[LLVMdev] Attribute to mark that function only access memory through it's arguments

[Philip Reames]

I haven't heard any objection to the proposed attribute.  Given that, I 
think we can go ahead and move forward.

Igor - Can you update the patch to use the name argmemonly (analogous to 
readonly, readnone)?  As discussed, we want to factor out the read/write 
part and use the existing attributes.  Make sure you update the docs to 
clarify the answer to Nick's question regarding following pointers.

Once that's done, I'll take a close look and probably LGTM.  We will 
need to find someone to review the bitcode/IR serialization though since 
I'm not familiar with those pieces.

After this goes in, a good cleanup would be to remove the special casing 
for intrinsics and have the td files drive the addition of the now 
standardized attributes.  This would be both a good cleanup and ensure 
test coverage of the newly introduced attribute.  As such, I'd like you 
(Igor) to commit to doing this after the first patch lands.

Philip

On 06/19/2015 10:57 AM, Philip Reames wrote:
>
>
> On 06/18/2015 09:15 PM, Nick Lewycky wrote:
>>
>>     Currently in AliasAnalysis we can model ModRef behaviour for
>>     functions which
>>     only access memory through their pointer arguments. However we can't
>>     express this propery as a function attribute.
>>
>>     For example, for intrinsics we can specify ReadWriteArgMem or
>>     ReadArgMem
>>     attributes in tablegen definitions. But due to the lack of the
>>     related function
>>     attributes on the llvm ir level, this intrinsics would be
>>     modelled as if they
>>     were clobbering arbitrary memory.
>>
>>     It feels very naturall to add new function attribute which can
>>     cover such cases.
>>
>>     I have a patch (http://reviews.llvm.org/D10398
>>     <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D10398&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=6QD3kOt2shxvhEw0pkmPGL_NzzjWw6s3ZTzBo-rwkUs&s=MJrrF5wf5KOXiTUsfpoJMSDYvyM_nZ3L79ZSsA9iXaA&e=>)
>>     in which I added this
>>     attribute. Currently there is some discussion on how to name it
>>     and how it should
>>     behave when defined together with other fucntion attributes.
>>
>>
>> What does it mean? Can it only touch memory that is directly referred 
>> to by an argument? Or if that argument points to another pointer, can 
>> we follow it?
> I just want to point out that the notion Igor is introducing as an 
> attribute is not a new one.  It's a prexisting notion which is already 
> implemented within LLVM today; it's simply been restricted to 
> intrinsics.  Here's the definition from Intrinsics.td:
> // IntrReadWriteArgMem - This intrinsic reads and writes only from 
> memory that
> // one of its arguments points to, but may access an unspecified 
> amount.  The
> // reads and writes may be volatile, but except for this it has no 
> other side
> // effects.
> def IntrReadWriteArgMem : IntrinsicProperty;
>
> I did point out in the review that I think the notion of ReadsArgMem 
> is redundant given the existing notions of ReadWriteArgMem and 
> ReadOnly, but that's somewhat orthogonal. It's true of the existing 
> implementation, not just Igor's patch. It may be worth settling on 
> this to clarify naming (i.e. are we ever going to need an attribute 
> like reads_arg_mem?  Or can we be more generic and use 
> accesses_arg_mem + readonly for the same purpose?), but I don't think 
> that really changes the semantics in major ways.
>
> Philip
>
>
>
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