Re: [PATCH] Fast String Hash

[KHMan <keinhong@...>]

David Olofson wrote:
> On Monday 17 December 2007, Mike Pall wrote:
> [...]
> [snip snip]
> Could probably be config/compile time detected and used only on 
> platforms that need it. Unfortunately though, there are CPU families 
> where older ones can't do unaligned access, whereas newer ones can; 
> for example MC68000..20 vs MC60030, but well... I guess you'd just 
> compile for the most restricted CPUs your code will be used on - just 
> as it is with the x86 family and SIMD extensions and whatnot.
> 
> BTW, have you considered the penalty of unaligned access in this 
> context? (Don't even know if it's significant on modern x86 in 
> situations like this...)

As far as manufacturers' claims go...

AMD #25112 PDF p. 111 says a minimum 1-cycle penalty for unaligned
access, more for 64-bit.

Intel #248966 PDF p. 122 doesn't say anything specific about
penalty, only the usual recommendation, but it does have a big
warning about accesses across cache lines.

I can't find any micro benchmark that checks for unaligned access
penalty. lmbench development seems more focused on cache and cores.

Well, it's only 2 unaligned accesses... Still, it would be
interesting and useful to quantify unaligned access behaviour, and
analyze and optimize the snippet.

-- 
Cheers,
Kein-Hong Man (esq.)
Kuala Lumpur, Malaysia