Re: FE Exception triggered by comparison

[Rich Felker <dalias@libc.org>]

On Wed, Feb 27, 2019 at 09:16:09PM +0100, Szabolcs Nagy wrote:
> * Alexander Monakov <amonakov@ispras.ru> [2019-02-27 22:48:02 +0300]:
> > On Wed, 27 Feb 2019, Rich Felker wrote:
> > > Ideally the compiler would be able to recognize portable (within IEEE)
> > > patterns for floating point representation examination and optimize
> > > them if there's a more efficient way to be able to do it for a
> > > particular machine.
> > 
> > There's a difference for sNaN operands: the bit-test version obviously
> > is not going to raise "invalid", while comparing the fpu register with
> > itself will.  So I'm afraid the compiler wouldn't do that for x86 (but
> > could for targets where an suitable instruction is available).
> 
> using -fsignaling-nan is extremely rare, by default the transformation
> is valid (but maybe tricky anyway).

I'm not sure if I'd call transforming non-floating-point
bit-manipulation code into a floating point instruction that alters
exception flags "valid" regardless of the -fsignaling-nan state. It's
one thing to say "this program isn't using and doesn't care about
signaling nans [as floating point values]"; it's completely different
to say "you can optimize integer operations as floating point without
regard for how that affects fenv".

Rich