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Add notes about SIMD optimization issue. [skip ci]

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#499
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Milo Yip 2016-02-02 22:49:33 +08:00
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doc/internals.md
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@ -199,6 +199,20 @@ To enable this optimization, need to define `RAPIDJSON_SSE2` or `RAPIDJSON_SSE42
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_load_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: