From: Rich Felker <dalias@aerifal.cx>
To: pinskia@gmail.com
Cc: "Maciej W. Rozycki" <macro@codesourcery.com>,
"libc-ports@sourceware.org" <libc-ports@sourceware.org>,
"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>,
Doug Gilmore <Doug.Gilmore@imgtec.com>
Subject: Re: [RFC][PATCH] MIPS: IEEE 754-2008 NaN encoding support
Date: Fri, 23 Aug 2013 04:12:00 -0000 [thread overview]
Message-ID: <20130823041222.GR20515@brightrain.aerifal.cx> (raw)
In-Reply-To: <6DDE6A09-5E4D-4FCF-B126-7021735F431F@gmail.com>
On Thu, Aug 22, 2013 at 09:07:34PM -0700, pinskia@gmail.com wrote:
> On Aug 22, 2013, at 8:38 PM, Rich Felker <dalias@aerifal.cx> wrote:
>
> > On Thu, Aug 22, 2013 at 08:10:49PM -0700, pinskia@gmail.com wrote:
> >>>> To give you a small example this:
> >>>>
> >>>> double foo = __builtin_nan ("");
> >>>>
> >>>> will compile to a different data pattern with opposite (qNaN vs sNaN)
> >>>> semantics depending on the NaN encoding mode selected in the compiler.
> >>>> Modules built with different NaN encodings are therefore not compatible,
> >>>
> >>> They are compatible except in the area of subtle exception-raising
> >>> semantics that GCC *DOES NOT GET CORRECT ANYWAY*. GCC is full of
> >>> incorrect optimizations that cause the exception flags to be wrong.
> >>> Until that's fixed, I don't see why this issue is so important to
> >>> merit flagging object files build with different modes as having an
> >>> incompatible ABI. The semantics are slightly different, but the type
> >>> sizes and the way they're passed are all the same, and programs that
> >>> don't use the GCC extension __builtin_nan() or the NAN macro from
> >>> math.h, or writing raw float values to/from disk, are completely
> >>> unaffected.
> >>
> >> Can you give an example and maybe a link to a GCC bug where this is
> >> recorded before spreading this kind of information. I really don't
> >> like blank statements without facts to back up them.
> >
> > int foo() { double x = 0; x /= 0.0; return 1; }
> >
> > While this is a stupid, trivial example, the issue has come up A LOT
> > for us in musl's implementation of the math library (based on fdlibm)
> > with nontrivial code.
>
> Wait you mean dead code? I think the standard/IEEE allows to remove
> it.
They don't. Anyway, here's a less trivial example:
double f(int y){ double x=0; x/=0; if(y) return 1; return x; }
> At -O0, GCC won't remove it though.
That's not helpful; -O0 is not usable for most purposes. The output is
gigantic and slow. And if I'm not mistaken, there are some cases where
even -O0 will mess it up...
Rich
next prev parent reply other threads:[~2013-08-23 4:12 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-22 23:53 Maciej W. Rozycki
2013-08-23 0:58 ` Rich Felker
2013-08-23 1:41 ` Maciej W. Rozycki
2013-08-23 1:57 ` Rich Felker
2013-08-23 3:11 ` pinskia
2013-08-23 3:38 ` Rich Felker
2013-08-23 4:08 ` pinskia
2013-08-23 4:12 ` Rich Felker [this message]
2013-08-23 4:10 ` Rich Felker
2013-08-23 3:46 ` Rich Felker
2013-08-23 10:53 ` Maciej W. Rozycki
2013-08-23 15:26 ` Joseph S. Myers
2013-09-05 22:47 ` [RFC][PATCH v2] " Maciej W. Rozycki
2013-09-06 16:24 ` Joseph S. Myers
2013-09-10 9:36 ` Maciej W. Rozycki
2013-09-10 23:24 ` Joseph S. Myers
2013-09-16 15:14 ` [PATCH v3] " Maciej W. Rozycki
2013-09-16 15:26 ` Joseph S. Myers
2013-09-18 20:48 ` Maciej W. Rozycki
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