Re: printf issues

[Morten Welinder <mwelinder@gmail.com>]

In fmt_fmt, the rounding decision is done using this test:

            /* Decide whether to round by probing round+small */
            if (round+small != round) { ...

Why is this done with long double?

The reason I ask is that the Valgrind situation improves a lot if
this is done with doubles.

(Valgrind situation: Valgrind emulates long doubles, poorly, by using
simple doubles.  See, for example, https://bugs.kde.org/show_bug.cgi?id=164298)

Morten




On Fri, Apr 4, 2014 at 2:54 PM, Szabolcs Nagy <nsz@port70.net> wrote:
> * 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);
> }