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llvm-project/flang/docs/OpenACC.md
Razvan Lupusoru c91ab61de7
[flang][acc] Disallow duplicate variables in use_device clause (#176217) 
Add a semantic check to detect when the same variable appears multiple
times in `use_device` clauses on the same `host_data` directive. While
the OpenACC specification does not explicitly prohibit this, allowing
duplicates is likely a user error and provides no additional semantics.

A similar restriction was already in place for `private`,
`firstprivate`, and `reduction` clauses on compute constructs. This
change extends that behavior to `use_device` on `host_data`.

Error message:
`'<var>' appears in more than one USE_DEVICE clause on the same
HOST_DATA directive`
2026-01-15 19:31:27 +00:00

2.0 KiB
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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.
  • The OpenACC specification does not prohibit the same variable from appearing in multiple data clauses, but this is disallowed for variables appearing in private, firstprivate, or reduction clauses.
  • The OpenACC specification does not prohibit the same variable from appearing multiple times in a use_device clause on a host_data construct, but this is disallowed.

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.