Make configurable size of internal storage in util::function

[hkaiser]

No description provided.

[hkaiser]

[16 Feb 16 08:20] * hkaiser * K-ballo: btw, may I ask you to look at #1987, please?
[16 Feb 16 08:21] * hkaiser * (if you have the time)
[16 Feb 16 08:21] * K-ballo * the one for fatter functions?
[16 Feb 16 08:21] * hkaiser * nod
16 Feb 16 08:22] * K-ballo * I don't think it's a good idea in the long term, but if you are gonna use that to search for the optimal size then it looks good
[16 Feb 16 08:22] * hkaiser * K-ballo: that's the idea, yes
[16 Feb 16 08:22] * hkaiser * I've seen some speedup because of reduced allocations, though
[16 Feb 16 08:22] * hkaiser * K-ballo: why do you think this is a bad idea?
[16 Feb 16 08:23] * hkaiser * move/copy-overheads?
[16 Feb 16 08:23] * K-ballo * having it configurable by the user, is another source of problems if they link the wrong builds
[16 Feb 16 08:24] * hkaiser * K-ballo: ahh, sure - I plan not to expose that number
[16 Feb 16 08:24] * K-ballo * use it to find the optimal size, which is likely to be ~3, and then drop the config flag
[16 Feb 16 08:24] * hkaiser * nod
[16 Feb 16 08:25] * hkaiser * K-ballo: do other implementations have fatter functions?
[16 Feb 16 08:25] * K-ballo * mmh, msvc uses "the size of a std::string" as heuristic...................
[16 Feb 16 08:26] * K-ballo * libc++ has a 3 pointer buffer I think, but it wastes some space too
[16 Feb 16 08:26] * hkaiser * so they went through the motions of optimizing small string optimization for std::string
[16 Feb 16 08:27] * hkaiser * and just adopted that value
[16 Feb 16 08:27] * K-ballo * libstdc++ uses the size of the biggest mem-fun-ptr
[16 Feb 16 08:30] * hkaiser * which is probably 3
[16 Feb 16 08:30] * K-ballo * they can get pretty fat for virtual functions of classes with multiple inheritance, let me see
[16 Feb 16 08:32] * K-ballo * it can go up to 4 or 5 pointers on some compilers
[16 Feb 16 08:33] * hkaiser * nod, figures - how do they figure that out?
[16 Feb 16 08:33] * hkaiser * at compile time, that is
[16 Feb 16 08:33] * K-ballo * sizeof this https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/functional#L1457
[16 Feb 16 08:34] * hkaiser * tks
[16 Feb 16 08:34] * K-ballo * a pointer to member function of incomplete class has to get the worst possible size

[hkaiser]

The latest commit sets the size for the internal function storage to 3*sizeof(void*) which seems to give best performance results.