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. Regards, R. -- Fedora http://fedoraproject.org/wiki/User:Rathann RPMFusion http://rpmfusion.org | MPlayer http://mplayerhq.hu "Faith manages." -- Delenn to Lennier in Babylon 5:"Confessions and Lamentations" -- fedora-devel-list mailing list fedora-devel-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/fedora-devel-list
