Re: 2.6.13/14 x86 Makefile - Pentiums penalized ?

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Wed, 14 Sep 2005, Denis Vlasenko wrote:

> > It's documented as being suboptimal to use inc/dec due to it modifying all 
> > of eflags resulting in dependency related stalls. add/sub only modifies 
> > one bit of eflags so is more optimal. However there is a problem of 
> 
> ?! add/sub doesn't modify "only one bit in eflags", it modifies all.
> In fact, it's dec/inc which does not modify all bits.
> It doesn't touch 'carry' bit (IIRC).
> 
> If inc/dec is slower on P4, it must be just another P4 quirk.

You're right about the add and the number of modified bits. The documented 
part is found in the P4 optimisation manual;

"The inc and dec instructions should always be avoided. Using add
 and sub instructions instead avoids data dependence and improves
 performance."

 -- 2-12 IA-32 Intel Architecture Optimization Reference Manual
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]
  Powered by Linux