Re: [RFC][PATCH] MIPS: IEEE 754-2008 NaN encoding support

[Rich Felker <dalias@aerifal.cx>]

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 ld.so 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
appropriately.

Rich