Re: [PATCH] ieeefp.c: Auto-detect _LDBL_EQ_DBL

On 09/06/2018 12:18 AM, Keith Packard wrote:
> Make configuring the library a bit simpler
>
> Signed-off-by: Keith Packard <[email protected]>
> ---
> This seems simpler than computing this value during configure, and
> will make using meson easier in the future.
>
>   newlib/libc/include/ieeefp.h | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/newlib/libc/include/ieeefp.h b/newlib/libc/include/ieeefp.h
> index 2d6421a4c..71749ccc0 100644
> --- a/newlib/libc/include/ieeefp.h
> +++ b/newlib/libc/include/ieeefp.h
> @@ -143,6 +143,11 @@ typedef union
>   
>   #endif /* __IEEE_LITTLE_ENDIAN */
>   
> +#if LDBL_MANT_DIG == DBL_MANT_DIG && LDBL_MIN_EXP == DBL_MIN_EXP && \
> +    LDBL_MAX_EXP == DBL_MAX_EXP
> +#define _LDBL_EQ_DBL
> +#endif
> +
>   #ifndef _LDBL_EQ_DBL
>   
>   #ifndef LDBL_MANT_DIG
     Back when _LDBL_EQ_DBL was added, the reason for a configure-time test was 
that there were compiler options that choose long double size.  (See 
https://sourceware.org/ml/newlib/2009/msg00497.html)  GCC does have 
-mlong-double options which allow the size of long double to be changed, so the 
original reason for the decision still applies.  So unless some kind of multilib 
magic can be added to take care of this, the proposal is a non-starter.
      However, even assuming that the configure-time problem can be solved, this 
patch by itself seems to be a bad idea because it is leaving the 
config-generated define in newlib.h.  I don't think the math routines all 
include include/ieeefp.h (they probably all get include/machine/ieeefp.h), so 
that the definitions could end up coming from different places.  (Realistically 
they should never give a different result, but it is not good practice have a 
decision like this in 2 places.)  So to do something like this, it 1) needs a 
place where all files using it can get it (or all files using it would need to 
be sure they included ieeefp.h), and 2)  get rid of the configured part of it 
and the definition in newlib.h.  (That is, this patch is the start of a 
configuration simplification, but is incomplete.)
      In addition, this could/would produce warnings on platforms without long 
double, so an added term would be needed if this bit of it is kept, something like:

#if defined(LDBL_MANT_DIG) && LDBL_MANT_DIG == DBL_MANT_DIG && ...



                 Craig