Re: Computer architects leaving Intel...

anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>

mitchalsup@aol.com (MitchAlsup1) writes:
>

So:
# define memcpy memomve

>
Incidentally, if one wants to do this, it's advisable to write
>
 #undef  memcpy
>
before the #define of memcpy.
>

and move forward with life--for the 2 extra cycles memmove costs
it saves everyone long term grief.

>
Is it two extra cycles?  Here are some data points from
<2017Sep23.174313@mips.complang.tuwien.ac.at>:
>
Haswell (Core i7-4790K), glibc 2.19
   1    8   32   64  128  256  512   1K  2K    4K   8K  16K block size
  14   14   15   15   17   30   48   85  150  281  570 1370 memmove
  15   16   13   16   19   32   48   86  161  327  631 1420 memcpy
>
Skylake (Core i5-6600K), glibc 2.19
   1    8   32   64  128  256  512   1K  2K    4K   8K  16K block size
  14   14   14   14   15   27   43   77  147  305  573 1417 memmove
  13   14   10   12   14   27   46   85  165  313  607 1350 memcpy
>
Zen (Ryzen 5 1600X), glibc 2.24
   1    8   32   64  128  256  512   1K  2K    4K   8K  16K block size
  16   16   16   17   32   43   66  107  177  328  601 1225 memmove
  13   13   14   13   38   49   73  116  188  336  610 1233 memcpy
>
I don't see a consistent speedup of memcpy over memmove here.
>
However, when one uses memcpy(&var,ptr,8) or the like to perform an
unaligned access, gcc transforms this into a load (or store) without
the redefinition of memcpy, but into much slower code with the
redefinition (i.e., when using memmove instead of memcpy).
>

Simply replacing memcpy() by memmove() of course will always
work, but there might be negative consequences beyond a cost
of 2 extra cycles -- for example, if a negative stride is
better performing than a positive stride, but the nature
of the compaction forces memmove() to always take the slower
choice.

>
If the two memory blocks don't overlap, memmove() can use the
fastest stride.

It /could/ use the fastest stride.  Whether it /does/ use the
fastest stride is a different question (and one that may have
different answers on different platforms).