[llvm-dev] RFC: Strong GC References in LLVM

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Andrew Trick" <atrick at apple.com>
> To: "Sanjoy Das" <sanjoy at playingwithpointers.com>
> Cc: "Daniel Berlin" <dberlin at dberlin.org>, "llvm-dev" <llvm-dev at lists.llvm.org>, "Joseph Tremoulet"
> <jotrem at microsoft.com>, "Oscar Blumberg" <oscar.blumberg at normalesup.org>, "Chandler Carruth" <chandlerc at gmail.com>,
> "Nick Lewycky" <nlewycky at google.com>, "Hal Finkel" <hfinkel at anl.gov>, "Philip Reames" <listmail at philipreames.com>,
> "Manuel Jacob" <me at manueljacob.de>, "Eli Friedman" <eli.friedman at gmail.com>, "David Majnemer"
> <david.majnemer at gmail.com>
> Sent: Friday, July 15, 2016 7:40:51 PM
> Subject: Re: RFC: Strong GC References in LLVM
> 
> 
> > On Jul 15, 2016, at 5:37 PM, Sanjoy Das
> > <sanjoy at playingwithpointers.com> wrote:
> > 
> > Hi Andy,
> > 
> > Andrew Trick wrote:
> > > At some point I stopped thinking about this as a bug and realized
> > > that
> > > you just need to think of LLVM as modeling speculative code
> > > barriers as
> > > memory dependence. In LLVM, it makes no sense to have a readonly
> > > may-throw call.
> > 
> > The problem is that that model breaks down with aggressive aliasing
> > like:
> > 
> > void foo(int* restrict ptr) {
> >  *ptr = 40;
> >  may_throw(); // read/write call
> >  *ptr = 50;
> > }
> > 
> > Now it is tempting to CSE the store of 40 to *ptr.  If we can't do
> > that then what does restrict/noalias even mean?
> 
> I thought it meant ‘ptr’ doesn’t alias with other ‘restrict’ pointer
> args. Not that it’s an exclusive way to access the memory. I could
> be wrong though...

It means that, within the scope of ptr, any object accessed via a pointer based on ptr is not accessed via a pointer not based on ptr.

 -Hal

> 
> In the same way you can’t have readonly-maythrow, you wouldn’t have
> TBAA+maythrow. Yeah, it’s not great.
> 
> -Andy

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory