On 9 October 2012 13:16, David Chisnall <thera...@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 reduces 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, including > 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 the > stack. If this is early on in the application, then it is most likely to be > 0. If it's later on, then there may be a value here, but it's still not 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. -- Eitan Adler Source & Ports committer X11, Bugbusting teams _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
