On Wed, Oct 30, 2013 at 08:19:12PM +0000, Ben Hutchings wrote:

On Wed, 2013-10-30 at 13:13 -0700, Paul E. McKenney wrote:

quoted

On Wed, Oct 30, 2013 at 07:35:48PM +0000, Ben Hutchings wrote:

quoted

On Wed, 2013-10-30 at 18:23 +0100, Jesper Dangaard Brouer wrote:

quoted

From: Jesper Dangaard Brouer <redacted>

As described in commit 5a581b367 (jiffies: Avoid undefined
behavior from signed overflow), according to the C standard
3.4.3p3, overflow of a signed integer results in undefined
behavior.

[...]

According to the real processors that Linux runs on, signed arithmetic
uses 2's complement representation and overflow wraps accordingly.  And
we rely on that behaviour in many places, so we use
'-fno-strict-overflow' to tell gcc not to assume we avoid signed
overflow.  (There is also '-fwrapv' which tells gcc to assume the
processor behaves this way, but shouldn't it already know how the target
machine works?)

We should still fix them as we come across them.  There are a few types
of loops where '-fno-strict-overflow' results in more instructions
being generated.

I realise there's an opportunity for optimisation, but if these cases
are fixed on an ad-hoc basis, how will we know we're ready to make the
switch?

I believe that there are some tools that check for code that relies on
signed integer overflow.  Probably not yet up to dealing with the
kernel.  In the meantime, fixing them as we come across them is not
a bad approach.

							Thanx, Paul