llvm dev - Feb 2015 - [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.
- 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.

*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.

Philip
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----- 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.
> - 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.
> 
> 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?

 -Hal
> 
> Philip
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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.

Philip