Re: mingw isnanl-nolibm failure

[Bruno Haible]

Eric Blake wrote:
> When compiling natively on mingw:
> ...
> checking where to find the exponent in a 'long double'... word 2 bit 0
> checking where to find the exponent in a 'long double'... (cached) word 2 bit > 0
> 
> and the resulting rpl_isnanl works.
> 
> But when cross-compiling:
> ...
> checking where to find the exponent in a 'long double'... unknown
> checking where to find the exponent in a 'long double'... unknown
> 
> And the resulting rpl_isnanl fails on:
> 
>   { /* Pseudo-Denormal.  */
>     static memory_long_double x =
>       { LDBL80_WORDS (0x0000, 0x83333333, 0x00000000) };
>     ASSERT (isnanl (x.value));
>   }
> 
> because gcc 3.4.4 emitted code for (x == x) that returns true for pseudo-
> denormal long doubles.

Ack. It's a bug in the isnanl module.

> I'm not sure how to fix this, but it seems like rpl_isnanl needs to check for 
> invalid x86 long double bit patterns before falling back to ==.

Either this, or ensure that the "checking where to find the exponent" tests
return the bit positions instead of "unknown".

But I don't see how to implement either of these two possible fixes for a
cross-compiler, in a way that works also for other platforms than mingw.
Any ideas?

Bruno