[Bug 126] Add support for attribute to mark data as volatile.

[Johannes Pfau via D.gnu]

Am Wed, 16 Jul 2014 14:41:42 +0100
schrieb "Iain Buclaw via D.gnu" <d.gnu at puremagic.com>:

> On 16 July 2014 14:10, Johannes Pfau via D.gnu <d.gnu at puremagic.com>
> wrote:
> > Am Wed, 16 Jul 2014 05:53:30 +0000
> > schrieb "Timo Sintonen" <t.sintonen at luukku.com>:
> >
> >> On Tuesday, 24 June 2014 at 14:14:18 UTC, Johannes Pfau wrote:
> >>
> >> > I think we should at least try to bring this to the main
> >> > newsgroup,
> >>
> >> I told you this is not going to work. The decision seems to be
> >> made even when the conversion is still going on. Lets just make
> >> this into gdc so we can continue the real work. When this is
> >> ready and working, we may try again.
> >>
> >> Unfortunately I am not skilled enough to implement this into the
> >> compiler so I have to ask somebody else to do it. I will test it
> >> as soon it is possible.
> >>
> >> This has even more importance now because I have understood that
> >> dmd will disallow read-modify-write access to shared variables. I
> >> hope this feature is not brought to gdc before volatile is
> >> working.
> >>
> >
> > I didn't have any high expectations, nevertheless we had to try.
> >
> > Well I could implement the DIP for GDC, but this is against the
> > vision of a shared frontend. In the end Iain has to decide what to
> > do.
> >
> > I'm not sure if I'd implement Volatile!T though. It's probably lots
> > of work and there's no guarantee it'll work at all in the end. The
> > more I think about it the more corner cases come to mind which all
> > need to be worked around.
> >
> > Keeping only peek/poke as Walter suggests of course also has
> > drawbacks. peek/poke plus Mike's wrapper is probably the best we'll
> > get from DMD.
> >
> > Btw: @Iain if we implement these peek/poke things I think
> > asm{"" : : :"memory";} is not good enough. At least it's being
> > removed in some tests and according to some sources it only affect
> > operations on volatile variables... So we'd have to implement
> > peek/poke on top of C/GCC volatile.
> 
> Wouldn't a reinterpret cast (VIEW_CONVERT_EXPR) to volatile be enough?
> 
> Something akin to:
> 
> T peek(T var) { return *(volatile T*)&var; }
> T poke(T var, T val) { return *(volatile T*)&var = *(volatile
> T*)&val; }

Yes I guess this should work. I just meant that a library solution with
compiler barriers would probably not work.