CPU dependent code: how to avoid function pointers?

Question

I have performance critical code written for multiple CPUs. I detect CPU at run-time and based on that I use appropriate function for the detected CPU. So, now I have to use fun

Answer 1

Lazy loading ELF symbols from shared libraries is described in section 1.5.5 of Ulrich Drepper's DSO How To (updated 2011-12-10). For ARM it is described in section 3.1.3 of ELF for ARM.

EDIT: With the STT_GNU_IFUNC extension mentioned by R. I forgot that was an extension. GNU Binutils supports that for ARM, apparently since March 2011, according to changelog.

If you want to call functions without the indirection of the PLT, I suggest function pointers or per-arch shared libraries inside which function calls don't go through PLTs (beware: calling an exported function is through the PLT).

I wouldn't patch the code at runtime. I mean, you can. You can add a build step: after compilation disassemble your binaries, find all offsets of calls to functions that have multi-arch alternatives, build table of patch locations, link that into your code. In main, remap the text segment writeable, patch the offsets according to the table you prepared, map it back to read-only, flush the instruction cache, and proceed. I'm sure it will work. How much performance do you expect to gain by this approach? I think loading different shared libraries at runtime is easier. And function pointers are easier still.