[U-Boot] [PATCH 1/3] spl: ram: Do not memcpy() identical buffers

[Marek Vasut]

On 5/28/19 5:04 AM, Tom Rini wrote:
> On Tue, May 28, 2019 at 04:44:52AM +0200, Marek Vasut wrote:
>> On 5/28/19 4:42 AM, Tom Rini wrote:
>>> On Tue, May 28, 2019 at 04:07:44AM +0200, Marek Vasut wrote:
>>>> On 5/28/19 4:06 AM, Tom Rini wrote:
>>>>> On Tue, May 28, 2019 at 03:49:13AM +0200, Marek Vasut wrote:
>>>>>
>>>>>> If the source and destination buffer address is identical, there is
>>>>>> no need to memcpy() the content. Skip the memcpy() in such a case.
>>>>>>
>>>>>> Signed-off-by: Marek Vasut <marex at denx.de>
>>>>>> Cc: Michal Simek <michal.simek at xilinx.com>
>>>>>> Cc: Tom Rini <trini at konsulko.com>
>>>>>
>>>>> Shouldn't memcpy catch that itself?
>>>>>
>>>> memcpy(3) says
>>>>        The memcpy() function copies n bytes from memory area src to
>>>> memory area dest.  The memory areas must not overlap.  Use memmove(3) if
>>>> the memory areas do overlap.
>>>
>>> OK, and shouldn't memcpy optimize that case?  Does it usually?
>>
>> As the manpage says "The memory areas must not overlap." , I would
>> expect it does not have to ?
> 
> I guess I'm not being clear enough, sorry.  Go look at how this is
> implemented in a few places please and report back to us.  Someone else,
> or many someone else, have probably already figured out if optimizing
> this case in general, in memcpy, is a good idea or not.  Thanks!

If even [1] says the behavior is undefined, then what does it matter
whether some implementation added an optimization for such undefined
behavior? We cannot depend on it, can we ?

[1] https://pubs.opengroup.org/onlinepubs/009695399/functions/memcpy.html

-- 
Best regards,
Marek Vasut