TITLE: Compiler-optimization LFS VERSION: any AUTHOR: Gerard Beekmans , SYNOPSIS: How to use compiler-optimization HINT: Thomas -Balu- Walter is equally the author of this hint, but due to format restrictions I had to remove one of the two email addresses from the AUTHOR field. -SP The origin of this text is the 2.4.3-version of the book - Chapter 6. I modified it a little to create this hint. Most programs and libraries by default are compiled with optimizing 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 those package 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=" && 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 that 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 unreliable 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 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-3.3.1/gcc/Submodel-Options.html /* * Ed. note * "Reboant" dropped a note about how using -Os (optimize for size) showed * incredibly good results. So if you want is small binary size rather than fast * execution time, you might want to take a look at this. */ 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 all together. 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 (most noticable in RedHat because RedHat often uses beta software which aren't always very stable. "RedHat likes living on the bleeding edge, but leaves the bleeding up to you" (quoted from somebody on the lfs-discuss mailinglist).