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llvm-project/flang/docs/OpenACC.md
Susan Tan (ス-ザン　タン) ada514b0af
[flang][acc][NFC] Add deviation to the spec that declaring the same variable is permitted (#148288) 
OpenACC spec says `A var may appear at most once in all the clauses of
declare directives for a function, subroutine, program, or module.` but
our implementation allows it with a warning generated. Add this to the
diviation list for record.
2025-07-14 16:55:40 -04:00

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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.
  • 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.