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