On 5/28/19 1:19 PM, 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 <ma...@denx.de> >>>>>>>> Cc: Michal Simek <michal.si...@xilinx.com> >>>>>>>> Cc: Tom Rini <tr...@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 will just drop this patch, since U-Boot memcpy() implementation does this check. But let me be very clear here, that check is part of undefined behavior (!) and I don't think it's the right thing to do in memcpy() itself. -- Best regards, Marek Vasut _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de https://lists.denx.de/listinfo/u-boot
