bcopy/memmove optimization broken ? [looks like you are correct to me, I give supporting detail]

[Warner Losh]

On Mon, Feb 13, 2017 at 9:04 PM, Mark Millard <markmi at dsl-only.net> wrote:
> On 2017-Feb-13, at 1:27 AM, Mark Millard <markmi at dsl-only.net> wrote:
>
>> As the decision about when to call the code that can
>> deal with overlapping memory regions is wrong, the code
>> that should only be used for non-overlaping regions likely
>> would handle some overlapping regions and so would operate
>> incorrectly in at least some cases.
>>
>> In other words, I think the bug is worse than just an
>> example of being sub-optimal: the code is wrong from what
>> I can tell. (I've no clue if the code is ever put to use
>> for any bad cases.)
>
> I was wrong about the error status, possibly for multiple reasons,
> but the following is sufficient:
>
> https://www.freebsd.org/cgi/man.cgi?query=memcpy&sektion=3
>
> says:
>
>      In this implementation memcpy() is implemented using bcopy(3), and there-
>      fore the strings may overlap.  On other systems, copying overlapping
>      strings may produce surprises.  Programs intended to be portable should
>      use memmove(3) when src and dst may overlap.
>
> so the branch taken case for:
>
> bcc PIC_SYM(_C_LABEL(memcpy), PLT)
>
> also deals with overlaps since FreeBSD criteria is
> that memcpy does so. (I had been thinking that it
> did not deal with such.)
>
>
> Side note:
>
> Notably the arm implementation of FreeBSD memcpy does not call
> bcopy (that would be recursive in the arm implementation).
> memcpy just needs to have some properties that bcopy also has.
>
> This suggests that memcpy vs. bcopy may have a performance
> Principle of Least Astonishment violation since memcpy may well
> perform differently than bcopy for some types of contexts but
> memcpy is supposed to use bcopy.
>
>
> [A varient of these notes are in the comments for bugzilla
> 217065.]

Seems like the memcpy man page should be softened to reflect the !x86
reality. If we provide different semantics between different arches,
we should consider carefully why and document in the code or change.

Warner