[Bug middle-end/34678] Optimization generates incorrect code with -frounding-math option (#pragma STDC FENV_ACCESS not implemented)

[nmm1 at cam dot ac.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678

--- Comment #23 from Nick Maclaren <nmm1 at cam dot ac.uk> ---
(In reply to Vincent Lefèvre from comment #21)
>
> > Richard Biener's approach to the default is the one that matches the C
> > standard and Vincent Lefèvre is mistaken.
> 
> No, I'm correct.
> 
> > Defining it to be 'off' and not setting __STDC_IEC_559__ is very reasonable.
> 
> No, this is really stupid. If the user *decides* to set the STDC FENV_ACCESS
> pragma to "on", then the compiler must not assume that it is "off" (this bug
> is not about the default state). At least it must behave in this way if
> -std=c99 (or c11) has been used. Otherwise a compilation failure may be
> better than getting wrong results.

If __STDC_IEC_559__ is unset or does not have the value 1, setting
STDC FENV_ACCESS to "on" is undefined behaviour (see 6.10.8.3, 7.6 and
Annex F), unless the implementation explicitly chooses to extend the
language to support it.  So the user would get what he so richly
deserves.


(In reply to joseph@codesourcery.com from comment #22)
> 
> So a key part of actually making rounding modes and exceptions work 
> reliably would be working out a definition of GCC's default mode that 
> allows more or less the same optimizations as at present, while allowing 
> users wanting the full support (and consequent optimization cost) to 
> specify the appropriate command-line options or FENV_ACCESS pragma to 
> enable it.

Yes.  That won't deal with the correctness problems of introducing
IEEE 754 support into code not set up to handle it, especially C++,
of course.  I tried to get WG21 to take a stand on that issue, but
failed :-(

Working out what on earth to do in such a case is likely to be a far
fouler task than merely dealing with the performance problems :-(