Re: [PATCH] Fix float to LONGEST conversion.

["Ali Tamur via gdb-patches" <gdb-patches at sourceware dot org>]

Updated the commit message:
>
    Fix float to LONGEST conversion.

    The code used to have undefined behaviour when template parameter is
float and
    host_float is NaN, because it attempted to convert NaN value to LONGEST
at the
    last statement. This frequently caused crashes on tests that checked
"info
    all-registers" (at least when the code is compiled with clang; I didn't
test
    with gdb).

On Wed, Aug 28, 2019 at 1:29 AM Andrew Burgess <andrew.burgess@embecosm.com>
wrote:

> * Ali Tamur via gdb-patches <gdb-patches@sourceware.org> [2019-08-27
> 20:28:08 -0700]:
>
> > The code used to have undefined behaviour.
>
> I would much prefer to see a more detailed explanation for _why_ the
> previous behaviour is undefined.  I ran my eye over the old code but
> didn't see anything obvious.
>
> If at a later date I come back and want to figure out why this patch
> went in it would be nice if the commit message could tell me
> everything I need to know.
>
> Thanks,
> Andrew
>
>
>
> >
> >
> > gdb/ChangeLog:
> >       *gdb/target-float.c (host_float_ops<T>::to_longest): Update
> >       implementation.
> > ---
> >  gdb/target-float.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/gdb/target-float.c b/gdb/target-float.c
> > index 39abb12696..0fd71c0dc3 100644
> > --- a/gdb/target-float.c
> > +++ b/gdb/target-float.c
> > @@ -1007,13 +1007,18 @@ host_float_ops<T>::to_longest (const gdb_byte
> *addr,
> >  {
> >    T host_float;
> >    from_target (type, addr, &host_float);
> > -  /* Converting an out-of-range value is undefined behavior in C, but we
> > -     prefer to return a defined value here.  */
> > -  if (host_float > std::numeric_limits<LONGEST>::max())
> > -    return std::numeric_limits<LONGEST>::max();
> > -  if (host_float < std::numeric_limits<LONGEST>::min())
> > +  T min_possible_range =
> static_cast<T>(std::numeric_limits<LONGEST>::min());
> > +  T max_possible_range = -min_possible_range;
> > +  /* host_float can be converted to an integer as long as it's in
> > +     the range [min_possible_range, max_possible_range). If not, it is
> either
> > +     too large, or too small, or is NaN; in this case return the
> maximum or
> > +     minimum possible value.  */
> > +  if (host_float < max_possible_range && host_float >=
> min_possible_range)
> > +    return static_cast<LONGEST> (host_float);
> > +  if (host_float < min_possible_range)
> >      return std::numeric_limits<LONGEST>::min();
> > -  return (LONGEST) host_float;
> > +  /* This line will be executed if host_float is NaN.  */
> > +  return std::numeric_limits<LONGEST>::max();
> >  }
> >
> >  /* Convert signed integer VAL to a target floating-number of type TYPE
> > --
> > 2.23.0.187.g17f5b7556c-goog
> >
>