Register allocation rules in code generated by major C/C++ compilers

Question

I remember some rules from a time ago (pre-32bit Intel processors), when was quite frequent (at least for me) having to analyze the assembly output generated by C/C++ compilers (in my case, Borland/Turbo at that time) to find performance bottlenecks, and to safely mix assembly routines with C/C++ code. Things like using the SI register for the this pointer, AX being used for return values, which registers should be preserved when an assembly routine returns, etc.

Now I was wondering if there's some reference for the more popular C/C++ compilers (Visual C++, GCC, Intel...) and processors (Intel, ARM, ...), and if not, where to find the pieces to create one. Ideas?

Answer 1

You are asking about "application binary interface" (ABI) and calling conventions. These are typically set by operating systems and libraries, and enforced by compilers and linkers. Google for "ABI" or "calling convention." Some starting points from Wikipedia and Debian for ARM.

Answer 2

Agner Fog's "Calling Conventions" document summarizes, amongst other things, the Windows and Linux 64 and 32-bit ABIs: http://www.agner.org/optimize/calling_conventions.pdf. See Table 4 on p.10 for a summary of register usage.

One warning from personal experience: don't embed assumptions about the ABI in inline assembly. If you write a function in inline assembly that assumes return and/or parameter transfer in particular registers (e.g. eax, rdi, rsi), it will break if/when the function is inlined by the compiler.