[LLVMdev] RFC: attribute for a pointer which is dereferenceable xor null

[Hal Finkel]

----- Original Message -----
> From: "Philip Reames" <listmail at philipreames.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Friday, February 13, 2015 11:48:12 AM
> Subject: Re: [LLVMdev] RFC: attribute for a pointer which is dereferenceable xor	null
> 
> On 02/12/2015 10:20 PM, Hal Finkel wrote:
> > ----- Original Message -----
> >> From: "Philip Reames" <listmail at philipreames.com>
> >> To: llvmdev at cs.uiuc.edu
> >> Sent: Thursday, February 12, 2015 11:59:17 AM
> >> Subject: [LLVMdev] RFC: attribute for a pointer which is
> >> dereferenceable xor	null
> >>
> >>
> >> I'd like to propose that we add an attribute which expresses the
> >> notion that the specified value is either null or dereferenceable
> >> up
> >> to a fixed size. (Note the xor.) Our current dereferenceable(n)
> >> attribute doesn't quite fit the bill, it implies that the pointer
> >> is
> >> non-null. Similarly, our nonnull attribute says nothing about
> >> dereferenceability.
> >>
> >> There are two syntax proposals below, but let's start with the
> >> motivation.
> >>
> >> These semantics arise in a number of common cases:
> >> - In C, malloc is defined to either return null, or a
> >> dereferenceable
> >> region of the size requested.
> > I think this is really only useful if we allowed 'n' to be a
> > runtime value.
> For malloc, you might have a point.  However, I believe that the same
> is
> true for operator new and the size will frequently be a compile time
> constant there.
> 
> I am not proposing adding a 'n' as a runtime value.  I am not opposed
> to
> it, but it's not part of this proposal.
> >
> >> - In Java, any reference is either null or dereferenceable to the
> >> size of the static type.
> >> - I suspect this will also be useful for Julia, Go, Rust, and
> >> others
> >> for similar reasons.
> >>
> >> With such an attribute available, we can increase the
> >> effectiveness
> >> of LICM. We can't move a load outside a loop if it might introduce
> >> a
> >> fault. Knowing that a pointer is deferefenceable(N) at a location
> >> (i.e. the loop preheader) allows us to satisfy this constraint. In
> >> the near term, we can simply add a case in the dereferenceability
> >> analysis that combines the new attribute and isKnownNonNull. This
> >> won't be too effective out of the box, but will enable testing
> >> with
> >> llvm.assumes and might catch some cases. I will probably also add
> >> a
> >> case to look at the controlling branch to the loop preheader since
> >> in practice that tends to be where a unswitched null check would
> >> live.
> >>
> >> Longer term, I plan on introducing a mechanism to have
> >> isKnownNonNull
> >> consider trivially dominating conditions. This will make the
> >> proposed attribute more powerful, but is explicitly not part of
> >> this
> >> proposal. That's a lot more work and will need a fair amount of
> >> discussion on its own.
> >>
> >> Now, on to possible syntax.
> >>
> >> Option 1
> >> We could simply redefine our current notion of dereferenceable(N)
> >> to
> >> allow the pointer to be null. Since we already have the nonnull
> >> attribute, this wouldn't loose any expressibility. Frontends would
> >> need to be modified to emit both dererefenceable(N) and nonnull if
> >> they want to preserve the same semantics. Most of the existing
> >> utility functions for dereferenceability in LLVM would be modified
> >> to just check both. There'd need to by a forward migration added
> >> to
> >> the bytecode parser to enable upgrade from the old semantics to
> >> the
> >> new.
> >>
> >> This is my preferred option, but in offline conversation, Hal
> >> objected to this change. I'll let him describe his objection since
> >> I
> >> was never quite clear on it.
> > I feel this would be all pain and no gain. We already have the
> > dereferenceable attribute, and a fair about of code now exists
> > which depends on the current semantics. Introducing a silent
> > semantic change now requires, at least, all producers to be
> > updated. Plus it would be confusing; we currently assume that
> > dereferenceable pointers in address-space zero are not null (and
> > optimize based on that). 'dereferenceable' is the terminology we
> > use for that (not 'dereferenceableAndNotNull'), and I don't like
> > the proposed inconsistency with our API. Lastly, it would be
> > inconsistent with its name: a null pointer in address-space zero
> > is not dereferenceable.
> I think this is a far smaller change than your indicating.  There's
> only
> a handful of places in the code base that directly access the
> attributes; we'd extend them to check 'new deref' and 'nonnull' at
> once.  As a result, most of the APIs would be semantically unchanged.
> We might want to rename them, but that's a separate and less risky
> change.
> 
> Your naming point is a reasonable one.  I'm more okay with the
> seperation between "this has a dereferenceable attribute (but might
> still be null)", and "this pointer is dereferenceable.  I think that
> in
> practice this confusion is likely to be less than introducing a
> parallel
> attribute.
> >
> >> Option 2
> >> We introduce a new attribute with the desired semantics. This
> >> results
> >> in a collection of confusing overlapping attributes, but is
> >> otherwise straight forward.
> >>
> >> My proposed strawman syntax would be: dereferenceable_or_null(N).
> >> (Bikeshedding welcomed.) This would be a legal parameter and
> >> return
> >> attribute on both function declarations and call sites (i.e. calls
> >> and invokes). As with above, we'd extend all the places that
> >> currently consider 'dereferenceable' to consider the new attribute
> >> in combination with isKnownNonNull.
> > Okay; I don't object to this attribute. Just so we're on the same
> > page, what is your use case? Is it like the Java case you
> > mentioned above? Also, I wonder: Are you satisfied with the static
> > size constraint, or do you also want runtime sizes?
> >
> My use case is the Java object case.  I do not need runtime sizes.

I'm fine with reviewing patches for dereferenceable_or_null(N), with N some constant. I'd like to address the runtime size issue at some point, but we can address that at some other point (it might require something other than an attribute anyway).

 -Hal

> 
> Philip
> 

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