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

On Thu, Aug 22, 2013 at 09:07:34PM -0700, wrote:
> On Aug 22, 2013, at 8:38 PM, Rich Felker <> wrote:
> > On Thu, Aug 22, 2013 at 08:10:49PM -0700, 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...


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