[FFmpeg-devel] [PATCH] Improve handling of cmov usage enablement

Dominik 'Rathann' Mierzejewski dominik
Sat Nov 1 14:43:51 CET 2008

On Saturday, 01 November 2008 at 14:05, Reimar D?ffinger wrote:
> On Sat, Nov 01, 2008 at 01:52:02PM +0100, Dominik 'Rathann' Mierzejewski wrote:
> > The attached patch enables CMOV by default on x86_64 arch, because
> > all x86_64 CPUs have it. It doesn't enable fast_cmov, because x86_64
> > includes some NetBurst-architecture CPUs from Intel, which are known
> > to have a slow implementation of CMOV.
> > 
> > The second part of the patch disables fast_cmov for the "i686" cpu
> > target, because that target might include NetBurst-arch CPUs.
> > 
> > OK to apply?
> Given the statements by Intel I saw, my opinion is that we should go
> with "enable fast_cmov unless we are really sure it is slow" - unless it
> is some really predictable condition (in which case cmov probably is a
> bad idea for all CPUs) cmov is likely to be faster on all CPUs except
> netburst, including future CPUs.
> Or in other words: Netburst is the special case, and our configure
> should treat it as such, and personally my opinion is: you bought a
> crappy CPU when everybody could have told you it is stupid (and going by
> my experience a lot of people have told and been ignored), now live with
> it, not our problem unless it is no effort for us. Complain to Intel if
> anything.

:) I'm fine with that, too. Updated patch attached.


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-------------- next part --------------
Index: configure
--- configure	(revision 15761)
+++ configure	(working copy)
@@ -1245,6 +1245,8 @@
+        enable cmov
+        enable fast_cmov
         enable fast_unaligned
         check_cc <<EOF && enable fast_64bit && arch="x86_64"
         int test[sizeof(char*) - 7];

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