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Message-ID: <20200120223204.GP23985@port70.net>
Date: Mon, 20 Jan 2020 23:32:07 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Triggering Overflow (or Underflow) without triggering
Inexact on i386
* Damian McGuckin <damianm@....com.au> [2020-01-21 08:11:50 +1100]:
> I am trying to write arithmetic expressions using only x87 instructions to
> trigger an exception. The ASM instruction to raise the flag in the status
> register has way too much overhead.
>
> My normal technique of doing either of these would also trigger Inexact.
>
> e.g.
> volatile long double t;
>
> Overflow: t = largest normal long double, t *= t;
> Underflow: t = smallest normal long double, t *= t;
>
> Can it be triggered WITHOUT also triggering INEXACT?
ieee operations raise inexact and underflow/overflow together.
so you cannot implement feraiseexcept by only doing arithmetic
ops (which can do individual underflow/overflow), but that's a
rare function so it can be slow fldenv/fwait etc.
note that there is __math_uflow/__math_oflow now for internal
math code (and they don't use the smallest/largest normal,
instead they use small/large numbers with simple representation
since sometimes those can be constructed as immediates on some
targets, but this is a micro-optimization).
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