1 << 31 redux

[Dimitry Andric]

On 11 Jan 2018, at 13:05, Ed Schouten <ed at nuxi.nl> wrote:
> 
> 2018-01-11 13:03 GMT+01:00 Eitan Adler <lists at 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.
> 
> Does Clang already offer a warning for this? If so, we should consider
> adding it to WARNS=6.

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=undefined, which can catch this kind
of thing at runtime.

-Dimitry

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