[llvm-dev] [RFC] Adding a globalmemonly attribute for functions that only access global memory

[Nicolai Hähnle via llvm-dev]

Hi Mugilan,

I think this is an interesting direction. I've recently put some thought
into the memory attributes you mention for unrelated reasons, and found
that the various XYZonly attributes don't compose well, while noXYZ
attributes compose better.The reason is simple. With what you're proposing,
we end up with:

* argmemonly
* inaccessiblememonly
* inaccessibleorargmemonly
* globalmemonly

There are 4 attributes talking about 3 types of memory. Clearly, there are
orthogonality issues. First of all, glancing at your patch, your new
attribute should really be called inaccessibleorglobalmemonly. What if
somebody wants argorglobalmemonly? Do they add yet another attribute? That
way lies madness.

If we instead take a step back, we can recognize that there are different
"paths" of accessing memory:

* arg
* global
* inaccessible
* indirect (i.e., via a pointer loaded from memory)
* other (e.g., pointer returned by black box function)

These could map very nicely onto noXYZ attributes. You end up 5 attributes:
noargmem, noglobalmem, noinaccessiblemem, noindirectmem, noothermem. Nicely
composable, in particular today's attribute are mapped as follows:

* argmemonly -> noglobalmem noinaccessiblemem noindirectmem noothermem
* inaccessiblememonly -> noargmem noglobalmem noindirectmem noothermem
* inaccessibleorargmemonly -> noglobalmem noindirectmem noothermem
* globalmemonly -> noargmem noindirectmem noothermem (remember, you seem to
want to allow access to inaccessible memory here)

More importantly, the 5 attributes can express any future desired
combination, e.g. a function that accesses memory through argument
pointers, and through pointers loaded from there (transitively) is:
noglobalmem noinaccessiblemem noothermem

Thoughts?

Cheers,
Nicolai

On Sat, Sep 11, 2021 at 9:16 PM Mugilan Ganesan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello everyone,
>
> We propose the addition of a GlobalMemOnly attribute to LLVM.
>
> Definition:
>
> This attribute indicates that the only memory accessed inside a function,
> that is visible to the callee, is global memory. If the function reads or
> writes other memory that is visible to the callee, the behavior is
> undefined.
>
> Motivation:
>
> Memory attributes already exist for specifying the memory locations
> accessed by a function, such as ArgMemOnly and InacessibleMemOnly.
> Similarly, there are attributes for describing memory behaviour, such as
> ReadOnly and WriteOnly.
>
> However, no attributes exist for differentiating the types of memory
> objects that functions can access, such as global objects and objects
> created by alloca. We would like to start by introducing an attribute to
> deal with functions that only access global objects.
>
> The addition of the GlobalMemOnly attribute can benefit LLVM as follows:
>
> 1. Improves Alias Analysis, since this information can be used to improve
> the determination of aliasing between functions and memory locations that
> point to global objects.
>
> 2. Provides Transforms with greater information: many math lib calls only
> modify the global variable errno. Unnecessary loads of the call arguments
> can therefore be removed, since the calls will have no side-effects on them.
>
> Patches Overview:
>
> 1. Definition of GlobalMemOnly in the AsmParser, IR, and LLVM Bitcode. It
> is also defined in the LangRef.
>
> https://reviews.llvm.org/D109644
>
> 2. Support is added to Alias Analysis. If a function is GlobalMemOnly and
> a memory location does not point to a global object, getModRefInfo can
> infer that no aliasing between the function and the location can occur.
> This patch also adds a regression test for BasicAA.
>
> https://reviews.llvm.org/D109647
>
> 3. Certain math lib calls are assigned GlobalMemOnly, since they only
> modify the global variable errno. These calls include common functions,
> such as atan, exp, and log.
>
> https://reviews.llvm.org/D109648
>
> Please let us know of any thoughts you may have regarding this attribute’s
> implementation and use cases. We believe that it can strengthen LLVM’s
> alias analysis and optimization passes if added.
>
> Thanks,
> Mugilan
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>


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