[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 #13 from toon at moene dot org  2009-11-20 19:45 -------
> The funny conditional initialization of countm1.6 makes the analysis of
> the number of iterations of this loop impossible (not to mention the
> conversions to character(kind=4)).

> Why does the frontend do induction variable "optimization" at all and
> not simply generate a loop with a non-unit counting IV?

It's not trying to be funny - it just follows the text of the Fortran Standard
(hey, what a concept !):

12   8.1.6.6.1    Loop initiation
13 1 When the DO statement is executed, the DO construct becomes active. If
loop-control is
14 2     [ , ] do-variable = scalar-int-expr 1 , scalar-int-expr 2 [ ,
scalar-int-expr 3 ]
15 3 the following steps are performed in sequence.
16          (1)    The initial parameter m1 , the terminal parameter m2 , and
the incrementation parameter m3 are
17                 of type integer with the same kind type parameter as the
do-variable. Their values are established
18                 by evaluating scalar-int-expr 1 , scalar-int-expr 2 , and
scalar-int-expr 3 , respectively, including, if ne-
19                 cessary, conversion to the kind type parameter of the
do-variable according to the rules for numeric
20                 conversion (Table 7.11). If scalar-int-expr 3 does not
appear, m3 has the value 1. The value of m3
21                 shall not be zero.
22          (2)    The DO variable becomes defined with the value of the
initial parameter m1 .
23          (3)    The iteration count is established and is the value of the
expression (m2 - m1 + m3 )/m3 , unless that
24                 value is negative, in which case the iteration count is 0.

Only interprocedural analysis can tell us that this is a simple loop only
executed 3 times (I got this wrong at first - it's *always* executed 3 times).


-- 


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