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Re: [RFC][PATCH] MIPS: IEEE 754-2008 NaN encoding support
- From: Rich Felker <dalias at aerifal dot cx>
- To: "Maciej W. Rozycki" <macro at codesourcery dot com>
- Cc: libc-ports at sourceware dot org, libc-alpha at sourceware dot org, Doug Gilmore <Doug dot Gilmore at imgtec dot com>
- Date: Fri, 23 Aug 2013 00:10:28 -0400
- Subject: Re: [RFC][PATCH] MIPS: IEEE 754-2008 NaN encoding support
- References: <alpine dot DEB dot 1 dot 10 dot 1308222343480 dot 8514 at tp dot orcam dot me dot uk> <20130823005820 dot GL20515 at brightrain dot aerifal dot cx> <alpine dot DEB dot 1 dot 10 dot 1308230213040 dot 8514 at tp dot orcam dot me dot uk>
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