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

["Rolf Campbell" <rcampbell@tropicnetworks.com>]

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

From: "Rolf Campbell" <rcampbell@tropicnetworks.com>
To: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>,
	<reichelt@igpm.rwth-aachen.de>,
	<gcc-bugs@gcc.gnu.org>,
	<gcc-prs@gcc.gnu.org>,
	<nobody@gcc.gnu.org>,
	<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 11:22:32 -0500

 But, this was compiled WITHOUT optimizations (gcc -Wunreachable-code -c
 a.c), so there should be no removal of superfluous code, or folding of
 break statements.
 
 -Rolf
 
 > -----Original Message-----
 > From: Christian Ehrhardt [mailto:ehrhardt@mathematik.uni-ulm.de]=20
 > Sent: Friday, December 06, 2002 11:14 AM
 > To: reichelt@igpm.rwth-aachen.de; gcc-bugs@gcc.gnu.org;=20
 > gcc-prs@gcc.gnu.org; nobody@gcc.gnu.org; Rolf Campbell;=20
 > gcc-gnats@gcc.gnu.org
 > Subject: Re: c/8828: gcc reports some code is unreachable=20
 > when it is not
 >=20
 >=20
 > On Fri, Dec 06, 2002 at 09:29:12AM -0000,=20
 > reichelt@igpm.rwth-aachen.de wrote:
 > >     An even shorter example is the following:
 > >    =20
 > >     -----------------snip here----------------
 > >     void foo(int i)
 > >     {
 > >       switch(i) {
 > >         case 0:
 > >           break;
 > >         case 1:
 > >           break;
 > >       }
 > >     }
 > >     -----------------snip here----------------
 > >    =20
 > >     Compiling this with gcc 3.2.1 or mainline I get the message
 > >    =20
 > >     PR8828.c: In function `foo':
 > >     PR8828.c:7: warning: will never be executed
 > >     PR8828.c:5: warning: will never be executed
 >=20
 > Looks like warnings removed by the optimizer. In this case=20
 > the optimizer will just remove both of the empty case labels=20
 > and warn that the instructions therein (the breaks) aren't=20
 > executed. In the original example the break is probably=20
 > folded into the for loop an then optimized away. I even=20
 > managed to get warnings for code like
 > this:
 >=20
 > switch (i) {
 > 	case 0:
 > 		x++;
 > 		break;
 > 	case 1:
 > 		x++;
 > 		break;
 > }
 >=20
 > where the optimizer tells me that it removed one of the x++=20
 > instructions probably because the two case labels were=20
 > combined. This is probably not a bug, the documentation=20
 > doesn't explicitly mention optimizations but it does mention=20
 > that inlined function may produce warning for code that is=20
 > unreachable only in a single inlined copy. This case looks=20
 > rather similar.
 >=20
 >    regards   Christian
 >=20
 > --=20
 > THAT'S ALL FOLKS!
 >=20