llvm dev - Jul 2016 - RFC: Strong GC References in LLVM

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?

-- Sanjoy

> 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...

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

-Andy

----- 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