$CFLAGES and $C++FLAGES what do I set these for to get the most out of my CPU

rob.rice · 08-09-2008, 11:43 AM

when I build something from source I want it to match the CPU and use as much of what it has as it can so here is my /proc/cpuinfo file

Code:

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 22
model name      : Intel(R) Celeron(R) CPU          540  @ 1.86GHz
stepping        : 1
cpu MHz         : 1862.100
cache size      : 1024 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2
 ss tm pbe nx lm constant_tsc up arch_perfmon pebs bts pni monitor ds_cpl tm2 ssse3 cx16 xtpr lahf_lm
bogomips        : 3728.21
clflush size    : 64

not the best CPU but it's best one I have ever had
so what do I set $CFLAGES and $C++FLAGES to

weibullguy · 08-09-2008, 12:14 PM

Depends to some extent on the version of gcc you use. Look at the gcc online docs, for the version of gcc you are using, --> http://gcc.gnu.org/onlinedocs/ in the section title "Intel 386 and AMD x86-64 Options" for processor-specific settings.

BTW, it's CXXFLAGS, not C++FLAGS.

rob.rice · 08-09-2008, 06:47 PM

"gcc --version"returns 4.2.3
I've been looking at info gcc but I don't know what options apply to this cpu
other than -mtune=_native_

weibullguy · 08-09-2008, 11:08 PM

Use the 4.2.4 version of the gcc manual.

With a quick search of Google, I can't find any reference to a family 6, model 22 Celeron so I don't know which microarchitecture yours is based on. --mtune=native or --march=native (if you are not building binaries for another machine) works. You can force the -mmmx, -msse, -msse2, and -msse3 since you have those registers. A reasonable optimization level is -O2. You could use -pipe so the compilation process uses pipe instead of intermediate files. Other than that, there's not much else unless you really know what the option is going to do to the resulting binary.

ErV · 08-10-2008, 12:06 AM

Quote:

Originally Posted by weibullguy

A reasonable optimization level is -O2.

Why not -O3 ?

H_TeXMeX_H · 08-10-2008, 04:24 AM

According to this:
http://gentoo-wiki.com/Safe_Cflags#I..._T20xx.2FT21xx

it should be:

Code:

CFLAGS="-march=prescott -O2 -pipe -fomit-frame-pointer"
CXXFLAGS="${CFLAGS}"

@ ErV

I also recommend against using '-O3' because it WILL break some programs. I've had this happen to me and it took a while to debug, and then I found out that changing '-O3' to '-O2' fixed the problem. You can take your chances with '-O3', maybe newer gcc versions will no longer break programs when given this option, but I wouldn't use it if you want to keep your system as stable as possible.

weibullguy · 08-10-2008, 10:00 AM

@H_TeXMeX_H, I'm a Gentoo user and I completely forgot about that page. Thanks!

@ErV, let me join H_TeXMeX_H in strongly discouraging the use of -O3. Some things will, without a doubt, break and you won't notice a difference over using -O2.

H_TeXMeX_H · 08-10-2008, 02:41 PM

Yeah, I like the gentoo-wiki, it has useful info for all Linux users, not just for Gentoo (although many topics are Gentoo-specific).