Fix "SSE 4.1 -> SSE 4.2" typo and add some comments about SIMD in internals and FAQ

doc/faq.md

@@ -198,7 +198,7 @@
 
 3. What is SIMD? How it is applied in RapidJSON?
 
-   [SIMD](http://en.wikipedia.org/wiki/SIMD) instructions can perform parallel computation in modern CPUs. RapidJSON support Intel's SSE2/SSE4.1 to accelerate whitespace skipping. This improves performance of parsing indent formatted JSON.
+   [SIMD](http://en.wikipedia.org/wiki/SIMD) instructions can perform parallel computation in modern CPUs. RapidJSON support Intel's SSE2/SSE4.2 to accelerate whitespace skipping. This improves performance of parsing indent formatted JSON. Define `RAPIDJSON_SSE2` or `RAPIDJSON_SSE42` macro to enable this feature. However, running the executable on a machine without such instruction set support will make it crash.
 
 4. Does it consume a lot of memory?
 

doc/faq.zh-cn.md

@@ -198,7 +198,7 @@
 
 3. 什是是SIMD？它如何用于RapidJSON？
 
-   [SIMD](http://en.wikipedia.org/wiki/SIMD)指令可以在现代CPU中执行并行运算。RapidJSON支持了Intel的SSE2/SSE4.1去加速跳过空白字符。在解析含缩进的JSON时，这能提升性能。
+   [SIMD](http://en.wikipedia.org/wiki/SIMD)指令可以在现代CPU中执行并行运算。RapidJSON支持了Intel的SSE2/SSE4.2去加速跳过空白字符。在解析含缩进的JSON时，这能提升性能。只要定义名为`RAPIDJSON_SSE2`或`RAPIDJSON_SSE42`的宏，就能启动这个功能。然而，若在不支持这些指令集的机器上执行这些可执行文件，会导致崩溃。
 
 4. 它会消耗许多内存么？
 

doc/features.md

@@ -15,7 +15,7 @@
 * High performance
  * Use template and inline functions to reduce function call overheads.
  * Internal optimized Grisu2 and floating point parsing implementations.
- * Optional SSE2/SSE4.1 support.
+ * Optional SSE2/SSE4.2 support.
 
 ## Standard compliance
 

doc/features.zh-cn.md

@@ -15,7 +15,7 @@
 * 高性能
  * 使用模版及内联函数去降低函数调用开销。
  * 内部经优化的Grisu2及浮点数解析实现。
- * 可选的SSE2/SSE4.1支持。
+ * 可选的SSE2/SSE4.2支持。
 
 ## 符合标准
 

doc/internals.md

@@ -183,7 +183,21 @@ void SkipWhitespace(InputStream& s) {
 
 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 SSE4.1 instructions for this. And it is only activated for UTF-8 memory streams, including string stream or *in situ* parsing. 
+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.
 
 ## Local Stream Copy {#LocalStreamCopy}
 

readme.md

@@ -28,7 +28,7 @@ RapidJSON is a JSON parser and generator for C++. It was inspired by [RapidXml](
 
 * RapidJSON is small but complete. It supports both SAX and DOM style API. The SAX parser is only a half thousand lines of code.
 
-* RapidJSON is fast. Its performance can be comparable to `strlen()`. It also optionally supports SSE2/SSE4.1 for acceleration.
+* RapidJSON is fast. Its performance can be comparable to `strlen()`. It also optionally supports SSE2/SSE4.2 for acceleration.
 
 * RapidJSON is self-contained. It does not depend on external libraries such as BOOST. It even does not depend on STL.
 

readme.zh-cn.md

@@ -16,7 +16,7 @@ RapidJSON是一个C++的JSON解析器及生成器。它的灵感来自[RapidXml]
 
 * RapidJSON小而全。它同时支持SAX和DOM风格的API。SAX解析器只有约500行代码。
 
-* RapidJSON快。它的性能可与`strlen()`相比。可支持SSE2/SSE4.1加速。
+* RapidJSON快。它的性能可与`strlen()`相比。可支持SSE2/SSE4.2加速。
 
 * RapidJSON独立。它不依赖于BOOST等外部库。它甚至不依赖于STL。