short string optimization #131

Since the payload (the `Data` union) of the current implementation of `GenericValue` is `12 bytes` (32 bit) or `16 bytes` (64 bit) it could store `UTF8`-encoded strings up to `10` or `14` chars plus the `terminating zero` character plus the string length: ``` C++ struct ShortString { enum { MaxSize = sizeof(GenericValue::String) / sizeof(Ch) - sizeof(unsigned char) }; Ch str[MaxSize]; unsigned char length; }; // at most as many bytes as "String" above => 12 bytes in 32-bit mode, 16 bytes in 64-bit mode ``` This is achieved by introducing additional `kInlineStrFlag` and `kShortStringFlag` flags. When setting a new string value in `SetStringRaw(s, alloc)` it is first checked if the string is short enough to fit into the `inline string buffer` and if so the given source string will be copied into the new `ShortString` target instead of allocating additional memory for it.

Instead of replicating the functionality of `GetString()` and `GetStringLength()` in `StringEqual()` it now calls these methods instead.

The `ShortString` can represent zero-terminated strings up to `MaxSize` chars (excluding the terminating zero) and store a value to determine the length of the contained string in the last character `str[LenPos]` by storing `MaxSize - length` there. If the string to store has the maximal length of `MaxSize` (excluding the terminating zero) then `str[LenPos]` will store `0` and therefore act as the string terminator as well. For getting the string length back from that value just use `MaxSize - str[LenPos]`. This allows to store `11`-chars strings in 32-bit mode and `15`-chars strings in 64-bit mode inline (for `UTF8`-encoded strings).

…g the string length

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short string optimization #131

short string optimization #131

Commits on Sep 1, 2014