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llvm-project/flang/docs/OpenACC.md
Andre Kuhlenschmidt 2e353a635b
[flang][openacc] Relax constraint on OpenACC declare statement (#135238) 
OpenACC declare statements are restricted from having having clauses
that reference assumed size arrays. It should be the case that we can
implement `deviceptr` and `present` clauses for assumed-size arrays.
This is a first step towards relaxing this restriction.

Note running flang on the following example results in an error in
lowering.
```
$ cat t.f90
subroutine vadd (a, b, c, n)
   real(8) :: a(*), b(*), c(*)
!$acc declare deviceptr(a, b, c)
!$acc parallel loop
   do i = 1,n
      c(i) = a(i) + b(i)
   enddo
end subroutine

$ flang -fopenacc -c t.f90
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":3:7): expect declare attribute on variable in declare operation
error: Lowering to LLVM IR failed
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): unsupported OpenACC operation: acc.private.recipe
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): LLVM Translation failed for operation: acc.private.recipe
error: failed to create the LLVM module
```

I would like to to share this code, because others are currently working
on the implementation of `deviceptr`, but it is obviously not running
end-to-end. I think the cleanest approach to this would be to put this
exception to the rule behind some feature flag, but I am not certain
what the precedence for that is.
2025-04-14 09:08:21 -07:00

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OpenACC in Flang

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local:
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Intentional deviation from the specification

  • The end directive for combined construct can omit the loop keyword.
  • An !$acc routine with no parallelism clause is treated as if the seq clause was present.
  • !$acc end loop does not trigger a parsing error and is just ignored.
  • The restriction on !$acc data required clauses is emitted as a portability warning instead of an error as other compiler accepts it.
  • The if clause accepts scalar integer expression in addition to scalar logical expression.
  • !$acc routine directive can be placed at the top level.
  • !$acc cache directive accepts scalar variable.
  • The !$acc declare directive accepts assumed size array arguments for deviceptr and present clauses.

Remarks about incompatibilities with other implementations

  • Array element references in the data clauses are equivalent to array sections consisting of this single element, i.e. copyin(a(n)) is equivalent to copyin(a(n:n)). Some other implementations have treated it as copyin(a(:n)), which does not correspond to OpenACC spec – Flang does not support this interpretation of an array element reference.