OpenACC Extensions in Flang

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Flang is more lenient than the OpenACC specification requires for purposes of compatibility. This document describes extensions to the OpenACC specification. There are a couple of known places where the Flang compiler intentionally deviates from the standard by being more strict than the specification; these are currently listed in OpenACC.md.

Extensions always active

These extensions require no flag.

  • The end directive for combined constructs can omit the loop keyword.
  • An !$acc routine with no parallelism clause is treated as if the seq clause were present.
  • !$acc end loop does not trigger a parsing error and is silently ignored.
  • The restriction on required clauses for !$acc data is emitted as a portability warning rather than an error, matching the behavior of other compilers.
  • The if clause accepts scalar integer expressions in addition to scalar logical expressions.
  • !$acc routine directives can be placed at the top level.
  • !$acc cache directives accept scalar variables.
  • !$acc cache directives are accepted outside of a loop construct.
  • The !$acc declare directive accepts assumed-size array arguments for deviceptr and present clauses.
  • The OpenACC specification disallows a variable from appearing multiple times in clauses of !$acc declare directives for a function, subroutine, program, or module, but Flang permits it with a warning when the same clause is used.
  • The REDUCTION clause accepts a MINUS “-” operator which is not permitted in the OpenACC specification. A warning is issued for this use.

Extensions enabled by default

-fopenacc-multiple-names-in-routine — !$acc routine(<name>[, <name>]*) <clause-list>

The ROUTINE directive accepts a parenthesized list of more than one name (e.g. !$acc routine(foo, bar) seq). The OpenACC specification permits only a single name; this extension is equivalent to writing one ROUTINE directive per name, each with identical clauses. A BIND clause may not be combined with multiple names. A warning is emitted for each such directive (-Wopenacc-multiple-names-in-routine; suppress with -Wno-openacc-multiple-names-in-routine). This extension may be disabled with -fno-openacc-multiple-names-in-routine.

Extensions enabled by flag

-fno-openacc-default-none-scalars-strict — pre-OpenACC-3.2 scalar behavior under DEFAULT(NONE)

OpenACC version 3.2 (section 1.16, change 733) clarified that the default(none) clause applies to scalar variables. Prior to version 3.2, default(none) did not impose a data-clause requirement on scalar variables.

By default, Flang enforces the OpenACC 3.2 behavior: scalar variables referenced inside a default(none) compute region without an explicit data clause produce an error.

When -fno-openacc-default-none-scalars-strict is specified, Flang reverts to the pre-3.2 behavior: scalar variables referenced inside a default(none) compute region without an explicit data clause do not produce an error. Instead, Flang infers implicit data attributes for those scalars via the same implicit-copy logic applied in regions without default(none).

Array variables always require an explicit data clause under default(none) regardless of this flag.

When a scalar is implicitly attributed under this extension no warning is emitted by default; explicit opt-in to the non-standard behavior is treated as acknowledgement. Use -Wopenacc-default-none-scalars-strict to enable per-use warnings for audit purposes.