369 lines
22 KiB
Markdown
369 lines
22 KiB
Markdown
# Internals
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This section records some design and implementation details.
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[TOC]
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# Architecture {#Architecture}
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## SAX and DOM
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The basic relationships of SAX and DOM is shown in the following UML diagram.
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The core of the relationship is the `Handler` concept. From the SAX side, `Reader` parses a JSON from a stream and publish events to a `Handler`. `Writer` implements the `Handler` concept to handle the same set of events. From the DOM side, `Document` implements the `Handler` concept to build a DOM according to the events. `Value` supports a `Value::Accept(Handler&)` function, which traverses the DOM to publish events.
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With this design, SAX is not dependent on DOM. Even `Reader` and `Writer` have no dependencies between them. This provides flexibility to chain event publisher and handlers. Besides, `Value` does not depends on SAX as well. So, in addition to stringify a DOM to JSON, user may also stringify it to a XML writer, or do anything else.
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## Utility Classes
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Both SAX and DOM APIs depends on 3 additional concepts: `Allocator`, `Encoding` and `Stream`. Their inheritance hierarchy is shown as below.
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# Value {#Value}
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`Value` (actually a typedef of `GenericValue<UTF8<>>`) is the core of DOM API. This section describes the design of it.
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## Data Layout {#DataLayout}
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`Value` is a [variant type](http://en.wikipedia.org/wiki/Variant_type). In RapidJSON's context, an instance of `Value` can contain 1 of 6 JSON value types. This is possible by using `union`. Each `Value` contains two members: `union Data data_` and a`unsigned flags_`. The `flags_` indicates the JSON type, and also additional information.
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The following tables show the data layout of each type. The 32-bit/64-bit columns indicates the size of the field in bytes.
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| Null | |32-bit|64-bit|
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|-------------------|----------------------------------|:----:|:----:|
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| (unused) | |4 |8 |
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| (unused) | |4 |4 |
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| (unused) | |4 |4 |
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| `unsigned flags_` | `kNullType kNullFlag` |4 |4 |
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| Bool | |32-bit|64-bit|
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|-------------------|----------------------------------------------------|:----:|:----:|
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| (unused) | |4 |8 |
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| (unused) | |4 |4 |
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| (unused) | |4 |4 |
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| `unsigned flags_` | `kBoolType` (either `kTrueFlag` or `kFalseFlag`) |4 |4 |
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| String | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `Ch* str` | Pointer to the string (may own) |4 |8 |
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| `SizeType length` | Length of string |4 |4 |
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| (unused) | |4 |4 |
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| `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 |
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| Object | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `Member* members` | Pointer to array of members (owned) |4 |8 |
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| `SizeType size` | Number of members |4 |4 |
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| `SizeType capacity` | Capacity of members |4 |4 |
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| `unsigned flags_` | `kObjectType kObjectFlag` |4 |4 |
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| Array | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `Value* values` | Pointer to array of values (owned) |4 |8 |
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| `SizeType size` | Number of values |4 |4 |
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| `SizeType capacity` | Capacity of values |4 |4 |
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| `unsigned flags_` | `kArrayType kArrayFlag` |4 |4 |
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| Number (Int) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `int i` | 32-bit signed integer |4 |4 |
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| (zero padding) | 0 |4 |4 |
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| (unused) | |4 |8 |
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| `unsigned flags_` | `kNumberType kNumberFlag kIntFlag kInt64Flag ...` |4 |4 |
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| Number (UInt) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `unsigned u` | 32-bit unsigned integer |4 |4 |
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| (zero padding) | 0 |4 |4 |
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| (unused) | |4 |8 |
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| `unsigned flags_` | `kNumberType kNumberFlag kUintFlag kUint64Flag ...` |4 |4 |
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| Number (Int64) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `int64_t i64` | 64-bit signed integer |8 |8 |
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| (unused) | |4 |8 |
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| `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 |
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| Number (Uint64) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `uint64_t i64` | 64-bit unsigned integer |8 |8 |
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| (unused) | |4 |8 |
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| `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 |
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| Number (Double) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `uint64_t i64` | Double precision floating-point |8 |8 |
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| (unused) | |4 |8 |
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| `unsigned flags_` | `kNumberType kNumberFlag kDoubleFlag` |4 |4 |
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Here are some notes:
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* To reduce memory consumption for 64-bit architecture, `SizeType` is typedef as `unsigned` instead of `size_t`.
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* Zero padding for 32-bit number may be placed after or before the actual type, according to the endianness. This makes possible for interpreting a 32-bit integer as a 64-bit integer, without any conversion.
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* An `Int` is always an `Int64`, but the converse is not always true.
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## Flags {#Flags}
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The 32-bit `flags_` contains both JSON type and other additional information. As shown in the above tables, each JSON type contains redundant `kXXXType` and `kXXXFlag`. This design is for optimizing the operation of testing bit-flags (`IsNumber()`) and obtaining a sequential number for each type (`GetType()`).
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String has two optional flags. `kCopyFlag` means that the string owns a copy of the string. `kInlineStrFlag` means using [Short-String Optimization](#ShortString).
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Number is a bit more complicated. For normal integer values, it can contains `kIntFlag`, `kUintFlag`, `kInt64Flag` and/or `kUint64Flag`, according to the range of the integer. For numbers with fraction, and integers larger than 64-bit range, they will be stored as `double` with `kDoubleFlag`.
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## Short-String Optimization {#ShortString}
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[Kosta](https://github.com/Kosta-Github) provided a very neat short-string optimization. The optimization idea is given as follow. Excluding the `flags_`, a `Value` has 12 or 16 bytes (32-bit or 64-bit) for storing actual data. Instead of storing a pointer to a string, it is possible to store short strings in these space internally. For encoding with 1-byte character type (e.g. `char`), it can store maximum 11 or 15 characters string inside the `Value` type.
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| ShortString (Ch=char) | |32-bit|64-bit|
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|---------------------|-------------------------------------|:----:|:----:|
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| `Ch str[MaxChars]` | String buffer |11 |15 |
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| `Ch invLength` | MaxChars - Length |1 |1 |
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| `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 |
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A special technique is applied. Instead of storing the length of string directly, it stores (MaxChars - length). This make it possible to store 11 characters with trailing `\0`.
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This optimization can reduce memory usage for copy-string. It can also improve cache-coherence thus improve runtime performance.
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# Allocator {#InternalAllocator}
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`Allocator` is a concept in RapidJSON:
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~~~cpp
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concept Allocator {
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static const bool kNeedFree; //!< Whether this allocator needs to call Free().
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// Allocate a memory block.
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// \param size of the memory block in bytes.
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// \returns pointer to the memory block.
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void* Malloc(size_t size);
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// Resize a memory block.
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// \param originalPtr The pointer to current memory block. Null pointer is permitted.
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// \param originalSize The current size in bytes. (Design issue: since some allocator may not book-keep this, explicitly pass to it can save memory.)
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// \param newSize the new size in bytes.
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void* Realloc(void* originalPtr, size_t originalSize, size_t newSize);
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// Free a memory block.
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// \param pointer to the memory block. Null pointer is permitted.
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static void Free(void *ptr);
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};
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~~~
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Note that `Malloc()` and `Realloc()` are member functions but `Free()` is static member function.
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## MemoryPoolAllocator {#MemoryPoolAllocator}
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`MemoryPoolAllocator` is the default allocator for DOM. It allocate but do not free memory. This is suitable for building a DOM tree.
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Internally, it allocates chunks of memory from the base allocator (by default `CrtAllocator`) and stores the chunks as a singly linked list. When user requests an allocation, it allocates memory from the following order:
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1. User supplied buffer if it is available. (See [User Buffer section in DOM](doc/dom.md))
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2. If user supplied buffer is full, use the current memory chunk.
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3. If the current block is full, allocate a new block of memory.
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# Parsing Optimization {#ParsingOptimization}
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## Skip Whitespaces with SIMD {#SkipwhitespaceWithSIMD}
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When parsing JSON from a stream, the parser need to skip 4 whitespace characters:
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1. Space (`U+0020`)
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2. Character Tabulation (`U+000B`)
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3. Line Feed (`U+000A`)
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4. Carriage Return (`U+000D`)
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A simple implementation will be simply:
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~~~cpp
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void SkipWhitespace(InputStream& s) {
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while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
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s.Take();
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}
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~~~
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However, this requires 4 comparisons and a few branching for each character. This was found to be a hot spot.
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To accelerate this process, SIMD was applied to compare 16 characters with 4 white spaces for each iteration. Currently RapidJSON supports SSE2, SSE4.2 and ARM Neon instructions for this. And it is only activated for UTF-8 memory streams, including string stream or *in situ* parsing.
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To enable this optimization, need to define `RAPIDJSON_SSE2`, `RAPIDJSON_SSE42` or `RAPIDJSON_NEON` before including `rapidjson.h`. Some compilers can detect the setting, as in `perftest.h`:
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~~~cpp
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// __SSE2__ and __SSE4_2__ are recognized by gcc, clang, and the Intel compiler.
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// We use -march=native with gmake to enable -msse2 and -msse4.2, if supported.
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// Likewise, __ARM_NEON is used to detect Neon.
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#if defined(__SSE4_2__)
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# define RAPIDJSON_SSE42
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#elif defined(__SSE2__)
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# define RAPIDJSON_SSE2
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#elif defined(__ARM_NEON)
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# define RAPIDJSON_NEON
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#endif
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~~~
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Note that, these are compile-time settings. Running the executable on a machine without such instruction set support will make it crash.
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### Page boundary issue
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In an early version of RapidJSON, [an issue](https://code.google.com/archive/p/rapidjson/issues/104) reported that the `SkipWhitespace_SIMD()` causes crash very rarely (around 1 in 500,000). After investigation, it is suspected that `_mm_loadu_si128()` accessed bytes after `'\0'`, and across a protected page boundary.
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In [Intel® 64 and IA-32 Architectures Optimization Reference Manual
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](http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-optimization-manual.html), section 10.2.1:
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> To support algorithms requiring unaligned 128-bit SIMD memory accesses, memory buffer allocation by a caller function should consider adding some pad space so that a callee function can safely use the address pointer safely with unaligned 128-bit SIMD memory operations.
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> The minimal padding size should be the width of the SIMD register that might be used in conjunction with unaligned SIMD memory access.
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This is not feasible as RapidJSON should not enforce such requirement.
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To fix this issue, currently the routine process bytes up to the next aligned address. After tha, use aligned read to perform SIMD processing. Also see [#85](https://github.com/Tencent/rapidjson/issues/85).
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## Local Stream Copy {#LocalStreamCopy}
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During optimization, it is found that some compilers cannot localize some member data access of streams into local variables or registers. Experimental results show that for some stream types, making a copy of the stream and used it in inner-loop can improve performance. For example, the actual (non-SIMD) implementation of `SkipWhitespace()` is implemented as:
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~~~cpp
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template<typename InputStream>
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void SkipWhitespace(InputStream& is) {
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internal::StreamLocalCopy<InputStream> copy(is);
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InputStream& s(copy.s);
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while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
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s.Take();
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}
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~~~
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Depending on the traits of stream, `StreamLocalCopy` will make (or not make) a copy of the stream object, use it locally and copy the states of stream back to the original stream.
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## Parsing to Double {#ParsingDouble}
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Parsing string into `double` is difficult. The standard library function `strtod()` can do the job but it is slow. By default, the parsers use normal precision setting. This has has maximum 3 [ULP](http://en.wikipedia.org/wiki/Unit_in_the_last_place) error and implemented in `internal::StrtodNormalPrecision()`.
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When using `kParseFullPrecisionFlag`, the parsers calls `internal::StrtodFullPrecision()` instead, and this function actually implemented 3 versions of conversion methods.
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1. [Fast-Path](http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/).
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2. Custom DIY-FP implementation as in [double-conversion](https://github.com/floitsch/double-conversion).
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3. Big Integer Method as in (Clinger, William D. How to read floating point numbers accurately. Vol. 25. No. 6. ACM, 1990).
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If the first conversion methods fail, it will try the second, and so on.
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# Generation Optimization {#GenerationOptimization}
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## Integer-to-String conversion {#itoa}
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The naive algorithm for integer-to-string conversion involves division per each decimal digit. We have implemented various implementations and evaluated them in [itoa-benchmark](https://github.com/miloyip/itoa-benchmark).
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Although SSE2 version is the fastest but the difference is minor by comparing to the first running-up `branchlut`. And `branchlut` is pure C++ implementation so we adopt `branchlut` in RapidJSON.
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## Double-to-String conversion {#dtoa}
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Originally RapidJSON uses `snprintf(..., ..., "%g")` to achieve double-to-string conversion. This is not accurate as the default precision is 6. Later we also find that this is slow and there is an alternative.
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Google's V8 [double-conversion](https://github.com/floitsch/double-conversion
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) implemented a newer, fast algorithm called Grisu3 (Loitsch, Florian. "Printing floating-point numbers quickly and accurately with integers." ACM Sigplan Notices 45.6 (2010): 233-243.).
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However, since it is not header-only so that we implemented a header-only version of Grisu2. This algorithm guarantees that the result is always accurate. And in most of cases it produces the shortest (optimal) string representation.
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The header-only conversion function has been evaluated in [dtoa-benchmark](https://github.com/miloyip/dtoa-benchmark).
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# Parser {#Parser}
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## Iterative Parser {#IterativeParser}
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The iterative parser is a recursive descent LL(1) parser
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implemented in a non-recursive manner.
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### Grammar {#IterativeParserGrammar}
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The grammar used for this parser is based on strict JSON syntax:
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~~~~~~~~~~
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S -> array | object
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array -> [ values ]
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object -> { members }
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values -> non-empty-values | ε
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non-empty-values -> value addition-values
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addition-values -> ε | , non-empty-values
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members -> non-empty-members | ε
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non-empty-members -> member addition-members
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addition-members -> ε | , non-empty-members
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member -> STRING : value
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value -> STRING | NUMBER | NULL | BOOLEAN | object | array
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~~~~~~~~~~
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Note that left factoring is applied to non-terminals `values` and `members`
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to make the grammar be LL(1).
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### Parsing Table {#IterativeParserParsingTable}
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Based on the grammar, we can construct the FIRST and FOLLOW set.
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The FIRST set of non-terminals is listed below:
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| NON-TERMINAL | FIRST |
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|:-----------------:|:--------------------------------:|
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| array | [ |
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| object | { |
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| values | ε STRING NUMBER NULL BOOLEAN { [ |
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| addition-values | ε COMMA |
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| members | ε STRING |
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| addition-members | ε COMMA |
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| member | STRING |
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| value | STRING NUMBER NULL BOOLEAN { [ |
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| S | [ { |
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| non-empty-members | STRING |
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| non-empty-values | STRING NUMBER NULL BOOLEAN { [ |
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The FOLLOW set is listed below:
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| NON-TERMINAL | FOLLOW |
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|:-----------------:|:-------:|
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| S | $ |
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| array | , $ } ] |
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| object | , $ } ] |
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| values | ] |
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| non-empty-values | ] |
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| addition-values | ] |
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| members | } |
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| non-empty-members | } |
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| addition-members | } |
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| member | , } |
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| value | , } ] |
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Finally the parsing table can be constructed from FIRST and FOLLOW set:
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| NON-TERMINAL | [ | { | , | : | ] | } | STRING | NUMBER | NULL | BOOLEAN |
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|:-----------------:|:---------------------:|:---------------------:|:-------------------:|:-:|:-:|:-:|:-----------------------:|:---------------------:|:---------------------:|:---------------------:|
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| S | array | object | | | | | | | | |
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| array | [ values ] | | | | | | | | | |
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| object | | { members } | | | | | | | | |
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| values | non-empty-values | non-empty-values | | | ε | | non-empty-values | non-empty-values | non-empty-values | non-empty-values |
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| non-empty-values | value addition-values | value addition-values | | | | | value addition-values | value addition-values | value addition-values | value addition-values |
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| addition-values | | | , non-empty-values | | ε | | | | | |
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| members | | | | | | ε | non-empty-members | | | |
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| non-empty-members | | | | | | | member addition-members | | | |
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| addition-members | | | , non-empty-members | | | ε | | | | |
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| member | | | | | | | STRING : value | | | |
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| value | array | object | | | | | STRING | NUMBER | NULL | BOOLEAN |
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There is a great [tool](http://hackingoff.com/compilers/predict-first-follow-set) for above grammar analysis.
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### Implementation {#IterativeParserImplementation}
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Based on the parsing table, a direct(or conventional) implementation
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that pushes the production body in reverse order
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while generating a production could work.
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In RapidJSON, several modifications(or adaptations to current design) are made to a direct implementation.
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First, the parsing table is encoded in a state machine in RapidJSON.
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States are constructed by the head and body of production.
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State transitions are constructed by production rules.
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Besides, extra states are added for productions involved with `array` and `object`.
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In this way the generation of array values or object members would be a single state transition,
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rather than several pop/push operations in the direct implementation.
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This also makes the estimation of stack size more easier.
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The state diagram is shown as follows:
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Second, the iterative parser also keeps track of array's value count and object's member count
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in its internal stack, which may be different from a conventional implementation.
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