------- Comment #15 from toon at moene dot org 2009-11-21 12:11 -------
> I don't see that the standard suggests the specific code the Frontend
> generates. In fact it should be valid to increment the DO variable
> by m3 and express the exit test in terms of the DO variable as well.
The Standard doesn't prescribe the code the Frontend generates - however, to be
sure one follows the Standard, it's most easy to simply implement the steps
given.
To illustrate this with a simple example:
DO I = M1, M2, M3
B(I) = A(I)
ENDDO
would be most easily, and atraightforwardly, implemented as follows:
IF (M3 > 0 .AND. M1 < M2) GOTO 200 ! Loop executed zero times
IF (M3 < 0 .AND. M1 > M2) GOTO 200 ! Ditto
ITEMP = (M2 - M1 + M3) / M3 ! Temporary loop count
I = M1
100 CONTINUE
B(I) = A(I)
ITEMP = ITEMP - 1 ! Adjust internal loop counter
I = I + M3 ! Adjust DO loop variable
IF (ITEMP > 0) GOTO 100
200 CONTINUE
That there are two induction variables in this loop is inconsequential - one of
them should be eliminated by induction variable elimination (at least, that was
the case with g77 and the RTL loop optimization pass).
If you think that the Frontend does something different / in addition to the
above, feel free to open a separate PR.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108
