Re: [Ksummit-discuss] [TECH TOPIC] Compiler shopping list

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2016-07-22 at 10:17 -0700, James Bottomley wrote:
> On Fri, 2016-07-22 at 19:05 +0200, Christian Borntraeger wrote:
> > On 07/22/2016 05:59 PM, David Woodhouse wrote:
> > > On Fri, 2016-07-22 at 17:52 +0200, Christian Borntraeger wrote:
> > > > On 07/22/2016 12:41 PM, David Howells wrote:
> > > > > Are there additional things we can get the compiler to do for 
> > > > > us?  Some things I've seen brought up:
> > > > > 
> > > > >  (1) Additional __atomic_*() ops could be useful.  Suggestions 
> > > > > I've heard      include direct LL/SC support - though the 
> > > > > compiler people don't seem so keen on that.
> > > > > 
> > > > >  (2) -mmodel=kernel flag so that the compiler can optimise 
> > > > > better for the kernel memory model.
> > > > 
> > > > Some years ago (actually many) Linus proposed to have an 
> > > > endianess attribute to data types, so that the compiler can do 
> > > > the bswap automatically. For some reason this was never
> > > > implemented, but this might be a good idea anyway.
> > > > 
> > > > e.g. 
> > > > 
> > > > unsigned long x[10] __attribute__(("bigendian"));
> > > 
> > > I'm not sure Linus proposed that. I certainly did, many times.
> > 
> > Yes, I know at least 3 people suggesting that and thinking this is 
> > useful ( Can you beat Linus' 2001 
> > https://gcc.gnu.org/ml/gcc/2001-12/msg00932.html? ;-) )
> 
> The fact that it's been discussed on and off for 15 years tends to
> suggest that where we've ended up is about good enough for everyday and
> no-one can really be bothered to take the extra effort.
> 
> So what's the overriding reason we should spend the extra effort now?

I don't think there's much more to be gained than what I've already
done. We *have* the compiler visibility into endianness changes, so it
can optimise them correctly.

What we *don't* have is the ease of programming, where we can happily
*forget* that certain storage is of a specific endianness, do a direct
assignment and let the compiler sort it out for us. But that's not
actually much use until we can depend on *everyone* having a compiler
that supports that. And we have the sparse attributes to catch mistakes
where we *forget* the explicit handling anyway, so I don't think
there's really much more to be gained.

-- 
dwmw2

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