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From: Dimitry Andric <dim@FreeBSD.org>
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Subject: Re: 1 << 31 redux
Date: Thu, 11 Jan 2018 14:37:25 +0100
In-Reply-To: <CABh_MKmy3ojYO3M6YwH2wWrEp5a4C-o2XC1icN+Nj3GDo4f+ZA@mail.gmail.com>
Cc: Eitan Adler <lists@eitanadler.com>,
 FreeBSD Hackers <freebsd-hackers@freebsd.org>
To: Ed Schouten <ed@nuxi.nl>
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On 11 Jan 2018, at 13:05, Ed Schouten <ed@nuxi.nl> wrote:
>=20
> 2018-01-11 13:03 GMT+01:00 Eitan Adler <lists@eitanadler.com>:
>> I'd also like to see if we could find some more general solution, be =
it a
>> compiler warning, bit set macro, or otherwise.
>=20
> Does Clang already offer a warning for this? If so, we should consider
> adding it to WARNS=3D6.

There is a -Wshift-sign-overflow flag, but it isn't enabled by default:

$ clang -Wshift-sign-overflow -c bar.c
bar.c:1:26: warning: signed shift result (0x80000000) sets the sign bit =
of the shift expression's type ('int') and becomes negative =
[-Wshift-sign-overflow]
int bar(void) { return 1 << 31; }
                       ~ ^  ~~

I would expect quite a lot of stuff to break if you enable it, though. =
:)

And of course, there is -fsanitize=3Dundefined, which can catch this =
kind
of thing at runtime.

-Dimitry


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