Re: Is gcc thread-unsafe?

Måns Rullgård
2007-10-25T18:45:57Z

Arjan van de Ven <arjan@infradead.org> writes:

> On Wed, 24 Oct 2007 21:29:56 -0700
> "David Schwartz" <davids@webmaster.com> wrote:
>
>>
>> > Well that's exactly right. For threaded programs (and maybe even
>> > real-world non-threaded ones in general), you don't want to be
>> > even _reading_ global variables if you don't need to. Cache misses
>> > and cacheline bouncing could easily cause performance to completely
>> > tank in some cases while only gaining a cycle or two in
>> > microbenchmarks for doing these funny x86 predication things.
>>
>> For some CPUs, replacing an conditional branch with a conditional
>> move is a *huge* win because it cannot be mispredicted.
>
> please name one...
> Hint: It's not one made by either Intel or AMD in the last 4 years...

ARM. On ARM1136 (used in the Nokia N800) a mispredicted branch takes
5-7 cycles (a correctly predicted branch takes 0-4 cycles), while a
conditional load, store or arithmetic instruction always takes one
cycle.

--
Måns Rullgård
mans@mansr.com