J3/15-131r1
To:        J3
Subject:   Variables in a BLOCK construct in a DO CONCURRENT construct
From:      Van Snyder
Date: 2015 February 18
Reference: 15-007

There was some correspondence about variables being separately private
to each iteration of a DO CONCURRENT construct.

Declaring them in a BLOCK construct within a DO CONCURRENT construct
makes them separately private to each iteration.

NOTE 8.11 mentions this, but the interesting case is a variable that is
conditionally defined within the construct.

[180:4+8-5-1 NOTE 8.11] Revise NOTE 8.11

"NOTE 8.11
 A variable that is effectively local to each iteration of a DO
 CONCURRENT construct can be declared in a BLOCK construct within it.
 For example:

 T = ...
 ...
 DO CONCURRENT ( I = 1 : N )
   BLOCK
     REAL :: T
     IF ( A(I) > 0 ) THEN
       T = SQRT(A(I))
       B(I) = T + T**3
     END IF
   END BLOCK
 END DO

 The variable T that is declared within the BLOCK construct does not
 exist outside the construct.  There is therefore no question whether
 the T that exists outside the DO CONCURRENT construct has been modified
 in one iteration (and therefore retains that value), or more than one
 iteration (and therefore becomes undefined).
"