mirror of
https://github.com/RetroDECK/ES-DE.git
synced 2024-11-25 15:45:38 +00:00
369 lines
22 KiB
Markdown
369 lines
22 KiB
Markdown
# Internals
|
||
|
||
This section records some design and implementation details.
|
||
|
||
[TOC]
|
||
|
||
# Architecture {#Architecture}
|
||
|
||
## SAX and DOM
|
||
|
||
The basic relationships of SAX and DOM is shown in the following UML diagram.
|
||
|
||
|
||
|
||
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.
|
||
|
||
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.
|
||
|
||
## Utility Classes
|
||
|
||
Both SAX and DOM APIs depends on 3 additional concepts: `Allocator`, `Encoding` and `Stream`. Their inheritance hierarchy is shown as below.
|
||
|
||
|
||
|
||
# Value {#Value}
|
||
|
||
`Value` (actually a typedef of `GenericValue<UTF8<>>`) is the core of DOM API. This section describes the design of it.
|
||
|
||
## Data Layout {#DataLayout}
|
||
|
||
`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.
|
||
|
||
The following tables show the data layout of each type. The 32-bit/64-bit columns indicates the size of the field in bytes.
|
||
|
||
| Null | |32-bit|64-bit|
|
||
|-------------------|----------------------------------|:----:|:----:|
|
||
| (unused) | |4 |8 |
|
||
| (unused) | |4 |4 |
|
||
| (unused) | |4 |4 |
|
||
| `unsigned flags_` | `kNullType kNullFlag` |4 |4 |
|
||
|
||
| Bool | |32-bit|64-bit|
|
||
|-------------------|----------------------------------------------------|:----:|:----:|
|
||
| (unused) | |4 |8 |
|
||
| (unused) | |4 |4 |
|
||
| (unused) | |4 |4 |
|
||
| `unsigned flags_` | `kBoolType` (either `kTrueFlag` or `kFalseFlag`) |4 |4 |
|
||
|
||
| String | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `Ch* str` | Pointer to the string (may own) |4 |8 |
|
||
| `SizeType length` | Length of string |4 |4 |
|
||
| (unused) | |4 |4 |
|
||
| `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 |
|
||
|
||
| Object | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `Member* members` | Pointer to array of members (owned) |4 |8 |
|
||
| `SizeType size` | Number of members |4 |4 |
|
||
| `SizeType capacity` | Capacity of members |4 |4 |
|
||
| `unsigned flags_` | `kObjectType kObjectFlag` |4 |4 |
|
||
|
||
| Array | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `Value* values` | Pointer to array of values (owned) |4 |8 |
|
||
| `SizeType size` | Number of values |4 |4 |
|
||
| `SizeType capacity` | Capacity of values |4 |4 |
|
||
| `unsigned flags_` | `kArrayType kArrayFlag` |4 |4 |
|
||
|
||
| Number (Int) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `int i` | 32-bit signed integer |4 |4 |
|
||
| (zero padding) | 0 |4 |4 |
|
||
| (unused) | |4 |8 |
|
||
| `unsigned flags_` | `kNumberType kNumberFlag kIntFlag kInt64Flag ...` |4 |4 |
|
||
|
||
| Number (UInt) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `unsigned u` | 32-bit unsigned integer |4 |4 |
|
||
| (zero padding) | 0 |4 |4 |
|
||
| (unused) | |4 |8 |
|
||
| `unsigned flags_` | `kNumberType kNumberFlag kUintFlag kUint64Flag ...` |4 |4 |
|
||
|
||
| Number (Int64) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `int64_t i64` | 64-bit signed integer |8 |8 |
|
||
| (unused) | |4 |8 |
|
||
| `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 |
|
||
|
||
| Number (Uint64) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `uint64_t i64` | 64-bit unsigned integer |8 |8 |
|
||
| (unused) | |4 |8 |
|
||
| `unsigned flags_` | `kNumberType kNumberFlag kInt64Flag ...` |4 |4 |
|
||
|
||
| Number (Double) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `uint64_t i64` | Double precision floating-point |8 |8 |
|
||
| (unused) | |4 |8 |
|
||
| `unsigned flags_` | `kNumberType kNumberFlag kDoubleFlag` |4 |4 |
|
||
|
||
Here are some notes:
|
||
* To reduce memory consumption for 64-bit architecture, `SizeType` is typedef as `unsigned` instead of `size_t`.
|
||
* 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.
|
||
* An `Int` is always an `Int64`, but the converse is not always true.
|
||
|
||
## Flags {#Flags}
|
||
|
||
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()`).
|
||
|
||
String has two optional flags. `kCopyFlag` means that the string owns a copy of the string. `kInlineStrFlag` means using [Short-String Optimization](#ShortString).
|
||
|
||
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`.
|
||
|
||
## Short-String Optimization {#ShortString}
|
||
|
||
[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.
|
||
|
||
| ShortString (Ch=char) | |32-bit|64-bit|
|
||
|---------------------|-------------------------------------|:----:|:----:|
|
||
| `Ch str[MaxChars]` | String buffer |11 |15 |
|
||
| `Ch invLength` | MaxChars - Length |1 |1 |
|
||
| `unsigned flags_` | `kStringType kStringFlag ...` |4 |4 |
|
||
|
||
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`.
|
||
|
||
This optimization can reduce memory usage for copy-string. It can also improve cache-coherence thus improve runtime performance.
|
||
|
||
# Allocator {#InternalAllocator}
|
||
|
||
`Allocator` is a concept in RapidJSON:
|
||
~~~cpp
|
||
concept Allocator {
|
||
static const bool kNeedFree; //!< Whether this allocator needs to call Free().
|
||
|
||
// Allocate a memory block.
|
||
// \param size of the memory block in bytes.
|
||
// \returns pointer to the memory block.
|
||
void* Malloc(size_t size);
|
||
|
||
// Resize a memory block.
|
||
// \param originalPtr The pointer to current memory block. Null pointer is permitted.
|
||
// \param originalSize The current size in bytes. (Design issue: since some allocator may not book-keep this, explicitly pass to it can save memory.)
|
||
// \param newSize the new size in bytes.
|
||
void* Realloc(void* originalPtr, size_t originalSize, size_t newSize);
|
||
|
||
// Free a memory block.
|
||
// \param pointer to the memory block. Null pointer is permitted.
|
||
static void Free(void *ptr);
|
||
};
|
||
~~~
|
||
|
||
Note that `Malloc()` and `Realloc()` are member functions but `Free()` is static member function.
|
||
|
||
## MemoryPoolAllocator {#MemoryPoolAllocator}
|
||
|
||
`MemoryPoolAllocator` is the default allocator for DOM. It allocate but do not free memory. This is suitable for building a DOM tree.
|
||
|
||
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:
|
||
|
||
1. User supplied buffer if it is available. (See [User Buffer section in DOM](doc/dom.md))
|
||
2. If user supplied buffer is full, use the current memory chunk.
|
||
3. If the current block is full, allocate a new block of memory.
|
||
|
||
# Parsing Optimization {#ParsingOptimization}
|
||
|
||
## Skip Whitespaces with SIMD {#SkipwhitespaceWithSIMD}
|
||
|
||
When parsing JSON from a stream, the parser need to skip 4 whitespace characters:
|
||
|
||
1. Space (`U+0020`)
|
||
2. Character Tabulation (`U+000B`)
|
||
3. Line Feed (`U+000A`)
|
||
4. Carriage Return (`U+000D`)
|
||
|
||
A simple implementation will be simply:
|
||
~~~cpp
|
||
void SkipWhitespace(InputStream& s) {
|
||
while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
|
||
s.Take();
|
||
}
|
||
~~~
|
||
|
||
However, this requires 4 comparisons and a few branching for each character. This was found to be a hot spot.
|
||
|
||
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.
|
||
|
||
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`:
|
||
|
||
~~~cpp
|
||
// __SSE2__ and __SSE4_2__ are recognized by gcc, clang, and the Intel compiler.
|
||
// We use -march=native with gmake to enable -msse2 and -msse4.2, if supported.
|
||
// Likewise, __ARM_NEON is used to detect Neon.
|
||
#if defined(__SSE4_2__)
|
||
# define RAPIDJSON_SSE42
|
||
#elif defined(__SSE2__)
|
||
# define RAPIDJSON_SSE2
|
||
#elif defined(__ARM_NEON)
|
||
# define RAPIDJSON_NEON
|
||
#endif
|
||
~~~
|
||
|
||
Note that, these are compile-time settings. Running the executable on a machine without such instruction set support will make it crash.
|
||
|
||
### Page boundary issue
|
||
|
||
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.
|
||
|
||
In [Intel® 64 and IA-32 Architectures Optimization Reference Manual
|
||
](http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-optimization-manual.html), section 10.2.1:
|
||
|
||
> 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.
|
||
> The minimal padding size should be the width of the SIMD register that might be used in conjunction with unaligned SIMD memory access.
|
||
|
||
This is not feasible as RapidJSON should not enforce such requirement.
|
||
|
||
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).
|
||
|
||
## Local Stream Copy {#LocalStreamCopy}
|
||
|
||
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:
|
||
|
||
~~~cpp
|
||
template<typename InputStream>
|
||
void SkipWhitespace(InputStream& is) {
|
||
internal::StreamLocalCopy<InputStream> copy(is);
|
||
InputStream& s(copy.s);
|
||
|
||
while (s.Peek() == ' ' || s.Peek() == '\n' || s.Peek() == '\r' || s.Peek() == '\t')
|
||
s.Take();
|
||
}
|
||
~~~
|
||
|
||
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.
|
||
|
||
## Parsing to Double {#ParsingDouble}
|
||
|
||
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()`.
|
||
|
||
When using `kParseFullPrecisionFlag`, the parsers calls `internal::StrtodFullPrecision()` instead, and this function actually implemented 3 versions of conversion methods.
|
||
1. [Fast-Path](http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/).
|
||
2. Custom DIY-FP implementation as in [double-conversion](https://github.com/floitsch/double-conversion).
|
||
3. Big Integer Method as in (Clinger, William D. How to read floating point numbers accurately. Vol. 25. No. 6. ACM, 1990).
|
||
|
||
If the first conversion methods fail, it will try the second, and so on.
|
||
|
||
# Generation Optimization {#GenerationOptimization}
|
||
|
||
## Integer-to-String conversion {#itoa}
|
||
|
||
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).
|
||
|
||
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.
|
||
|
||
## Double-to-String conversion {#dtoa}
|
||
|
||
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.
|
||
|
||
Google's V8 [double-conversion](https://github.com/floitsch/double-conversion
|
||
) 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.).
|
||
|
||
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.
|
||
|
||
The header-only conversion function has been evaluated in [dtoa-benchmark](https://github.com/miloyip/dtoa-benchmark).
|
||
|
||
# Parser {#Parser}
|
||
|
||
## Iterative Parser {#IterativeParser}
|
||
|
||
The iterative parser is a recursive descent LL(1) parser
|
||
implemented in a non-recursive manner.
|
||
|
||
### Grammar {#IterativeParserGrammar}
|
||
|
||
The grammar used for this parser is based on strict JSON syntax:
|
||
~~~~~~~~~~
|
||
S -> array | object
|
||
array -> [ values ]
|
||
object -> { members }
|
||
values -> non-empty-values | ε
|
||
non-empty-values -> value addition-values
|
||
addition-values -> ε | , non-empty-values
|
||
members -> non-empty-members | ε
|
||
non-empty-members -> member addition-members
|
||
addition-members -> ε | , non-empty-members
|
||
member -> STRING : value
|
||
value -> STRING | NUMBER | NULL | BOOLEAN | object | array
|
||
~~~~~~~~~~
|
||
|
||
Note that left factoring is applied to non-terminals `values` and `members`
|
||
to make the grammar be LL(1).
|
||
|
||
### Parsing Table {#IterativeParserParsingTable}
|
||
|
||
Based on the grammar, we can construct the FIRST and FOLLOW set.
|
||
|
||
The FIRST set of non-terminals is listed below:
|
||
|
||
| NON-TERMINAL | FIRST |
|
||
|:-----------------:|:--------------------------------:|
|
||
| array | [ |
|
||
| object | { |
|
||
| values | ε STRING NUMBER NULL BOOLEAN { [ |
|
||
| addition-values | ε COMMA |
|
||
| members | ε STRING |
|
||
| addition-members | ε COMMA |
|
||
| member | STRING |
|
||
| value | STRING NUMBER NULL BOOLEAN { [ |
|
||
| S | [ { |
|
||
| non-empty-members | STRING |
|
||
| non-empty-values | STRING NUMBER NULL BOOLEAN { [ |
|
||
|
||
The FOLLOW set is listed below:
|
||
|
||
| NON-TERMINAL | FOLLOW |
|
||
|:-----------------:|:-------:|
|
||
| S | $ |
|
||
| array | , $ } ] |
|
||
| object | , $ } ] |
|
||
| values | ] |
|
||
| non-empty-values | ] |
|
||
| addition-values | ] |
|
||
| members | } |
|
||
| non-empty-members | } |
|
||
| addition-members | } |
|
||
| member | , } |
|
||
| value | , } ] |
|
||
|
||
Finally the parsing table can be constructed from FIRST and FOLLOW set:
|
||
|
||
| NON-TERMINAL | [ | { | , | : | ] | } | STRING | NUMBER | NULL | BOOLEAN |
|
||
|:-----------------:|:---------------------:|:---------------------:|:-------------------:|:-:|:-:|:-:|:-----------------------:|:---------------------:|:---------------------:|:---------------------:|
|
||
| S | array | object | | | | | | | | |
|
||
| array | [ values ] | | | | | | | | | |
|
||
| object | | { members } | | | | | | | | |
|
||
| values | non-empty-values | non-empty-values | | | ε | | non-empty-values | non-empty-values | non-empty-values | non-empty-values |
|
||
| non-empty-values | value addition-values | value addition-values | | | | | value addition-values | value addition-values | value addition-values | value addition-values |
|
||
| addition-values | | | , non-empty-values | | ε | | | | | |
|
||
| members | | | | | | ε | non-empty-members | | | |
|
||
| non-empty-members | | | | | | | member addition-members | | | |
|
||
| addition-members | | | , non-empty-members | | | ε | | | | |
|
||
| member | | | | | | | STRING : value | | | |
|
||
| value | array | object | | | | | STRING | NUMBER | NULL | BOOLEAN |
|
||
|
||
There is a great [tool](http://hackingoff.com/compilers/predict-first-follow-set) for above grammar analysis.
|
||
|
||
### Implementation {#IterativeParserImplementation}
|
||
|
||
Based on the parsing table, a direct(or conventional) implementation
|
||
that pushes the production body in reverse order
|
||
while generating a production could work.
|
||
|
||
In RapidJSON, several modifications(or adaptations to current design) are made to a direct implementation.
|
||
|
||
First, the parsing table is encoded in a state machine in RapidJSON.
|
||
States are constructed by the head and body of production.
|
||
State transitions are constructed by production rules.
|
||
Besides, extra states are added for productions involved with `array` and `object`.
|
||
In this way the generation of array values or object members would be a single state transition,
|
||
rather than several pop/push operations in the direct implementation.
|
||
This also makes the estimation of stack size more easier.
|
||
|
||
The state diagram is shown as follows:
|
||
|
||
|
||
|
||
Second, the iterative parser also keeps track of array's value count and object's member count
|
||
in its internal stack, which may be different from a conventional implementation.
|