maria-discuss team mailing list archive

[Gordan Bobic <gordan.bobic@xxxxxxxxx>]

For what it's worth, unaligned access does come with a performance penalty,
typically somewhere in the 1-10% range on x86, depending on the generation
of chip used. It has been _mostly_ mitigated on recent x86 chips, and IIRC
Intel's C compiler does have an option to align all structs and arrays to a
16 byte boundary.
I would be very interested to see some tests data on unalighed access cost
on various aarch64 chips. On various 32-bit ARM chips (including those >=
ARMv6) the unaligned access performance hit was quite dramatic.

On Wed, Nov 27, 2019 at 11:36 AM Marko Mäkelä <marko.makela@xxxxxxxxxxx>
wrote:

> Hi Daniel,
>
> You seem to be right that the compilers are already mostly doing the
> right thing. Here is a notable exception where GCC lags behind clang
> (unnecessary use of stack):
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89804
>
> I created another test program, checking how mach_read_from_4() gets
> compiled. It turns out that on Aarch64 and POWER, unaligned reads are
> being used by default:
> https://godbolt.org/z/ZcavM4
>
> For 32-bit ARM, -march=armv6 seems to enable unaligned reads. For
> RISC-V and WebAssembly, the code is rather ugly. :-)
>
> So, indeed, there does not appear to be much to micro-optimize here.
>
> Marko
> --
> Marko Mäkelä, Lead Developer InnoDB
> MariaDB Corporation
>
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