Test promotion

Test promotion involves promoting a test out of the loop that encloses it by replicating the containing loop for each branch of the test. The replicated loops contain fewer tests than the originals, or no tests at all, so the loops execute much faster. Multiple tests are promoted, and copies of the loop are made for each test.

DO I=1, 100
  DO J=1, 100
    IF(FOO .EQ. BAR) THEN
      A(I,J) = I + J
    ELSE
      A(I,J) = 0
    ENDIF
  ENDDO
ENDDO

Test promotion (and loop interchange) produces the following code:

IF(FOO .EQ. BAR) THEN
  DO J=1, 100    
    DO I=1, 100
      A(I,J) = I + J
    ENDDO
  ENDDO
ELSE
  DO J=1, 100    
    DO I=1, 100
      A(I,J) = 0
    ENDDO
  ENDDO
ENDIF

For loops containing large numbers of tests, loop replication can greatly increase the size of the code.

Each DO loop in Fortran and for loop in C and C++ whose bounds are not known at compile-time is implicitly tested to check that the loop iterates at least once. This test may be promoted, with the promotion noted in the Optimization Report. If you see unexpected promotions in the report, this implicit testing may be the cause. For more information on the Optimization Report, see Chapter 8 “Optimization Report”.