[llvm-dev] [AA] Question on the full restrict patch

[Kelvin Li via llvm-dev]

Hi Jeroen,

In working on a scheme to represent Fortran alias information precisely,
we encounter a situation in which two alias sets partially overlap.  We 
would
like to seek any idea of representing such alias sets, particular, in the 
scheme of
the full restrict alias approach in D68484.

Consider the following Fortran code, 

module m
  implicit none
!set A: variables *with* the TARGET attribute
  integer, allocatable, target :: tgtA01, tgtA02, tgtA03
  integer, pointer :: p_1_SetA, p_2_SetA, p_3_SetA

!set B: variables *without* the TARGET attribute
  integer :: tgtB01, tgtB02, tgtB03

  contains
  subroutine init_ptr()
    p_1_SetA = 0 ; p_2_SetA = 0 ; p_3_SetA = 0
  end subroutine init_ptr

  subroutine compute(n)
    integer :: idx, n

    call init_var(tgtB01)
    call init_var(tgtB02)
    call init_var(tgtB03)

    associate(assoc_tgtA01 => tgtA01, assoc_tgtA02 => tgtA02, assoc_tgtB02 
=> tgtB02)
      do idx = 1, n
        assoc_tgtA01 = assoc_tgtA01 * tgtB03 + assoc_tgtA02 * tgtA03 + 
tgtB01 * assoc_tgtB02
      end do
    end associate
  end subroutine compute
end module

The alias analysis in frontend can generate the following alias sets for
"tgtA01" and "tgtA02".

 -------------------------------
 "tgtA01" is aliased to:
    "__m_NMOD__&&_m" "__m_NMOD_init_ptr" "__m_NMOD_compute" "init_var"
    "p_1_SetA" "p_2_SetA" "p_3_SetA"
    "assoc_tgtA01" "tgtA01"
 -------------------------------
 "tgtA02" is aliased to:
    "__m_NMOD__&&_m" "__m_NMOD_init_ptr" "__m_NMOD_compute" "init_var"
    "p_1_SetA" "p_2_SetA" "p_3_SetA"
    "assoc_tgtA02" "tgtA02"
 -------------------------------

If we express this alias info with the scheme that we presented in the 
last LLVM
Developers' Meeting (https://youtu.be/Vp4GklTXDys), the optimizer manages 
to
move the loop invariant code out of the loop body.  We think that the 
optimizer
can make use of the alias information even though there is an overlap in 
the two
alias sets.  With the D69542 patch (https://reviews.llvm.org/D69542), we 
would
like to know if there is any way to express that with the intrinsics 
proposed in
the full restrict patch.

We attach the ll files for reference.

Note: The options --unroll-max-count=0 --interleave-loops=false
-passes=default<O3> -aa-pipeline=default are used in opt.

Attachments
0. alias-sets-from-fe.txt - full alias sets generated from Fortran 
frontend
1. before-opt-without-alias-from-fe.ll - IR before opt (NO alias info is 
emitted)
2. after-opt-without-alias-from-fe.ll - IR after opt (NO alias info is 
emitted)
3. before-opt-with-alias-from-fe.ll - IR before opt (noalias/alias.scope 
metadata is emitted)
4. after-opt-with-alias-from-fe.ll - IR after opt (noalias/alias.scope 
metadata is emitted)






Thanks,
Kelvin

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