Messages in this thread | | | Date | Thu, 25 Oct 2007 17:27:23 -0600 | From | Robert Hancock <> | Subject | Re: Is gcc thread-unsafe? |
| |
Arjan van de Ven wrote: > 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...
It is a win if the branch cannot be effectively predicted, i.e. if the outcome is essentially random, as may occur with data-dependent conditionals. I've seen a doubling of performance on one workload using a predicated instruction instead of a branch on newer Xeons in such a case.
I suspect that if branch prediction fails often, the data dependency created by the cmov, etc. is less expensive than the pipeline flush required by mispredicts..
-- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from hancockr@nospamshaw.ca Home Page: http://www.roberthancock.com/
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |