Re: [PATCH 01/14] Prepare vfprintf to use __printf_fp/__printf_fphex with float128 arg

[Florian Weimer <fweimer at redhat dot com>]

* Gabriel F. T. Gomes:

> On Tue, 18 Dec 2018, Florian Weimer wrote:
>
>>* Gabriel F. T. Gomes:
>>>
>>> @@ -1887,7 +1930,12 @@ printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
>>>  	      (args_value[cnt].pa_user, ap_savep);
>>>  	  }
>>>  	else
>>> -	  args_value[cnt].pa_long_double = 0.0;
>>> +	  {
>>> +	    args_value[cnt].pa_long_double = 0.0;
>>> +#if __HAVE_FLOAT128_UNLIKE_LDBL
>>> +	    args_value[cnt].pa_float128 = 0;
>>> +#endif
>>> +	  }  
>>
>>This bit doesn't look right to me because args_value[cnt] is a union.
>>You need to assign to the right member, or perhaps zero-initialize using
>>memset.
>
> Hmm, the original code doesn't seem to be dealing with anything particular
> to the long double type.  It seems that assigning to .pa_long_double,
> rather than to other members, was an arbitrary decision.  Do you know
> why .pa_long_double was chosen?

Not really.  My guess it was expected that it would be the largest
member.

> So, if this code is just zero-initializing the memory, do you think I
> should zero-initialize the whole of args_value/argsbuf.data when it gets
> expanded (see below)?  Then I could remove the else block entirely.
>
> Like this:
>
> diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
> index 61769e0ce1..17f1bae796 100644
> --- a/stdio-common/vfprintf-internal.c
> +++ b/stdio-common/vfprintf-internal.c
> @@ -1789,10 +1789,11 @@ printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
>      if (!scratch_buffer_set_array_size (&argsbuf, nargs, bytes_per_arg))
>        {
>         done = -1;
>         goto all_done;
>        }
> +    memset (argsbuf.data, 0, argsbuf.size);
>      args_value = argsbuf.data;
>      /* Set up the remaining two arrays to each point past the end of
>         the prior array, since space for all three has been allocated
>         now.  */
>      args_size = &args_value[nargs].pa_int;
> @@ -1884,12 +1885,10 @@ printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
>           {
>             args_value[cnt].pa_user = alloca (args_size[cnt]);
>             (*__printf_va_arg_table[args_type[cnt] - PA_LAST])
>               (args_value[cnt].pa_user, ap_savep);
>           }
> -       else
> -         args_value[cnt].pa_long_double = 0.0;

I would haved expected

            memset (&args_value[cnt], 0, sizeof (args_value[cnt]));

under thelse branch.

Thanks,
Florian