Re: printf issues

[Szabolcs Nagy <nsz@port70.net>]

* Morten Welinder <mwelinder@gmail.com> [2014-04-04 13:42:30 -0400]:
> It looks like the LDBL_EPSILON version could be used in
> 
>     roundl.c
>     modfl.c
>     ceill.c
>     floorl.c
> 
> in the definition of TOINT instead of enumerating choices for
> LDBL_MANT_DIG.  It's basically the same thing going on

yes, that would be a bit nicer
(although other long double formats won't be supported anytime soon)

(note that in the future these implementations may need to change
the current versions raise inexact flag if result!=input, but the
next version of the floating-point extension standard for c
will require suppressing inexact, which i dont know how to do
with simple arithmetics efficiently without accessing the fenv..)

> While I was looking for that, I noticed that this modfl fallback looks
> problematic.  Even if long double and double are the same thing
> under the hood, I don't think you can cast pointers like that and
> assume it works.  It needs a temporary.
> 
> #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
> long double modfl(long double x, long double *iptr)
> {
> return modf(x, (double *)iptr);
> }

yes, this is an aliasing violation, nice catch

the original idea was to allow tail call opt for these wrappers,
so they are a single branch instruction, we should fix it but
i think we can rely on that the ptr representations are the same:

long double modfl(long double x, long double *iptr)
{
	union {long double *ld; double *d;} u = {iptr};
	return modf(x, u.d);
}