Re: [PATCH 3/9] Avoid undefined behavior in extract_integer

[Pedro Alves <palves@redhat.com>]

On 08/27/2018 03:56 PM, Tom Tromey wrote:
> -fsanitize=undefined showed that extract_integer could left-shift a
> negative value, which is undefined.  This patch fixes the problem by
> doing all the work in an unsigned type, and then using a static_cast
> at the end of the function.  This relies on implementation-defined
> behavior, but I tend to think we are on safe ground there.  (Also, if
> need be, violations of this could probably be detected, either by
> configure or by a static_assert.)
> 
> ChangeLog
> 2018-08-27  Tom Tromey  <tom@tromey.com>
> 
> 	* findvar.c (extract_integer): Do work in an unsigned type and
> 	cast at the end.

LGTM.

I suspect we assume two's complement in a good number of
places, and I don't think it's worth it to bother with anything
else.  There's even been discussion in the C++ committee about
baking the the assumption into the language.

Is the cast really necessary, though?  What error do you get?

Thanks,
Pedro Alves