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

[Nick Lewycky]

On 18 June 2015 at 10:58, Igor Laevsky <igor at azulsystems.com> wrote:

> Hi,
>
> 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) 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 wanted to get some feedback on this. Is this attribute required at all?
> Maybe
> there is already a way of expressing such things?
>

The other way today is a custom AA pass. That can either use deduction, or
special knowledge about named functions in your runtime.
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