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

[J. William Campbell]

On 5/28/2019 4:19 AM, Tom Rini wrote:
> On Tue, May 28, 2019 at 05:24:34AM +0200, Marek Vasut wrote:
>> 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
> Yes, yes it would be worth seeing if other groups that have looked into
> the performance impact here have also decided to optimize this undefined
> behavior or not, thanks.

I don't think this is an optimization question, really. Calling memcpy 
with overlapping areas is an error. The result is explicitly undefined. 
It may well be that all the existing implementations effectively do 
nothing, either by explicit check or by actually copying the data over 
itself. However, to rely on that behavior is asking for trouble down the 
line. Undefined behavior is exactly that. Don't do it.


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