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Date: Sat, 17 Mar 2012 19:48:01 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: i386 fegetround bug
* Szabolcs Nagy <nsz@...t70.net> [2012-03-17 15:42:44 +0100]:
> fegetround had a bug: eax was not cleared properly
>
> while fixing it i also changed fesetround see my commit message
>
> http://nsz.repo.hu/git/?p=musl;a=commit;h=88cfaf8a142a8c57beccb89398a6421c4bbf121a
>
i realized my analysis was not entirely correct after rereading
Intel 64 and IA-32 .. Manual, vol.1. 8.6 last paragraph and 8.7.1
it suggests that pending unmasked exceptions can occur
with normal operation (assuming there are unmasked exceptions)
(i just couldn't produce such case from c as an arithmetic op
is usually followed by an fld which traped before i could test
my theory with an fnstenv)
so if we want to be unmasked-exception-friendly then
fnstenv/fldenv pair should be used
(maybe with some extra check for unmasked exceptions first:
fnstcw x
and $0x3f,x
sub $0x3f,x
jnz unmasked_exceptions
...
and the unmasked_exceptions case uses fnstenv/fldenv, otherwise
plain fldcw is used)
(fnstenv/fldenv can probably hurt those who change rounding
mode frequently, i haven't seen such code but in theory
it makes sense for interval arithmetics)
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