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From: Rich Felker <>
To: "Maciej W. Rozycki" <>
	Doug Gilmore <>
Subject: Re: [RFC][PATCH] MIPS: IEEE 754-2008 NaN encoding support
Date: Fri, 23 Aug 2013 04:10:00 -0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Aug 23, 2013 at 02:41:21AM +0100, Maciej W. Rozycki wrote:
> > >  As from revision 3.50 [2][3] the MIPS Architecture provides for 
> > > processors that support the IEEE 754-2008 preferred NaN encoding format.  
> > > As the two formats (further referred to as "legacy NaN" and "2008 NaN") 
> > > are incompatible to each other, tools will have to provide support for the 
> > > two formats to help people avoid using incompatible binary modules.  Here 
> > > is the glibc part.
> > 
> > Can you elaborate on why you think this is an ABI issue? IMO it's just
> > s runtime issue unless you're considering raw floating point data
> > written to disk. In any case this seems like such a small issue that
> > it should just be silently fixed, rather than adding huge amounts of
> > ABI ugliness.
>  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, 
> either at the link or at the load time, as applicable.  LD (from binutils) 
> and take care of the link-time and the load-time integrity check, 
> respectively.

If you really care about supporting this, I have an alternate idea
that doesn't lead to incompatible ABIs. Have GCC put static objects
containing NANs in the writable data section, and add a new R_MIPS_NAN
relocation type that flips the high bit of the NAN to match the
runtime environment. The only difficulty would be making it work with
static linking; you could choose to just have the static linker
resolve all static objects containing NANs to a single, fixed NAN
format at link time, or have some startup code to process them.

For __builtin_nan() that's not used as a constant expression for
initializing static objects, GCC can just generate a call to libgcc,
or direct access to a global object, to flip the high bit


  parent reply	other threads:[~2013-08-23  4:10 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
2013-08-23  4:10     ` Rich Felker [this message]
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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