[D-runtime] Precise garbage collection

[Daniel Nielsen]

Walter Bright wrote:
>>
>> Another option might be to make it a link time option, but that would mean the 
>> "standard" gc could not be part of the runtime library to be exchangeable.
>
> Users really only want one gc.

I respectfully disagree, I the opposite for the following reasons:

1) Many(?) large corporations(it applies at least for the one I work for.), will not build their own versions of their compilers/libs but prefer to use an official binary distribution, this gives obvious support benefits.

2) I expect there will be a small(?) subset of programs which will run better with the old GC. In particular,  changes to memory layout might introduce caching issues and thus even potentially negatively impact programs which toggle the GC on/off in performance critical sections.

3) I suspect it will be popular to benchmark all old programs with both GC:s to see the gain, it will be the first I do at least, in order to do a proper comparison, it would force everyone to build 2 versions of the runtime by themselves, even if they previously only used the binary distribution.

I believe link-time is sufficient though...