'version'-based code selection

[Yatheendra]

Hi people.

The 'version' keyword sounds like a fantastic capability, but how 
far does DMD take it (and does GDC take it equally far)? This is 
not a "D Improvement Proposal", I am just asking how it is now.

Can code of multiple versions be compiled into the same 
executable or library, and a particular one selected from 
"later"? I guess not without some name mangling and wrangling.

Can such selection be done at link-time for standard as well as 
user-defined library code? Maybe some library file naming 
conventions are followed to allow the compiler to select the 
corresponding file?

Wishful thinking, but can the selection be at runtime? That would 
be language support for making some things easy, e.g. picking 
from assembly-coded routines based on runtime CPU id (the video 
player MPlayer picks routines that way, and I read that the D 
library, Mir, has CPU id support).

Thanks.

P.S: I am a brand-new C++'y asylum seeker who may have missed 
seeing some documentation, so RTFM is a valid response.

P.P.S: This question was triggered by the D GSoC project for 
independent-of-C implementations of malloc/free/memcpy/memset. 
Could a common malloc be exposed to the D 
runtime/Phobos/programs, with the C or D implementations 
selectable at link-time (using a mechanism available to user code 
too)?