Re: [PATCH 1/2] avoid shifting signed integers 31 bits

From: Jeff King <peff@peff.net>
Date: Dec 31, 2015, 05:20 UTC
Message-ID: <20151231052029.GA10238@sigill.intra.peff.net>
In-Reply-To: <CACsJy8CAAqZFQCfadUov7DFhSgh=dtGfE-onbXZQXO-0Y2652g@mail.gmail.com>

On Thu, Dec 31, 2015 at 12:10:33PM +0700, Duy Nguyen wrote:

Show 13 quoted lines

> On Tue, Dec 29, 2015 at 1:35 PM, Jeff King <peff@peff.net> wrote:
> > We sometimes use 32-bit unsigned integers as bit-fields.
> > It's fine to access the MSB, because it's unsigned. However,
> > doing so as "1 << 31" is wrong, because the constant "1" is
> > a signed int, and we shift into the sign bit, causing
> > undefined behavior.
> >
> > We can fix this by using "1U" as the constant.
> 
> We have this in cache.h, should it be fixed as well?
> 
> /* CE_EXTENDED2 is for future extension */
> #define CE_EXTENDED2         (1 << 31)

Sort of. We don't actually use it, and since it's a macro, that means it never even hits the compiler proper itself. So it's not a bug, but it's a bug waiting to happen. :)

-Peff