[PATCH] Add support for nonnull metadata on Loads

[Philip Reames]

On 09/08/2014 04:32 AM, Hal Finkel wrote:
> ----- Original Message -----
>> From: "Philip Reames" <listmail at philipreames.com>
>> To: listmail at philipreames.com, hfinkel at anl.gov
>> Cc: llvm-commits at cs.uiuc.edu
>> Sent: Friday, September 5, 2014 4:42:25 PM
>> Subject: [PATCH] Add support for nonnull metadata on Loads
>>
>> Hi hfinkel,
>>
>> This patch simply adds support for marking a load as returning a
>> non-null pointer.  This is analogous to the existing nonnull return
>> attribute, but it applies to specific load instructions instead.
>>   The value of the load is allowed to vary between successive loads,
>> but null is not a valid value to be loaded by any load marked
>> nonnull.
>>
>> Hal - I'd particularly like your input here.  Is adding a parallel
>> metadata construct to the existing attribute a reasonable idea?
>>   Should we be uniform and accept the metadata on calls too?  Is
>> there a better way to approach this.
> Now that the @llvm.assume infrastructure is in place, I can legitimately respond by saying: I think that we can model this by @llvm.assume(x != null). If that currently does not work, then we should fix it.
A couple of counter points:
1) We already use metadata for optimization hints in multiple ways. It 
works.  While having this represented via assumes is nice, I don't want 
to block forward progress in a search for perfection.
2) I don't think assumes and metadata should be considered an 
either-or.  If something is widely useful enough, having it supported by 
metadata is actually a good thing.  As we've discussed previously, using 
assumes has some downsides.
3) If anything, having assume(x != null) canonicalize to a nonnull 
attribute or metadata (depending on the type of x), would seem like a 
very reasonable implementation.

Philip
>> http://reviews.llvm.org/D5220
>>
>> Files:
>>    lib/Analysis/ValueTracking.cpp
>>    test/Transforms/InstSimplify/compare.ll
>>