Optimization
Most programs and libraries by default are compiled with optimization level 2 (gcc options -g and -O2) and are compiled for a specific CPU. On Intel platforms software is compiled for i386 processors by default. If you don't wish to run software on other machines other than your own, you might want to change the default compiler options so that they will be compiled with a higher optimization level, and generate code for your specific architecture.
There are a few ways to change the default compiler options. One way is to edit every Makefile file you can find in a package, look for the CFLAGS and CXXFLAGS variables (a well designed package uses the CFLAGS variable to define gcc compiler options and CXXFLAGS to define g++ compiler options) and change their values. Packages like binutils, gcc, glibc and others have a lot of Makefile files in a lot of subdirectories so this would take a lot of time to do. Instead there's an easier way to do things: create the CFLAGS and CXXFLAGS environment variables. Most configure scripts read the CFLAGS and CXXFLAGS variables and use them in the Makefile files. A few packages don't follow this convention and require manual editing.
To set those variables you can do the following commands in bash (or in your .bashrc if you want them to be there all the time):
export CFLAGS="-O3 -march=<architecture>" && CXXFLAGS=$CFLAGS
This is a minimal set of optimizations that ensures it works on almost all platforms. The option -march will compile the binaries with specific instructions for the CPU you have specified. This means you can't copy this binary to a lower class CPU and execute it. It will either work very unreliably or not at all (it will give errors like "Illegal Instruction, core dumped"). You'll have to read the GCC Info page to find more possible optimization flags. In the above environment variable you have to replace <architecture> with the appropriate CPU identifiers such as i586, i686, powerpc and others. I suggest to have a look at the gcc-manual at http://gcc.gnu.org/onlinedocs/gcc_toc.html "Hardware Models and Configurations".
Please keep in mind that if you find that a package doesn't compile and gives errors like "segmentation fault, core dumped" it's most likely got to do with these compiler optimizations. Try lowering the optimizing level by changing -O3 to -O2. If that doesn't work try -O or leave it out altogether. Also try changing the -march variable. Compilers are very sensitive to certain hardware too. Bad memory can cause compilation problems when a high level of optimization is used, like the -O3 setting. The fact that I don't have any problems compiling everything with -O3 doesn't mean you won't have any problems either. Another problem can be the Binutils version that's installed on your system which often causes compilation problems in Glibc. Development builds often use new software which isn't always very stable.
Contents |
Definitions of Flags
For more information on compiler optimization flags see the GCC Commands page in the Online GCC 4.3.2 Manual at:
http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Submodel-Options.html#Submodel-Options
-s
A linker option that removes all symbol table and relocation information from the binary.
-O3
This flag sets the optimizing level for the binary.
3 Highest level, machine specific code is generated.
Auto-magically adds the -finline-functions and
-frename-registers flags.
2 Most make files have this set up as Default, performs all
supported optimizations that do not involve a space-speed
tradeoff. Adds the -fforce-mem flag auto-magically.
1 Minimal optimizations are performed. Default for the compiler,
if nothing is given.
0 Don't optimize.
s Same as O2 but does additional optimizations for size.
-fomit-frame-pointer
Tells the compiler not to keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.
-march={cpu_type}
Defines the instruction set to use when compiling. When only -march is used, -mpcu is implied be the same as -march.[BR] i486 Intel/AMD 486 Processor pentium Intel Pentium Processor pentiumpro Intel Pentium Pro Processor pentium2 Intel PentiumII/Celeron Processor pentium3 Intel PentiumIII/Celeron Processor pentium4 Intel Pentium 4/Celeron Processor k6 AMD K6 Processor k6-2 AMD K6-2 Processor k6-3 AMD K6-3 Processor athlon AMD Athlon/Duron Processor athlon-tbird AMD Athlon Thunderbird Processor athlon-4 AMD Athlon Version 4 Processor athlon-xp AMD Athlon XP Processor athlon-mp AMD Athlon MP Processor winchip-c6 Winchip C6 Processor winchip2 Winchip 2 Processor c3 VIA C3 Cyrix Processor
-mmmx -msse -msse2 -m3dnow
These switches enable or disable the use of built-in functions that allow direct access to the MMX, SSE and 3Dnow extensions of the instruction set.
Optimization Links
Safe flags to use for Gentoo http://www.gentoo.org/doc/en/gcc-optimization.xml
Optimization in GCC http://www.linuxjournal.com/article/7269
Personal Experience
I have tried using all optimization levels, but to my disappointment, results varied from package to package. Using -O (any number) with any GCC version can give unpredictable responses.
Some of those unpredictable responses can be seen with the following bugs sent to GCC.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=1259
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10655
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=8440
Release by Jim Gifford
I Jim Gifford < jim at cross-lfs dot org> release this document for publication into the Cross-LFS Hints Wiki on 1/12/2009
Previous Authors
Gerard Beekmans < gerard at linuxfromscratch dot org > Thomas Balu-Walter < tw at itreff dot de > Eric Olinger <eric at supertux dot com>
License: This material may be distributed only subject to the terms and conditions set forth in the Open Publication License v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/).
