What to do about alignment of ELF objects

[Rafael Espíndola]

On Apr 23, 2015 5:44 PM, "Sean Silva" <chisophugis at gmail.com> wrote:
>
>
>
> On Thu, Apr 23, 2015 at 2:34 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
>>
>> On 23 April 2015 at 17:22, Sean Silva <chisophugis at gmail.com> wrote:
>> >
>> >
>> > On Thu, Apr 23, 2015 at 12:12 PM, Rafael Espíndola
>> > <rafael.espindola at gmail.com> wrote:
>> >>
>> >> On 23 April 2015 at 14:17, Rui Ueyama <ruiu at google.com> wrote:
>> >> > I think the patch for LLVM looks okay, but not sure for the other
one.
>> >> >
>> >> > Your patch makes the linker to not be able to handle archive files
>> >> > containing unaligned objects, or just makes it slower? If you
cross-link
>> >> > an
>> >> > executable for machines generous for unaligned accesses, say x86, on
>> >> > not-so-generous machines, PowerPC for example, does it link fine?
>> >>
>> >> Not difference on X86 (we avoid the copy).
>> >
>> >
>> > This has the potential to radically change LLD's physical/virtual
memory
>> > usage characteristics depending on LLVM_IS_UNALIGNED_ACCESS_FAST
(LIUAF)
>> > along with total memory traffic profile and disk access patterns. For
>> > example, this patch causes the entire file to be faulted in and read up
>> > front on !LIUAF whereas the file might be faulted and touched on disk
>> > sparsely and/or in a random order when LIUAF. Realistically most
>> > benchmarking and optimization work is going to happen on x86 and so
>> > performance on !LIUAF is likely to "bit rot" (we currently don't have
any
>> > type of performance CI to avoid this; this is on my TODO list).
>> >
>> > Have you tried copying the buffers on x86? Also, if you make sure that
the
>> > incoming archives are aligned so you can avoid the copy on ppc, how
much
>> > faster does it get? I.e. does (time saved from your patch on ppc)  ==
(time
>> > copying buffers with your patch on ppc) + (time saved if we use aligned
>> > archives and avoid the copy with your patch (for testing purposes))?
>> >
>> > Can you dig in a bit deeper and figure out where this speedup is coming
>> > from? As it stands right now, this patch seems like a very
opportunistic
>> > "seems to work on my machine" speedup.
>>
>> At this I don't think ti is worth it. We don't support powerpc, which
>> is why I had to do a cross linking to benchmark it.
>
>
> Could you at least test if eagerly faulting in/loading archives speeds up
x86?
>

Sure,  I can definitely test that. I will probably do it tomorrow.

>>
>>
>> The main issue is deleting a bunch of complicated dead code (on x86)
>
>
> Ok, that I agree with. However it seems like the ownership issue would be
completely sidestepped by just using alignment 2 everywhere. It might not
be any slower.
>
> -- Sean Silva
>
>>
>> that just slows down other architectures.
>>
>> For what it is worth, gold copies data when the buffer is not
>> sufficiently aligned, so this is know to work.
>>
>> Cheers,
>> Rafael
>
>
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