[Bug tree-optimization/37449] [4.4 Regression] calculix gets wrong answer for -O1 -ffast-math

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #23 from rguenther at suse dot de  2008-10-15 18:43 -------
Subject: Re:  [4.4 Regression] calculix gets
 wrong answer for -O1 -ffast-math

On Wed, 15 Oct 2008, rguenther at suse dot de wrote:

> ------- Comment #22 from rguenther at suse dot de  2008-10-15 18:33 -------
> Subject: Re:  [4.4 Regression] calculix gets
>  wrong answer for -O1 -ffast-math
> 
> On Wed, 15 Oct 2008, dberlin at dberlin dot org wrote:
> 
> > ------- Comment #21 from dberlin at gcc dot gnu dot org  2008-10-15 17:55 -------
> > Subject: Re:  [4.4 Regression] calculix gets wrong answer for -O1 -ffast-math
> > 
> > >
> > > It already does (I fixed that recently), but we only phi-translate during
> > > insertion and we
> > > don't insert for that case, as obviously there is no partial redundancy.
> > 
> > True, but if it discovered all the new phi arguments would be constant
> > it used to create a new phi node with the new constant values and let
> > eliminate replace the old calculation with the new phi node.
> > 
> > Maybe it only did this if all the constants ended up the same value,
> > but it would be trivial to do it if all the arguments are constant,
> > regardless of whether they are the same value.
> > :)
> 
> Well, we already do for
> 
> int foo (int b)
> {
>   double i;
>   if (b)
>     i = 4;
>   else
>     i = 9;
>   return __builtin_sqrt(i);
> }
> 
> <bb 4>:
>   # i_1 = PHI <4.0e+0(5), 9.0e+0(3)>
>   # prephitmp.11_7 = PHI <2.0e+0(5), 3.0e+0(3)>
>   # prephitmp.12_8 = PHI <2(5), 3(3)>
>   D.1238_5 = prephitmp.11_7;
>   D.1237_6 = prephitmp.12_8;
>   return D.1237_6;

Ok, for return __builtin_pow (i, i) it doesn't work because we
do not register phi-translations that result in constants (translating
i does) and then we run into the if (seen) guard and fail to 
phi-translate.

Either we should register phi-translations for them or not do that
seen test for expr->kind == NAME (it shouldn't recurse for that, no?).

Richard.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37449