Using dbx

To use dbx on a parallel loop, temporarily rewrite the program as follows:

• Isolate the body of the loop in a file and subroutine of its own.

• In the original routine, replace loop body with a call to the new subroutine.

• Compile the new subroutine with -g and no parallelization options.

• Compile the changed original routine with parallelization and no -g.

  Example: Manually transform a loop to allow using dbx in parallel:

  ---

  Original code:
  demo% cat loop.f
  C$PAR DOALL
        DO i = 1,10
              WRITE(0,*) 'Iteration ', i
        END DO
        END
  Split into two parts: caller loop and loop body as a subroutine
  demo% cat loop1.f
  C$PAR DOALL
        DO i = 1,10
              k = i
              CALL loop_body ( k )
        END DO
        END
  
  demo% cat loop2.f
        SUBROUTINE loop_body ( k )
        WRITE(0,*) 'Iteration ', k 
        RETURN
              END
  Compile caller loop with parallelization but no debugging
  demo% f77 -O3 -c -explicitpar loop1.f
  Compile the subprogram with debugging but not parallelized
  demo% f77 -c -g loop2.f
  Link together both parts into a.out
  demo% f77 loop1.o loop2.o -explicitpar
  Run a.out under dbx and put breakpoint into loop body subroutine
  demo% dbx a.out          ¨ Various 
  dbx messages not shown
  (dbx) stop in loop_body
  (2) stop in loop_body
  (dbx) run
  Running: a.out
  (process id 28163)
  dbx stops at breakpoint
  t@1 (l@1) stopped in loop_body at line 2 in file  
      "loop2.f"
      2           write(0,*) 'Iteration ', k
  Now show value of k
  (dbx) print k
  k = 1                  ¨ Various values other than 1  are possible
  (dbx)

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