Re: Where does the C standard describe overflow of signed integers?

[Paul Koning <pkoning@equallogic.com>]

>>>>> "Paul" == Paul Schlie <schlie@comcast.net> writes:

 >> From: Robert Dewar <dewar@adacore.com>
 >>> Paul Schlie wrote:
 >>>> What about optimising x*2/2 to x?
 >>> Given that "C" requires the above be evaluated as (x*2)/2, as the
 >>> language specifies that the syntax defines the precedence of the
 >>> operations, and that no optimization should alter the behavior as
 >>> specified by the program; I'd say that unless it was known that
 >>> the value range of x was between 0 and INT_MAX, the optimization
 >>> is simply invalid.
 >> the optimization is indeed valid
 >> 
 >> optimizations may most certainly alter behavior of undefined
 >> code. think about uninitialized local variables.

 Paul> I don't contest that it may, I simply don't believe it should.

In that case you may want to stick with -O0.  There are *lots* of
things GCC does that alter undefined cases.  How about the undefined
behavior when aliasing rules are violated?  Would you want to make
-fno-strict-aliasing be the only supported setting?

     paul