[Bug middle-end/71614] ppc __gcc_qmul is mis-optimized not to be commutative

[timshen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71614

--- Comment #3 from Tim Shen <timshen at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> Two things, try -fno-strict-aliasing first.
> Since your main violates C aliasing rules.
> Second thing to try is -ffp-contract=off as I am suspecting:
>   v = a*d;
>   w = b*c;
>   tau += v + w;	    /* Add in other second-order terms.	 */
>   u = t + tau;
> 
> is using an fmadd here.

Passing -fno-strict-aliasing doesn't change the -O2 result; -ffp-contract=off
does give the consistent result as -O1.

What result is __gcc_qmul supposed to give? As a user I'd expect the
commutativity here.