[llvm-dev] Can someone give me some pointer on alias analysis ?

Amaury SECHET via llvm-dev
Mon, 04 Jan 2016 17:55:31 -0000

2016-01-04 18:21 GMT+01:00 Philip Reames <listmail at philipreames.com>:

> On 01/04/2016 07:32 AM, Amaury SECHET wrote:
>
> After a bit more investigation, it turns out that because %0 is stored
> into %1 (after bitcast) and so %3 may have access to it and clobber it.
>
> Can you give a bit more context?  I'm not sure which of the examples
> you're talking about.
>
>
Sure. Let's look at http://pastebin.com/K0J9yGq1

Because of the store line 7, it is assumed that the call line 8 may see %0
and even modify the memory it points to. As a result, it is assumed that
the load line 11 may not be eliminated.

Which seems actually correct in the general case.


>
> After a bit of thought, it is correct in the general case, but
> definitively something stricter is needed here. Looking at
> inaccessiblememonly I'm not sure this is what is needed. What if the
> memory allocator is defined is the current module ?
>
> At the moment, inaccessiblememonly would require separate compilation of
> the allocation function.
>
>
> This leads me to conclude this is way more linked to the memory allocation
> pass than I expected it to be in the first place. Can I ask what you plan
> to use inaccessiblememonly for ? Should the semantic be refined to fit
> the bill better ?
>
> Well, I didn't introduce the attribute, so I can't speak for the
original
> intent.  For me, I plan on applying it to some of our out of line
> allocation functions and other helper routines which modify runtime state,
> but not java visible state.
>
> If you have specific suggestions for how to refine the semantics, please
> make them.  Getting the details right is always the hard part.  :)
>
> You might also consider using a variant of your allocation function which
> takes a pointer to the global state it needs to modify.  Doing this would
> allow you to use argmemonly to restrict the aliasing while still allowing
> whole program optimization.  I haven't tried this in practice, but it
seems
> like it would probably work...
>

I do not wish to make suggestion before I understand where this is coming
from. So far, from what I've collected, use cases are:
 - Memory allocation
 - Runtime isolation for managed languages.

I have some more though to put into this, but to boot, would that be
possible to only use this attribute on method that are declared, but not
defined and remove it when merging modules ? It doesn't look like it is
necessary to have it when the function may be exposed depending on the way
the software is built.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20160104/dd27bbf7/attachment.html>