http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53957
Tobias Burnus <burnus at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |burnus at gcc dot gnu.org
--- Comment #4 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-07-18
12:46:29 UTC ---
(In reply to comment #3)
> It would be better to have this similar to other loops I see,
>
> bool flag = end-value == i;
> i = i + 1;
> if (flag) goto loop_exit;
That's not that simple as one might not reach the end value due to the step. If
"step" is (plus or minus) unity and if one has integers (and not reals, added
in Fortran 77, deleted in Fortran 90), it is simple.
But if abs(step) != 1 or if the loop variable is not an integer, one either
needs to calculate the number of trips beforehand, or has to use ">" or "<"
rather than "==". The problem with "<" / ">" is that one has to do another
comparison, unless the sign of "step" is known:
if (step > 0 ? dovar > to : dovar < to)
goto exit_label;
I don't see whether that version is better than the current version.
Suggestions or comments?
The current code is (comment from trans-stmt.c's gfc_trans_do):
------------<cut>-----------------
We translate a do loop from:
DO dovar = from, to, step
body
END DO
to:
[evaluate loop bounds and step]
empty = (step > 0 ? to < from : to > from);
countm1 = (to - from) / step;
dovar = from;
if (empty) goto exit_label;
for (;;)
{
body;
cycle_label:
dovar += step
if (countm1 ==0) goto exit_label;
countm1--;
}
exit_label:
countm1 is an unsigned integer. It is equal to the loop count minus one,
because the loop count itself can overflow. */
------------</cut>-----------------
