Andres Freund <andres@anarazel.de> writes:
> On 2017-11-11 12:41:40 -0500, Tom Lane wrote:
>> Yeah, changing MAXALIGN is out of the question. I'm thinking about
>> another flag bit for MemoryContextAllocExtended. Do we need to think
>> about other use-cases besides int128? Should we just force 16-byte
>> alignment on all architectures, or does it need to be platform-specific?
> I'm not sure we want to
> a) Rely on one alignment being enough for everybody.
> b) Additionally burden already hot code paths with a growing number of
> alignment flag tests, and the necessary math.
Well, (a) we could have more flag bits later if there are other use-cases
that require even stricter alignment, and (b) I do not believe that
MemoryContextAllocExtended is a hot code path at present; there are not
enough call sites.
> How about a MemoryContextAllocAligned(context, size, alignto, flags)
> that passes on most flags but adds the necessary overhead to size, and
> padds the result appropriately?
This'd result in hard-wiring the alignment requirement at call sites,
which I think might not be a great idea. For example, one plausible
future use-case is "align on cacheline boundaries". I think that would
be better served by a flag like MCXT_ALLOC_ALIGN_CACHELINE than by
having the callers demand a specific numeric alignment value --- it'd
be a lot easier to make the alignment match actual hardware requirements
if it were being inserted at one specific place.
regards, tom lane
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