[Bug c/45831] 0 = 10 (with -O0, 0 = 0 with -O1, but 10 = 10 expected)

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45831

--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-09-29 15:43:02 UTC ---
On Wed, 29 Sep 2010, manu at gcc dot gnu.org wrote:

> Why don't we just not compile programs with evident undefined behaviour? I
> think the standard allows us to do so.

http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_109.html

   A conforming implementation must not fail to translate a strictly
   conforming program simply because some possible execution of that
   program would result in undefined behavior. Because foo might never be
   called, the example given must be successfully translated by a
   conforming implementation.

The standard never says explicitly whether particular undefined behavior 
is compile-time undefined (a property of the program) or run-time 
undefined (a property of a particular path of execution through the 
program), so you have to read it carefully to figure this out in each 
case, but when it is run-time undefined (as here) the program must be 
accepted unless all executions must involve the undefined behavior.  I 
don't think the cases where the undefined behavior is in main and can be 
shown always to occur are sufficiently common (in real code rather than 
artificial testcases) to make it worthwhile to give errors specially for 
them.