[Nouveau] [Bug 80266] Undefined operation in tgsi_ureg.c left shift of 1 by 31 places cannot be represented in type 'int'

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=80266

--- Comment #9 from Vittorio <zeccav at gmail.com> ---
(In reply to comment #6)
> I'm sure that technically shifting any bits into the sign bit of a signed
> number produces undefined behavior according to C99, so yes, (1<<31)
> technically is undefined. As a matter of practicality, on basically any CPU,
> 1<<31 will produce a valid negative number, but the compiler sees it as
> trying to assign the constant 2147483648 to a 32-bit signed container, which
> is outside of the 2^31-1 range that it can accept.
> 
> In the C abstract machine, this would be a serious problem. Obviously you
> can make such an assignment in hardware, but the result (when interpreted as
> signed) is not a value of 2147483648, but a value of -2147483648, which is
> why the compiler gives a warning. It's weird to read that (1<<31) is less
> than 0. If you don't really mean to assign a value of -2147483648 or
> 2147483648 and just want a bit pattern, using unsigned is almost always the
> way to go.
> 
> I know that Solaris Studio happens to give a warning for the same case, if
> that means anything.

Thanks for the appreciated explanation.
Maybe substituting 1<<31 with 1U<<31 clears any undefined behaviour on the many
architectures where mesa runs.

-- 
You are receiving this mail because:
You are the assignee for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/nouveau/attachments/20140620/c401573f/attachment.html>