On Tue, Feb 03, 2009 at 09:45:46PM +0100, Dominik 'Rathann' Mierzejewski wrote: >On Tuesday, 03 February 2009 at 21:01, Ulrich Drepper wrote: >> Dominik 'Rathann' Mierzejewski wrote: >> > I'd like to see a case (not involving Pentium 4) where using cmov is slower >> > than not using it. It definitely is faster for decoding H.264 in FFmpeg >> > for example. >> >> I don't have a specific test case. But I do talk to the CPU >> architectures at Intel regularly. > >I didn't know architectures could talk. ;) > >> They always say the cmov should be >> avoided. Especially with the introduction of the fused micro-ops the >> various cmp+jcc pairs are likely move faster. >> >> And from the code generation perspective using cmp+jcc is also more >> flexible. With cmov you have to tie up two registers. This is >> particularly bad with the x86 ABI. >> >> There are certainly cases where cmov can be faster. Perhaps exclusively >> on older micro architectures (P4s, early Core2, maybe AMD, haven't >> checked). But in general it's no win. > >Well, I talk to people who write hand-optimized assembly and care to >squeeze every cycle out of various CPUs and they say it's definitely >a win. So please, show me some code instead of hand-waving. If they can do that, then why can't they rebuild things themselves? josh -- fedora-devel-list mailing list fedora-devel-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/fedora-devel-list
