On 2013/01/16 14:24, Max Kellermann <m...@duempel.org> wrote:
> class Path : private std::string { ... };
I did a few experiments here to judge the overhead of this. I started
with a "char*" attribute instead of deriving from std::string, to
avoid the overhead of duplicating the g_convert() result again for
std::string construction.
The preliminary patch is in my branch "path":
http://git.musicpd.org/cgit/cirrus/mpd.git/log/?h=path
My goal was to avoid overhead, but I failed, while at the same time
learning something about C++ again.
First thing I learned: free() is better than g_free(). The compiler
seems to "know" about the nature of free(), and when the compiler
figures out that the parameter is NULL, it will not generate a call to
free(), because the compiler knows it's a no-op anyway. The compiler
does not know about g_free(). (Tested with clang)
Second: structs/classes can be passed/returned in a simple register
UNLESS the class has a non-trivial destructor or a non-trivial copy
constructor. My patch explicitly "deletes" the copy constructor, but
the destructor must be non-trivial because it must call free().
(The copy constructor limitation doesn't seem to make sense in the
presence of a trivial move constructor, could be a clang bug)
Unfortunately, this means that a Path instance can never be passed in
a register, it must always go the round-trip to memory and back. This
adds overhead. Admittedly not much, but useless, and enough to annoy
me.
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