[PATCH] Add support for nonnull metadata on Loads

[Philip Reames]

On 10/22/2014 11:17 AM, Pete Cooper wrote:
> Hey Philip
>
> Sorry to go back to a committed patch, but it seemed like the best place for these questions.
Questions and comments are always welcome.
>
> So firstly I really like the patch here, I have no problems with it.
>
> I’m curious what your plans are for this in terms of expanding it to other instructions.  I’d like to see this on PHIs for example.
I'm not planning to do so currently.  The philosophy around metadata 
appears to be that we add it only to "source" instructions - those which 
can introduce a value from the outside world.  A PHI (and most other 
instructions) can be analysed and the metadata isn't needed.

If you can come up with a strong case which would motivate metadata on a 
phi, I'd be interested in seeing it.
>
> You also mentioned call returns.  I think that this complements function return attributes as I could do something like this:
>
> %0 = load … !nonnull
> call foo(%0)
>
> void* foo(void*p) { return p }
>
> You can’t put the nonnull attribute on the function return itself as you can’t prove that its always nonnull for all callers, but your metadata can be put on a specific call site like the one above.
You can put the nonnull attribute on the return value of the callsite.  
(I think.  I haven't actually checked.)  Given this, the metadata form 
would be redundant.


>
> Finally, any plans for clang to emit these?  vtable reads for example are probably guaranteed to be nonnull…probably :)
I'm not working on clang, but if someone else wanted to do so, I'd 
encourage it.
>
> Thanks,
> Pete
>> On Oct 20, 2014, at 3:51 PM, Philip Reames <listmail at philipreames.com> wrote:
>>
>> Closed by commit rL220240 (authored by @reames).
>>
>> REPOSITORY
>>   rL LLVM
>>
>> http://reviews.llvm.org/D5220
>>
>> Files:
>>   llvm/trunk/docs/LangRef.rst
>>   llvm/trunk/lib/Analysis/ValueTracking.cpp
>>   llvm/trunk/test/Transforms/InstSimplify/compare.ll
>> <D5220.15157.patch>_______________________________________________
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