Date: Sat, 17 Mar 2012 16:10:52 -0400 From: Rich Felker <dalias@...ifal.cx> To: musl@...ts.openwall.com Subject: Re: i386 fegetround bug On Sat, Mar 17, 2012 at 07:48:01PM +0100, Szabolcs Nagy wrote: > * 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 I'm not in favor of hurting performance or other useful characteristics for the sake of a nonstandard feature of dubious usefulness. Unless there's a demand to change it, I'm fine with your new (fastest) version of the code. > (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) I'm not opposed to this solution, but also don't really find it necessary. By the way, I think shifting right the low 6 bits and checking carry flag would be more efficient. :) Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.