http://gcc.gnu.org/onlinedocs/gcc-3.4.2/gcc/i386-and-x86_002d64-Options.html#i386-and-x86_002d64-Options
Puna
Puna Tannehill wrote:
James Green wrote:
Hi Puna,
I had a k6-2 a few years back (before discovering BSD :) and did a lot of Linux From Scratch work on it. I found that passing -march=i586 -mcpu=i686 produced by far the best results for pretty much any C/C++ code. Of course the code produced will not run on anything but a k6-2, which as I understand it is a 686 core with 586 interface/timings, and likewise if memory serves specifying only -march=i586 or -march=i686 (implying -mcpu=i586 or -mcpu=i686 respectively) won't run on the k6-2 either. Definitely a trade off between speed and (total lack of) portablility. Again that was gcc-2.9x days...
Interesting. Was there an option for -march=k6-2 at that time? Were the results based on a comparison of that setting and the ones you mention above?
Do you happen to know if there is a particular benchmarking program that might be useful to testing different compiles in FreeBSD?
I've also seen recommendations using '586/mmx' and 'k7', but it seems interesting that someone would create a 'k6-2' flag if there were not significant and benefitial optimizations that would be applied. Of course, whether anyone coded for that particular processor is probably extemely rare, so I can see how the -march -mcpu combination you suggested would probably be a better choice.
Here are the relavent bits from dmesg (Compaq Presario 1692):
CPU: AMD-K6(tm) 3D processor (432.98-MHz 586-class CPU) Origin = "AuthenticAMD" Id = 0x58c Stepping = 12 Features=0x8021bf<FPU,VME,DE,PSE,TSC,MSR,MCE,CX8,PGE,MMX> AMD Features=0x80000800<SYSCALL,3DNow!> K6-family MTRR support enabled (2 registers)
As far as ports such as Xorg/Xfree86, I am not entirely clear on CFLAGS inheritance, but AFAIK Xorg/Xfree don't gain much/anything from optimisation over than your usual -O2 and friends. I understand that this is down to whether they have been written to make use of these cpu functions/optimisations. On the other hand though, it is the specific applications that run under X, such as mplayer that tend to be written to make use of mmx, sse, 3dnow etc. because for graphics it makes a _big_ difference. Generally you find toggles in the Makefile to enable/force certain optimisation. Definitely worth looking at.
According to the latest GCC, you can use -m3dnow -mmmx and it is of
some benefit when comiling XF86 (and hopefully Xorg). I can't find the page offhand, but it was in the GCC Documentation, and I posted it in other responses of this same thread. I haven't been
able to test it yet, as I'm still compiling Xorg as we speak.
Puna
On Mon, 2004-09-13 at 17:07, Puna Tannehill wrote:
I've been looking for possible flags, optimizations, really anything that would help me setup my laptop to use mmx and 3dnow. I've updated /etc/make.conf to -march to the drum of a k6-2, but I'm not even sure if mmx and 3dnow are being taken into consideration for compiling and such, especially for Xorg.
I did some googling and found people who used CFLAGS like -mmmx and -m3dnow, but when I run with those options, they fail and said to be invalid. they don't appear in 'man gcc' which should have been the first place i looked. I'm not finding anything in terms of compiling or configuring Xorg to use 3dnow or mmx, or even how to check to see if they are automatically detected and used.
Any thoughts?
Puna
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