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

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

The following reply was made to PR c/8828; it has been noted by GNATS.

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: reichelt@igpm.rwth-aachen.de, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org,
  nobody@gcc.gnu.org, rcampbell@tropicnetworks.com, gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c/8828: gcc reports some code is unreachable when it is not
Date: Fri, 6 Dec 2002 17:13:43 +0100

 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!