Re: Architectures where unaligned access is (not) OK?
On 23.11.2014 23:11, Simon McVittie wrote:
> On 23/11/14 17:55, Simon McVittie wrote:
>> Unfortunately, on my x86-64 laptop, my patched liblzo2 with
>> -DLZO_CFG_NO_UNALIGNED on all architectures seems to be half as fast as
>> the unpatched one
> [...]
>> I'm trying out a slightly different approach: keeping the unaligned
>> accesses via casts like *(uint16_t *) on architectures where lzodefs.h
>> specifically allows them, but disabling the casts via
>> struct { char[n] } conditional on alignof(that struct) == 1, which seem
>> to be the problematic ones.
>
> That fixed the performance regression on amd64 while still working
> correctly on armv5tel, so I've uploaded it as a DELAYED/7 NMU. See
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=757037 for nmudiff.
>
> If anyone has better ideas, I'm happy to cancel the delayed upload and
> let someone take over fixing the bug.
>
what works well is just replacing the offending memory loads with the
memcpy call. As the size of the memcpy call is constant the compiler
will take care of emitting code appropriate for the platform.
It doesn't help speed up the code on the trapping arches but at least
one does not penalize the ones where it is allowed.
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