Warning for converting (possibly) negative float/double to unsigned int

[Manuel López-Ibáñez]

On 02/26/2016 09:28 PM, Bradley Lucier wrote:
> Perhaps this question is appropriate for the gcc mail list.
>
> Converting a float/double to unsigned int is undefined if the result would be
> negative when converted to a signed int.
>
> x86-64 and arm treat this condition differently---x86-64 returns a value whose
> bit pattern is the same as the bit pattern for converting to signed int, and
> arm returns zero.  So it would be nice to have a warning that this will (or
> could) happen.
>
> I couldn't find such a warning in the GCC manual or in the GCC code base.

prog.cc:4:23: warning: overflow in implicit constant conversion [-Woverflow]
      unsigned int z = -2.0;
                        ^~~
prog.cc:5:12: warning: conversion to 'unsigned int' from 'double' may alter its 
value [-Wfloat-conversion]
      return x;
             ^

Both warnings exist since quite a long time (10 years?), the latter under the 
more general -Wconversion:
https://gcc.gnu.org/wiki/NewWconversion#Frequently_Asked_Questions

Cheers,

	Manuel.