llvm/flang/docs/OpenACC.md

OpenACC in Flang

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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.
• 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.

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.