[llvm-dev] RFC phantom memory intrinsic

[Dinar Temirbulatov via llvm-dev]

Hi,
For PR21780 solution, I plan to add a new functionality to restore
memory operations that was once deleted, in this particular case it is
the load operations that were deleted by InstCombine, please note that
once the load was removed there is no way to restore it back and that
prevents us from vectorizing the shuffle operation. There are probably
more similar issues where this approach could be applied.
I added phatom_mem(llvm_anyptr_ty, llvm_i64_ty) intrinsic for that,
indicating that for particular pointer let's call it %ptr we observed
maximum possible offset at which there was reference by its type in a
function. After InstCombine deleted the load operation, it could be
restored in SLPVectorizer and we could restore chains of GEPs, Loads
and Inserts in case we encounter phatom_mem intrinsic.

 Here is two part review:
          https://reviews.llvm.org/D37579 - InstCombine part.
          https://reviews.llvm.org/D37648 - SLP part.

Also, there might be different approaches in describing deleted memory
operations, for example, for my case: phantom_load(llvm_anyptr_ty,
llvm_i64_ty). First parameter describes pointer and second parameter
offset from pointer this loaded was deleted, for example. This two
operations:

  %arrayidx1 = getelementptr inbounds double, double* %ptr, i64 1
  %ld1 = load double, double* %arrayidx1

could be represented in the IR with this one: "void phantom_load(%ptr,
1)" after removal. But, the approach that is already implemented in
both reviews looks better to me since we don't need to add intrinsic
for every removed operation in the IR. Also, while constructing such
form in the IR we have to be careful since some pointer operations
might be in loops and as the result we might end up construction an
incorrect IR.  So, I just avoid to notice any pointer operation if it
is belong to a loop, except those where the the whole chain of
operations pointer origin, GEP, Load, Shuffle operation are in the
same loop and in the same basic block.
                                         Thanks, Dinar.

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