BC ABI: handling volatile

[Bryce McKinlay]

Andrew Haley wrote:
> There is one other idea that I am somewhat reluctant to contemplate:
> always emit a fence when accessing volatiles declared outside the
> current class.  This might not be so awful, because accessing fields
> outside the current class is not much done in well-written
> object-oriented code.  However, these fences can be quite slow on
> some architectures.
>   

As ugly as it sounds at first, this actually seems to me to be the most 
simple and attractive solution. I agree that it shouldn't have a huge 
impact on most applications since accessing fields across classes is 
rare. Also, while the barrier/fence instructions may be slow, they are 
probably always going to be faster than a "synchronized".

Also, going this route in no way precludes switching to a more 
sophisticated implementation later, should it prove necessary - the old 
code will still work just fine.

Perhaps we can add an -fno-volatiles flag so that the impact of this can 
be easily measured, or so users who know their code does not use 
volatiles can optimize.

Bryce