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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: FE Exception triggered by comparison
Date: Wed, 27 Feb 2019 21:16:09 +0100
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* 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).