svn commit: r333240 - in head/sys: powerpc/powerpc sys

[Mateusz Guzik]

On Sat, May 5, 2018 at 2:00 PM, Alexey Dokuchaev <danfe at freebsd.org> wrote:

> On Sat, May 05, 2018 at 10:38:29AM +1000, Bruce Evans wrote:
> > ...
> > Summary: this change wouldn't have passed my review.  I have used similar
> > changes for 15-20 years but never made them production quality since
> there
> > are too many variations to consider and testing showed insignificant
> > improvements except for micro-benchmarks.
>
> Perhaps Foundation could sponsor your work to make them production quality
> Bruce.  :-)
>
> Seriously though, I'm a bit worried to see these commits happening at the
> same time correctness of the implementation is still under discussion and
> disputes.  Shall I expect that after my next -CURRENT update things would
> suddenly stop working? [1]
>
> ./danfe
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=638477
>
>
Nothing of the sort was done here. They had a memcpy function which
internally behaved like memmove. They changed to behave like mere
memcpy (i.e. assume non-overlapping buffers). This caused buggy code
to run into trouble.

bcopy has memmove semantics and this change keeps that. The only
difference is that instead of always generating a call, the compiler is
allowed to perform the copy in place in certain cases. This is precisely
what happens almost everywhere and it was not happening in the
kernel because of a giant hammer -fno-builtin flag (added to reduce
changes the compiler will do stuff the kernel does not want to deal
with).

-- 
Mateusz Guzik <mjguzik gmail.com>