[Bug rtl-optimization/50904] [4.7 regression] pessimization when -fno-protect-parens is enabled by -Ofast

["dominiq at lps dot ens.fr" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904

--- Comment #36 from Dominique d'Humieres <dominiq at lps dot ens.fr> 2011-12-03 14:54:40 UTC ---
> Kind of, though, -ffast-math by itself already is on the verge of violating the
> standard. 

I disagree with this statement at least for codes that does not use IEEE
intrinsic modules (not yet implemented in gfortran).  Indeed the interaction
between -ffast-math and IEEE intrinsic modules will have to be discussed and
documented when these modules will be implemented (see pr50724 for the kind of
problems).

> I think -fno-protect-parens could be enabled by -ffast-math as that
> means that one does not really care about the exact value.

PLEEEASE DON'T. The situation is bad enough with -Ofast to not make it worse:
with -fno-protect-parens gfortran no longer complies with the Fortran standard
(7.1.5.2.4 quoted in comment #23) and IMO SHOULD NOT be part of any compound
option.

Note that if I am using -ffast-math, it is not because I do "not really care
about the exact value". It is mostly because I KNOW that exceptions will be the
signature of either a bug in my code and/or a bad choice of the parameters
leading to numerical instabilities. In top of that, I think that the concept of
"exact value" for floating-point numbers is ill-posed and as a consequence I do
accept that the least significant digits may depend on the way I write the code
or it is optimized (small fluctuations for well-posed methods, large ones
otherwise).

> If we want to add add extra protection for
>      tem = x + 2.d0**52
>      x = tem - 2.d0**52
> we probably need to add yet another flag as there are surely users, which want
> to have protected parentheses but allow for optimizations in the 'tmp' case.

If I need an extra protection, I'll put parentheses and I don't need yet
another flag. However, since the logic of the optimization is surprising the
first time you hit it, it could (should) be documented.