--- /dev/null
+# 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.
projects / c11concurrency-benchmarks.git / blobdiff