[Bug middle-end/100839] -O2 does dangerous optimizations using FMA (please don't break my cross product)

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100839

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |INVALID

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Daniel Gibson from comment #4)
> Nope.
> 
> 1. -std=c++11 doesn't make a difference, just tested it on compiler explorer.
> 2. What's the point of -Ofast ("It also enables optimizations that are not
> valid for all standard-compliant programs.") if the normal optimization
> levels already violate the standard?

Oh that is because C++ allows it while C does not.
  /* ISO C restricts floating-point expression contraction to within
     source-language expressions (-ffp-contract=on, currently an alias
     for -ffp-contract=off).  */
  if (flag_iso
      && !c_dialect_cxx ()
      && (global_options_set.x_flag_fp_contract_mode
          == (enum fp_contract_mode) 0)
      && flag_unsafe_math_optimizations == 0)
    flag_fp_contract_mode = FP_CONTRACT_OFF;