}
}
return secondary_match;
}
bool GVNOptimization::Run() {
GlobalValueNumberer gvn(graph_, side_effects_);
return gvn.Run();
}
} // namespace art