Re: gcc will become the best optimizing x86 compiler

[Agner Fog <agner at agner dot org>]

Denys Vlasenko wrote:
> I tend to doubt that odd-byte aligned large memcpys are anywhere
> near typical. malloc and mmap both return well-aligned buffers
> (say, 8 byte aligned). Static and on-stack objects are also
> at least word-aligned 99% of the time.
>
> memcpy can just use "relatively simple" code for copies in which
> either src or dst is not word aligned. This cuts possibilities down
> from 16 to 4 (or even 2?).
>   
The XMM code is still more than 3 times faster than rep movsl when data 
are aligned by 4 or 8, but not by 16.
Even if odd addresses are rare, they must be supported, but we can put 
the most common cases first.
strcpy and strcat can be implemented efficiently simply by calling 
strlen and memcpy, since both strlen and memcpy can be optimized very 
well. This can give unaligned addresses.

Dennis Clarke wrote:
> You forgot to look at PowerPC :
>
> http://cvs.opensolaris.org/source/xref/ppc-dev/ppc-dev/usr/src/lib/libc/ppc/gen/memcpy.s
>
> is that nice and small ?
>   
.. and slow. Why doesn't it use Altivec?