10.5 Execution control [mostly 6.0]

• [5.3] The BLOCK construct allows declarations of entities within executable code. For example,

    Do i=1,n
      Block
        Real tmp
        tmp = a(i)**3
        If (tmp>b(i)) b(i) = tmp
      End Block
    End Do
  

  Here the variable tmp has its scope limited to the BLOCK construct, so will not affect anything outside it. This is particularly useful when including code by INCLUDE or by macro preprocessing.

  All declarations are allowed within a BLOCK construct except for COMMON, EQUIVALENCE, IMPLICIT, INTENT, NAMELIST, OPTIONAL and VALUE; also, statement function definitions are not permitted.

  BLOCK constructs may be nested; like other constructs, branches into a BLOCK construct from outside are not permitted. A branch out of a BLOCK construct “completes” execution of the construct.

  Entities within a BLOCK construct that do not have the SAVE attribute (including implicitly via initialisation), will cease to exist when execution of the construct is completed. For example, an allocated ALLOCATABLE variable will be automatically deallocated, and a variable with a FINAL procedure will be finalised.

• The EXIT statement is no longer restricted to exiting from a DO construct; it can now be used to jump to the end of a named ASSOCIATE, BLOCK, IF, SELECT CASE or SELECT TYPE construct (i.e. any named construct except FORALL and WHERE). Note that an EXIT statement with no construct-name still exits from the innermost DO construct, disregarding any other named constructs it might be within.
• In a STOP statement, the stop-code may be any scalar constant expression of type integer or default character. (In the NAG Fortran Compiler this also applies to the PAUSE statement, but that statement is no longer standard Fortran.) Additionally, the STOP statement with an integer stop-code now returns that value as the process exit status (on most operating systems there are limits on the value that can be returned, so for the NAG Fortran Compiler this returns only the lower eight bits of the value).
• The ERROR STOP statement has been added. This is similar to the STOP statement, but causes error termination rather than normal termination. The syntax is identical to that of the STOP statement apart from the extra keyword ‘ERROR’ at the beginning. Also, the default process exit status is zero for normal termination, and non-zero for error termination.

  For example,

     IF (x<=0) ERROR STOP 'x must be positive'
  

• [6.1] The FORALL construct now has an optional type specifier in the initial statement of the construct, which can be used to specify the type (which must be INTEGER) and kind of the index variables. When this is specified, the existence or otherwise of any entity in the outer scope that has the same name as an index variable does not affect the index variable in any way. For example,

    Complex i(100)
    Real x(200)
    ...
    Forall (Integer :: i=1:Size(x)) x(i) = i
  

  Note that the FORALL construct is still not recommended for high performance, as the semantics imply evaluating the right-hand sides into array temps the size of the iteration space, and then assigning to the variables; this usually performs worse than ordinary DO loops.

• [6.1] The DO CONCURRENT construct is a DO loop with restrictions and semantics intended to allow efficient execution. The iterations of a DO CONCURRENT construct may be executed in any order, and possibly even in parallel. The loop index variables are local to the construct.

  The DO CONCURRENT header has similar syntax to the FORALL header, including the ability to explicitly specify the type and kind of the loop index variables, and including the scalar mask.

  The restrictions on the DO CONCURRENT construct are:

  • no branch is allowed from within the construct to outside of it (this includes the RETURN and STOP statements, but ERROR STOP is allowed);
  • the EXIT statement cannot be used to terminate the loop;
  • the CYCLE statement cannot refer to an outer loop;
  • there must be no dependencies between loop iterations, and if a variable is assigned to by any iteration, it is not allowed to be referenced by another iteration unless that iteration assigns it a value first;
  • all procedures referenced within the construct must be pure;
  • no image control statements can appear within the loop;
  • no reference to IEEE_GET_FLAG or IEEE_SET_HALTING_MODE is allowed.

  For example,

    Integer vsub(n)
    ...
    Do Concurrent (i=1:n)
      ! Safe because vsub has no duplicate values.
      x(vsub(i)) = i
    End Do
  

  The full syntax of the DO CONCURRENT statement is:

  [ do-construct-name : ] DO [ label ] [, ] CONCURRENT forall-header

  where forall-header is

  ( [ integer-type-spec :: ] triplet-spec [, triplet-spec ]... [, mask-expr ] )

  where mask-expr is a scalar logical expression, and triplet-spec is

  name = expr : expr [ : expr ]