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. David _______________________________________________ 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"
