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Date: Wed, 27 Feb 2019 15:35:03 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: FE Exception triggered by comparison
On Wed, Feb 27, 2019 at 09:16:09PM +0100, Szabolcs Nagy wrote:
> * Alexander Monakov <amonakov@...ras.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
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