ATTRIBUTES VECTOR

!DIR$ ATTRIBUTES [att,] VECTOR [:clause] [, att]... :: routine-name

!DIR$ ATTRIBUTES [att,] VECTOR :(clause [, clause]...) [, att] :: routine-name

If you specify more than one clause, they must be separated by commas and enclosed in parentheses.

When you specify the ATTRIBUTES VECTOR directive, the compiler provides data parallel semantics by combining with the vectorized operations or loops at the call site. When multiple instances of the vector declaration are invoked in a parallel context, the execution order among them is not sequenced. If you specify one or more clauses, they affect the data parallel semantics provided by the compiler.

If you specify the ATTRIBUTES VECTOR directive with no VECTORLENGTH clause, a default VECTORLENGTH is computed based on efficiency heuristics of the vectorizer and the following:

• The return type of the function, if the function has a return type.

• The data type of the first non-scalar argument (that is, the first argument that is specified in the scalar clause), if any.

• Default integer type, if neither of the above is supplied.

If you do not explicitly specify a VECTORLENGTH clause, the compiler will choose a VECTORLENGTH using its own cost model.

If you specify the ATTRIBUTES VECTOR directive with no clause, the compiler will generate vector code based on compiler efficiency heuristics and whatever processor compiler options are specified.

The VECTOR attribute implies the C attribute, so that when you specify the VECTOR attribute on a routine, the C attribute is automatically also set on the same routine. This changes how the routine name is decorated and how arguments are passed.

The Fortran Standard keyword ELEMENTAL specifies that a procedure written with scalar arguments can be extended to conforming array arguments by processing the array elements one at a time in any order. The ATTRIBUTES VECTOR directive tells the optimizer to produce versions of the procedure routine-name that execute with contiguous slices of the array arguments as defined by the VECTORLENGTH clause in an "elemental" fashion. routine-name does not need to be defined as ELEMENTAL to be given the VECTOR attribute.

The VECTOR attribute causes the compiler to generate a short vector form of the procedure, which can perform the procedure's operation on multiple elements of its array arguments in a single invocation. The short vector version may be able to perform multiple operations as fast as the regular implementation performs a single operation by using the vector instruction set in the CPU.

In addition, when invoked from an OMP construct, the compiler may assign different copies of the elemental procedures to different threads, executing them concurrently. The end result is that your data parallel operation executes on the CPU using both the parallelism available in the multiple cores and the parallelism available in the vector instruction set. If the short vector procedure is called inside a parallel loop or an auto-parallelized loop that is vectorized, you can achieve both vector-level and thread-level parallelism.

The INTENT(OUT) or INTENT(INOUT) attribute is not allowed for arguments of a procedure with the VECTOR attribute since the VECTOR attribute forces the procedure to receive its arguments by value.

The Intel C/C++ compiler built in function __intel_simd_lane() may be helpful in removing certain performance penalties caused by non-unit stride vector access. Consider the following:

interface
! returns a number between 0 and vectorlength – 1 that reflects the current “lane id” within the SIMD vector
! __intel_simd_lane() will return zero if the loop is not vectorized
    function for_simd_lane () bind (C, name = “__intel_simd_lane”) 
        integer (kind=4) :: for_simd_lane 
        !DEC$ attributes known_intrinsic, default :: for_simd_lane 
    end function for_simd_lane 
end interface

For more details, see the Intel C++ documentation.

Example

The ATTRIBUTES VECTOR directive must be accessible in the caller, either via an INTERFACE block or by USE association.

The following shows an example of an external function with an INTERFACE block:

!... function definition

  function f(x)
 !dir$ attributes vector :: f
  real :: f, x
...
 !  attribute vector explicit in calling procedure using an INTERFACE

interface
   function f(x)
  !dir$ attributes vector :: f
  real :: f, x
  end
end interface
...
do i=1,n
  z(i) = f( x(i) )
end do

The ATTRIBUTES VECTOR directive can be brought into the caller by USE association if the vector function is a module procedure; for example:

!  attribute vector in definition of module procedure
 
module use_vect
 
contains
  function f(x)
  !dir$ attributes vector :: f
    real :: f, x
...
  end function
end module use_vect

!  USE and call of f(x) from another procedure with a module USE statement

 USE use_vect     !brings in ATTRIBUTE VECTOR for f(x)
...
 
! now simply call f(x)

do i=1,n
  z(i) = f( x(i) )
end do

You can specify more than one SCALAR or LINEAR clause in an ATTRIBUTES VECTOR directive. For example, all of the following are valid:

!DIR$ ATTRIBUTES VECTOR:PROCESSOR(atom) :: f
!DIR$ ATTRIBUTES VECTOR:(SCALAR(a), SCALAR(b)) :: f
!DIR$ ATTRIBUTES VECTOR:(LINEAR(x:1), LINEAR(y:1)) :: f

The three directives above are equivalent to specifying a single, continued, directive in fixed-form source, as follows:

!DIR$ ATTRIBUTES VECTOR:( PROCESSOR(atom),
!DIR$& SCALAR(a, b), 
!DIR$& LINEAR(x:1, y:1) ) :: f

The following example for Intel® 64 architecture targeting the Intel® Xeon Phi™ product family x200 (formerly code name Knights Landing) tells the compiler to generate vector functions for that processor:

!DIR$ ATTRIBUTES VECTOR : PROCESSOR(mic_avx512) :: f
integer function f(a, b)
integer :: a, b
f = a + b
end function f