Implementation of fast lookup table to search class_def by descriptor
Lookup table is a hash table which built at compile time and stored
into oat file. At runtime the table is restored and used in the
method DexFile::FindClassDef(const char*) to perform fast search of
the class_def_idx by class descriptor. Advantages of the lookup table
over the HashSet (runtime/base/hash_set.h) are:
1. Lookup table is built at compile time and uses read-only memory at
runtime
2. Lookup table uses less memory then DexFile::Index (less by 80% for
/system/framework/framework.jar on Nexus5)
3. Lookup table does less string comparisons compared with HashSet
(less by 70% for zygote process on Nexus5)
The disadvantage of the lookup table is it increased boot.oat size by
0.2% on Nexus5 and application .oat file by 0.3% in average on Nexus5.
mathieuc changes:
Create lookup table in dex2oat to speed up compilation. Clean up code
to follow style guide and use less static functions. Added
performance measurements.
Compile ~100 APKs 5 times with filter interpret-only:
Before:
real 1m8.989s
user 0m59.318s
sys 0m7.773s
After:
real 1m1.493s
user 0m52.055s
sys 0m7.581s
App launch (AOSP N5 maps, average of 45 runs):
Before: 966.84ms
After: 923.733ms
Launch speedup is 4.7%
Memory usage compared to HashSet index on 50 various APK:
32 bit: HashSet ~625694b vs TypeLookupTable ~404268b
64 bit: HashSet ~1251390b vs TypeLookupTable ~404268b
Bug: 10921004
Bug: 20269715
Change-Id: I7246c1d9ad9fe81fe5c5907a4bf70396d8f9242a
diff --git a/runtime/type_lookup_table.h b/runtime/type_lookup_table.h
new file mode 100644
index 0000000..3c2295c
--- /dev/null
+++ b/runtime/type_lookup_table.h
@@ -0,0 +1,162 @@
+/*
+ * Copyright (C) 2015 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.
+ */
+
+#ifndef ART_RUNTIME_TYPE_LOOKUP_TABLE_H_
+#define ART_RUNTIME_TYPE_LOOKUP_TABLE_H_
+
+#include "dex_file.h"
+#include "leb128.h"
+#include "utf.h"
+
+namespace art {
+
+/**
+ * TypeLookupTable used to find class_def_idx by class descriptor quickly.
+ * Implementation of TypeLookupTable is based on hash table.
+ * This class instantiated at compile time by calling Create() method and written into OAT file.
+ * At runtime, the raw data is read from memory-mapped file by calling Open() method. The table
+ * memory remains clean.
+ */
+class TypeLookupTable {
+ public:
+ ~TypeLookupTable();
+
+ // Return the number of buckets in the lookup table.
+ uint32_t Size() const {
+ return mask_ + 1;
+ }
+
+ // Method search class_def_idx by class descriptor and it's hash.
+ // If no data found then the method returns DexFile::kDexNoIndex
+ ALWAYS_INLINE uint32_t Lookup(const char* str, uint32_t hash) const {
+ uint32_t pos = hash & GetSizeMask();
+ // Thanks to special insertion algorithm, the element at position pos can be empty or start of
+ // bucket.
+ const Entry* entry = &entries_[pos];
+ while (!entry->IsEmpty()) {
+ if (CmpHashBits(entry->data, hash) && IsStringsEquals(str, entry->str_offset)) {
+ return GetClassDefIdx(entry->data);
+ }
+ if (entry->IsLast()) {
+ return DexFile::kDexNoIndex;
+ }
+ pos = (pos + entry->next_pos_delta) & GetSizeMask();
+ entry = &entries_[pos];
+ }
+ return DexFile::kDexNoIndex;
+ }
+
+ // Method creates lookup table for dex file
+ static TypeLookupTable* Create(const DexFile& dex_file);
+
+ // Method opens lookup table from binary data. Lookup table does not owns binary data.
+ static TypeLookupTable* Open(const uint8_t* raw_data, const DexFile& dex_file);
+
+ // Method returns pointer to binary data of lookup table. Used by the oat writer.
+ const uint8_t* RawData() const {
+ return reinterpret_cast<const uint8_t*>(entries_.get());
+ }
+
+ // Method returns length of binary data. Used by the oat writer.
+ uint32_t RawDataLength() const;
+
+ // Method returns length of binary data for the specified dex file.
+ static uint32_t RawDataLength(const DexFile& dex_file);
+
+ private:
+ /**
+ * To find element we need to compare strings.
+ * It is faster to compare first hashes and then strings itself.
+ * But we have no full hash of element of table. But we can use 2 ideas.
+ * 1. All minor bits of hash inside one bucket are equals.
+ * 2. If dex file contains N classes and size of hash table is 2^n (where N <= 2^n)
+ * then 16-n bits are free. So we can encode part of element's hash into these bits.
+ * So hash of element can be divided on three parts:
+ * XXXX XXXX XXXX YYYY YZZZ ZZZZ ZZZZZ
+ * Z - a part of hash encoded in bucket (these bits of has are same for all elements in bucket) -
+ * n bits
+ * Y - a part of hash that we can write into free 16-n bits (because only n bits used to store
+ * class_def_idx)
+ * X - a part of has that we can't use without increasing increase
+ * So the data element of Entry used to store class_def_idx and part of hash of the entry.
+ */
+ struct Entry {
+ uint32_t str_offset;
+ uint16_t data;
+ uint16_t next_pos_delta;
+
+ Entry() : str_offset(0), data(0), next_pos_delta(0) {}
+
+ bool IsEmpty() const {
+ return str_offset == 0;
+ }
+
+ bool IsLast() const {
+ return next_pos_delta == 0;
+ }
+ };
+
+ // Construct from a dex file.
+ explicit TypeLookupTable(const DexFile& dex_file);
+
+ // Construct from a dex file with existing data.
+ TypeLookupTable(const uint8_t* raw_data, const DexFile& dex_file);
+
+ bool IsStringsEquals(const char* str, uint32_t str_offset) const {
+ const uint8_t* ptr = dex_file_.Begin() + str_offset;
+ // Skip string length.
+ DecodeUnsignedLeb128(&ptr);
+ return CompareModifiedUtf8ToModifiedUtf8AsUtf16CodePointValues(
+ str, reinterpret_cast<const char*>(ptr)) == 0;
+ }
+
+ // Method extracts hash bits from element's data and compare them with
+ // the corresponding bits of the specified hash
+ bool CmpHashBits(uint32_t data, uint32_t hash) const {
+ uint32_t mask = static_cast<uint16_t>(~GetSizeMask());
+ return (hash & mask) == (data & mask);
+ }
+
+ uint32_t GetClassDefIdx(uint32_t data) const {
+ return data & mask_;
+ }
+
+ uint32_t GetSizeMask() const {
+ return mask_;
+ }
+
+ // Attempt to set an entry on it's hash' slot. If there is alrady something there, return false.
+ // Otherwise return true.
+ bool SetOnInitialPos(const Entry& entry, uint32_t hash);
+
+ // Insert an entry, probes until there is an empty slot.
+ void Insert(const Entry& entry, uint32_t hash);
+
+ // Find the last entry in a chain.
+ uint32_t FindLastEntryInBucket(uint32_t cur_pos) const;
+
+ const DexFile& dex_file_;
+ const uint32_t mask_;
+ std::unique_ptr<Entry[]> entries_;
+ // owns_entries_ specifies if the lookup table owns the entries_ array.
+ const bool owns_entries_;
+
+ DISALLOW_IMPLICIT_CONSTRUCTORS(TypeLookupTable);
+};
+
+} // namespace art
+
+#endif // ART_RUNTIME_TYPE_LOOKUP_TABLE_H_