[Bug tree-optimization/42108] [4.4/4.5 Regression] Vectorizer cannot deal with PAREN_EXPR gracefully, 50% performance regression

[toon at moene dot org]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108