OpenACC in Flang¶

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.
The OpenACC specification disallows a variable appearing multiple times in clauses of !$acc declare directives for a function, subroutine, program, or module, but it is allowed with a warning when same clause is used.

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.