Re: c/8828: gcc reports some code is unreachable when it is not

["Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>]

On Fri, Dec 06, 2002 at 09:29:12AM -0000, reichelt@igpm.rwth-aachen.de wrote:
>     An even shorter example is the following:
>     
>     -----------------snip here----------------
>     void foo(int i)
>     {
>       switch(i) {
>         case 0:
>           break;
>         case 1:
>           break;
>       }
>     }
>     -----------------snip here----------------
>     
>     Compiling this with gcc 3.2.1 or mainline I get the message
>     
>     PR8828.c: In function `foo':
>     PR8828.c:7: warning: will never be executed
>     PR8828.c:5: warning: will never be executed

Looks like warnings removed by the optimizer. In this case the
optimizer will just remove both of the empty case labels and warn
that the instructions therein (the breaks) aren't executed. In
the original example the break is probably folded into the for loop
an then optimized away. I even managed to get warnings for code like
this:

switch (i) {
	case 0:
		x++;
		break;
	case 1:
		x++;
		break;
}

where the optimizer tells me that it removed one of the x++ instructions
probably because the two case labels were combined. This is probably not
a bug, the documentation doesn't explicitly mention optimizations but
it does mention that inlined function may produce warning for code that
is unreachable only in a single inlined copy. This case looks rather
similar.

   regards   Christian

-- 
THAT'S ALL FOLKS!