Date: Tue, 9 Oct 2012 13:50:24 -0400 From: Eitan Adler <eadler@freebsd.org> To: David Chisnall <theraven@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Andrey Chernov <ache@freebsd.org> Subject: Re: svn commit: r241373 - head/lib/libc/stdlib Message-ID: <CAF6rxgnR%2Bqr0jEqKtSBKcE0oAESrwGKQED%2BN7LJSfUeuGryRYw@mail.gmail.com> In-Reply-To: <977E1107-46D4-476F-A04D-AEFD87D1DE53@FreeBSD.org> References: <201210091425.q99EPFS6020787@svn.freebsd.org> <507451DE.9060909@freebsd.org> <977E1107-46D4-476F-A04D-AEFD87D1DE53@FreeBSD.org>
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On 9 October 2012 13:16, David Chisnall <theraven@freebsd.org> wrote: > On 9 Oct 2012, at 17:33, Andrey Chernov wrote: > >> Do you check assembler output for _both_ cases? >> In my testing clang and gcc xor's 'junk' properly in case it have >> 'volatile' keyword (as in srandomdev()) and elide it without 'volatile'. >> IMHO this change should be backed out for srandomdev() and adding >> 'volatile' for sranddev() instead. > > In it's original form, it is very dangerous - the whole expression reduce= s to undefined and so the LLVM IR for the call is: > > call void @srand(i32 undef) > > The back end is then free to use any value for the call argument, includi= ng any register value or 0. In fact, the backend is free to jump to a random location and potentially kill kittens. There is *no* guarantee when it comes to undefined behavior. > Adding the volatile means that we are doing an XOR with a value left on t= he stack. If this is early on in the application, then it is most likely t= o be 0. If it's later on, then there may be a value here, but it's still n= ot very likely to be something particularly unpredictable. volatile only helps by mistake because clang is overly aggressive is turning off optimizers. The code is still undefined. --=20 Eitan Adler Source & Ports committer X11, Bugbusting teams
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