Re: Rethinking MemoryContext creation

Robert Haas <robertmhaas(at)gmail(dot)com> writes:

> On Mon, Dec 11, 2017 at 11:59 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>>> On 12/11/2017 05:27 PM, Tom Lane wrote:
>>>> However, unless we want to run around and touch all the ~ 150 calls
>>>> with constant arguments, we'd have to set things up so that the default
>>>> behavior for AllocSetContextCreate is to not copy. This risks breaking
>>>> callers in extensions. Not entirely sure if it's worth that --- any
>>>> thoughts?
>>
>>> I don't think silently breaking extensions is particularly attractive
>>> option, so I guess we'll have to run around and tweak the ~150 calls.
>>
>> Meh. I suppose that of the ~150 call sites, there are probably only
>> a dozen or two where it would actually make a performance difference,
>> so maybe this needn't be quite as invasive as I first thought.
>
> I think changing only a subset of the call sites is unappealing
> because, even though it may not make a measurable performance
> difference in other cases, it may get cargo-culted into some place
> where it does make a difference.

Would it be acceptable to only get this optimisation on compilers that
support __builtin_constant_p or similar? If so, the wrapper macro could
use that to automatically pass the no-copy flag when called with a
literal string.

- ilmari
--
"I use RMS as a guide in the same way that a boat captain would use
a lighthouse. It's good to know where it is, but you generally
don't want to find yourself in the same spot." - Tollef Fog Heen