Re: [PATCH] slob: push the min alignment to long long

[Matt Mackall <mpm@selenic.com>]

On Thu, 2011-06-16 at 09:59 +0300, Pekka Enberg wrote:
> On Thu, Jun 16, 2011 at 1:53 AM, Matt Mackall <mpm@selenic.com> wrote:
> >> Blink... because the compiler doesn't provide a portable way to
> >> do this, right? :-)
> >
> > Because I, on x86, cannot deduce the alignment requirements of, say,
> > CRIS without doing significant research. So answering a question like
> > "are there any architectures where assumption X fails" is obnoxiously
> > hard, rather than being a grep.
> >
> > I also don't think it's a given there's a portable way to deduce the
> > alignment requirements due to the existence of arch-specific quirks. If
> > an arch wants to kmalloc its weird crypto or SIMD context and those want
> > 128-bit alignment, we're not going to want to embed that knowledge in
> > the generic code, but instead tweak an arch define.
> >
> > Also note that not having generic defaults forces each new architectures
> > to (nominally) examine each assumption rather than discover they
> > inherited an incorrect default somewhere down the road.
> 
> I don't agree. I think we should either provide defaults that work for
> everyone and let architectures override them (which AFAICT Christoph's
> patch does) or we flat out #error if architectures don't specify
> alignment requirements.

Uh, isn't the latter precisely what I say above?

>  The current solution seems to be the worst one
> from practical point of view.

Good, because no one's advocating for it.

> This doesn't seem to be a *regression* per se so I'll queue
> Christoph's patch for 3.1 and mark it for 3.0-stable.

>                             Pekka


-- 
Mathematics is the supreme nostalgia of our time.


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