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diff --git a/src/rapidjson/doc/internals.md b/src/rapidjson/doc/internals.md new file mode 100644 index 00000000..49802a0f --- /dev/null +++ b/src/rapidjson/doc/internals.md @@ -0,0 +1,365 @@ +# 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_` indiciates 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 endianess. 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 only supports SSE2 and SSE4.2 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` or `RAPIDJSON_SSE42` 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. +#if defined(__SSE4_2__) +# define RAPIDJSON_SSE42 +#elif defined(__SSE2__) +# define RAPIDJSON_SSE2 +#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/miloyip/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. |