------- Comment #2 from anthony dot penniston at hotmail dot com 2010-06-17
01:14 -------
(In reply to comment #1)
> Value range-propagation (VRP) does not work on disjoint ranges, so the
> compiler
> does not actually know that argc can only be 1, 2 or 4. I think there is
> already a PR about this but I cannot find it right now.
>
I'm not sure VRP is the issue here (the values being range-like was merely a
coincidence). Consider the following two equivalent examples:
int main( int argc, char *argv[] )
{
if( argc == 111 || argc == 999 )
{
int n;
if( argc == 111 || argc == 999 )
n = argc;
return n;
}
}
int main( int argc, char *argv[] )
{
switch( argc )
{
case 111:
case 999:
{
int n;
switch( argc )
{
case 111:
case 999:
n = argc;
}
return n;
}
}
}
In both examples - as with the example in the original report - the compiler
is given the exact same information, namely that after the first switch/if
statement, the values on argc are constrained to either 111 or 999 (and thus
the following switch/if must necessarily cover all values of argc and n is
always used initialized). The only difference is that, in the example using
switch statements, the compiler seems to "forget" these constraints on argc.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44547
