Date: Wed, 17 Mar 2004 01:54:17 +0200 From: Ruslan Ermilov <ru@freebsd.org> To: Dag-Erling Sm?rgrav <des@des.no> Cc: net@freebsd.org Subject: Re: libalias patch for review / testing Message-ID: <20040316235417.GC65466@ip.net.ua> In-Reply-To: <xzpn06ggycp.fsf@dwp.des.no> References: <xzp7jxkjv98.fsf@dwp.des.no> <xzp3c88jv7k.fsf@dwp.des.no> <20040316225230.GO3462@ip.net.ua> <xzpllm0ie5z.fsf@dwp.des.no> <20040316232848.GR3462@ip.net.ua> <xzpn06ggycp.fsf@dwp.des.no>
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--zCKi3GIZzVBPywwA Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Mar 17, 2004 at 12:34:46AM +0100, Dag-Erling Sm?rgrav wrote: > Ruslan Ermilov <ru@freebsd.org> writes: > > : `-fstrict-aliasing' > > : Allows the compiler to assume the strictest aliasing rules > > : applicable to the language being compiled. For C (and C++), this > > : activates optimizations based on the type of expressions. In > > : particular, an object of one type is assumed never to reside at > > : the same address as an object of a different type, unless the > > : types are almost the same. For example, an `unsigned int' can > > : alias an `int', but not a `void*' or a `double'. A character ty= pe > > : may alias any other type. > > > > And asking myself a question: should those (void *)'s in your patch > > be (char *)'s instead, e.g., in twowords() and DifferentialChecksum(), > > or am I misreading the above? >=20 > You're misreading, we're doing u_short * <-> void * (both pointers) > but the man page speaks about int <-> void * (scalar vs pointer) >=20 OK, I stand corrected. ;) > Also, I doubt DifferentialChecksum() is a problem, since it's a > function call. I think the problem may be in the code I've replaced > with calls to twowords(). >=20 Hmm, now that I think about it more, since -O2 turns -fstrict-aliasing, and the latter may produce broken code if strict aliasing rules are broken by the source, I think people (and tinderboxes!) should compile with ``-O2 -Wstrict-aliasing'' in CFLAGS rather than just -O2. For WARNS > 1 compiled code, this will be a no-op (as -Wall implies -Wstrict-aliasing), but it should help catch bugs related to breaking strict aliasing like in libalias, in code that is otherwise compiled without warnings. Cheers, --=20 Ruslan Ermilov FreeBSD committer ru@FreeBSD.org --zCKi3GIZzVBPywwA Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAV5OpUkv4P6juNwoRAkGQAJ4sQy7mMiNCgX4b7vsc2HS+ZKYbHgCfcApJ Sbn6Z6pnxETzw2ZoQphZWXk= =tR29 -----END PGP SIGNATURE----- --zCKi3GIZzVBPywwA--
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