Re: [TRIVIAL] kstrdup

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    Jeff Garzik <[e-mail]>
In newsgroup: linux.dev.kernel
> 
> That's kinda cute.  Why not submit a patch to the strcpy implementation 
> in include/asm-i386/string.h?  :)  Ours is shorter, but does have a jump:
>          "1:\tlodsb\n\t"
>          "stosb\n\t"
>          "testb %%al,%%al\n\t"
>          "jne 1b"
> 
> Which is better?  I don't know; I'm still learning the performance 
> eccentricities of x86 insns on various processors.
> 

It varies from porocessor to processor, and also depends on usage.

> 
> Related x86 question:  if the memory buffer is not dword-aligned, is 
> 'rep movsl' the best idea?  On RISC it's usually smarter to unroll the 
> head of the loop to avoid unaligned accesses; but from reading x86 asm 
> code in the kernel, nobody seems to care about that.  Is the 
> unaligned-access penalty so small that the increased code size of the 
> head-unroll is never worth it?
> 

A lot of the newer x86 processors will perform this unrolling in
microcode.

	-hpa
-- 
<[e-mail]> at work, <[e-mail]> in private!
"Unix gives you enough rope to shoot yourself in the foot."
Architectures needed: ia64 m68k mips64 ppc ppc64 s390 s390x sh v850 x86-64