PRB: Large BLOCK DATA Subprogram Compiles Slowly (106731)

SYMPTOMS

Compilation of a large BLOCK DATA subprogram can take a very long time using the FORTRAN PowerStation. In tests, a BLOCK DATA subprogram with 32,000 DATA statements took approximately 45 minutes to compile on a 50 megahertz (MHz) 486 machine.

CAUSE

This slow compilation is caused by initializing a large array in a BLOCK DATA subprogram with several DATA statements. One DATA statement for each array element produces the slowest compile. The array elements must also be initialized in the order that they appear in the array for the slow compilation to occur.

FORTRAN PowerStation uses the COFF object file format. Because COFF requires that every element of an array be enumerated exactly once, all the data records (which are generated for each DATA statement in the BLOCK DATA) are sorted. The sorting algorithm is least efficient when the records are already sorted and most efficient when they are in reverse order.

RESOLUTION

If array elements are initialized in the reverse order that they appear in the array, compilation time will be significantly reduced (for example, reduced from 45 minutes to 3 minutes). Reducing the number of DATA statements by initializing thousands of array elements in a single DATA statement also reduces compile time.

MORE INFORMATION

The following sample BLOCK DATA subprograms illustrate the problem and one possible resolution:

Sample Code #1

C This sample illustrates the problem by initializing a large array,
C one element at a time, in the order the elements appear in the array.
      BLOCK DATA INITME
      COMMON /BIGCOM/ ARRAY(40000)
      DATA ARRAY(1)/1.5/ 
      DATA ARRAY(2)/7.4/ 
      .
      .                     ! 39996 DATA statements go here
      .
      DATA ARRAY(39999)/0.4/ 
      DATA ARRAY(40000)/12.0/ 
      END

Sample Code #2

C This sample initializes the array elements in reverse order to
C alleviate the problem.
      BLOCK DATA INITME
      COMMON /BIGCOM/ ARRAY(40000)
      DATA ARRAY(40000)/12.0/ 
      DATA ARRAY(39999)/0.4/ 
      .
      .                      ! 39996 DATA statements go here
      .
      DATA ARRAY(2)/7.4/ 
      DATA ARRAY(1)/1.5/ 
      END