I am working on toolchain upgrade recently, from gcc 3.4.6 to gcc 4.1.1.
Thing I learned during the upgrade.
1. libgcc contains compiler specific library, usually used for floating point computation. For example, 64 bit floating point has no support on the host system, the libgcc has to convert the computation into some native instructions.
You can use "gcc -v" to figure out the default library path "-L xxx" used by gcc, besides the path passed by your Makefile. "gcc -v" can also give you the exact command called by gcc, as gcc itself is an umbrella program, which calls "cc1", "collect2" etc.
2. -ffreestanding, flag may be used for kernel compilation. It implies that standard library may not exist and the program startup may not necessarily be at "main". To use gcc 4.1.1 to compile linux kernel 2.6.17, we need to add -ffreestanding in our makerules.
3. Optimization, gcc optimize the code differently in each version. Some functions were optimized away in kernel, but called in bootloader. I have to create a dummy function in bootloader to avoid linking problem.
4. -std=gnu99. I've met several preprocessing error when I was building gcc in buildroot. cpp was complaining about the unknown labels in assembly code. It turns out the the cpp using gnu standard is not 100% compatible with the iso c standard. By removing the -std=gnu99 flag, I can get gcc compiled.
5. -sysroot. This option will enable you to use a different set of "/include", "/lib" directories in a different root. I think that it may be useful in cross-compiling environment.
