flang/docs/OpenACC.md

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# 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.