r175906 - Replace some typically large vectors with SmallVector.

[Benjamin Kramer]

On 22.02.2013, at 22:42, Jordan Rose <jordan_rose at apple.com> wrote:

> 
> On Feb 22, 2013, at 13:40 , Sean Silva <silvas at purdue.edu> wrote:
> 
>> On Fri, Feb 22, 2013 at 1:29 PM, Benjamin Kramer
>> <benny.kra at googlemail.com> wrote:
>>> This may seem counter-intuitive but the POD-like optimization helps when the
>>> vectors grow into multimegabyte buffers. SmallVector calls realloc which knows
>>> how to twiddle virtual memory bits and avoids large copies.
>> 
>> Is this optimization something that libc++ should be doing?
> 
> I haven't looked but I would guess that libc++ uses is_trivially_movable/is_trivially_copyable instead of is_pod, and all of our isPODLike cases should be trivially copyable if not trivially movable.

Yes, our isPODLike is roughly equivalent to is_trivially_copyable (it actually uses that type trait as a baseline when compiling with clang). I'm not sure if libc++ is allowed to do the optimization, the standard sets tighter constraints. SmallVector does the resizing with realloc(3) when the type is pod-like (trivially copyable), std::vector uses operator new which doesn't have a realloc equivalent.

- Ben