mirror-llvm-project/flang/docs/OpenACC-extensions.md

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.
• !$acc routine directives can be placed directly within an interface block (i.e. as an interface-specification, such as preceding the interface body they name). The OpenACC specification only permits the named routine directive in the specification part of a subroutine, function, or module, and the unnamed form within an interface body; Flang additionally accepts a routine directive between the INTERFACE statement and the interface bodies, applying a named directive to the interface body it names.
• !$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.
• The collapse clause may be applied to a DO CONCURRENT loop, which the OpenACC specification does not permit. The collapse value must equal the number of DO CONCURRENT controls; the construct then lowers like the equivalent perfectly-nested DO loops. A portability warning is issued for this use (-Wportability, also enabled by -pedantic; suppress with -Wno-portability).

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.

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 uses 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 extension.

When a scalar is implicitly attributed under this extension, a warning is emitted by default (-Wopenacc-default-none-scalars-strict; suppress with -Wno-openacc-default-none-scalars-strict). Use -fopenacc-default-none-scalars-strict to enforce the OpenACC 3.2 behavior and produce an error for scalar variables that are not listed in an explicit data clause. -fno-openacc-default-none-scalars-strict preserves the default pre-3.2 behavior explicitly.