Hi list,
gcc-4.3 appears to make quite heavy use of cmov to eliminate
conditional branches on x86(-64) architecture, even for those
branches that are determined to be predictable.
The problem with this is that the data dependancy introduced
by the cmov can restrict execution, wheras a predicted b
On Tuesday 26 February 2008 21:14, Jan Hubicka wrote:
> Hi,
>
> > Core2 follows a similar pattern, although it's not seeing any
> > slowdown in the "no deps, predictable, jmp" case like K8 does.
> >
> > Any comments? (please cc me) Should gcc be using conditional jumps
> > more often eg. in the cas
On Wednesday 27 February 2008 03:06, J.C. Pizarro wrote:
> Compiling and executing the code of Nick Piggin at
> http://gcc.gnu.org/ml/gcc/2008-02/msg00601.html
>
> in my old Athlon64 Venice 3200+ 2.0 GHz,
> 3 GiB DDR400, 32-bit kernel, gcc 3.4.6, i got
>
> $ gcc -O3 -falign
On Monday 03 March 2008 22:38, Jan Hubicka wrote:
> Hi,
> I had to tweak the testcase a bit to not compute minimum: GCC optimizes
> this early into MIN_EXPR throwing away any profile information. If we
> get serious here we can maintain it via histogram, but I am not sure it
> is worth the effort
On Tuesday 04 March 2008 00:01, Jan Hubicka wrote:
> > On Monday 03 March 2008 22:38, Jan Hubicka wrote:
> > I hope so too. For the kernel we have some parts where
> > __builtin_expect is used quite a lot and noticably helps, and could
> > help even more if we cut down the use of cmov too. I guess
