[PATCH] Fix float to LONGEST conversion.

Andrew Burgess andrew.burgess@embecosm.com
Wed Aug 28 08:29:00 GMT 2019


* 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
> 



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