On Tue, Oct 9, 2012 at 10:16 AM, 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. Since the value is passed in a register, it will > probably just use whatever the last value there is, which may or may not be > anything sensible. On MIPS, for example, this is most likely to be &tv, and > so is 100% deterministic. > > 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. >
The original behavior can be recovered by using inline assembly to fetch the value from a register into a local C variable; this would at least not rely on undefined behavior. But I agree it's of dubious value anyways. _______________________________________________ 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"
