op 27-10-13 18:28, Linus Torvalds schreef: > On Sat, Oct 26, 2013 at 5:19 AM, Ingo Molnar <[email protected]> wrote: >> This tree fixes a boot crash in CONFIG_DEBUG_MUTEXES=y kernels, on >> kernels built with GCC 3.x. (There are still such distros.) > Btw, it's really not just gcc 3.x. That code was (a) incomprehensible, > (b) wrong and (c) caused problems for LLVM too. > > It was wrong because "__builtin_constant_p(ww_ctx == NULL)" simply > makes no sense. > > Why? > > That expression is largely equivalent to > "__builtin_constant_p(ww_ctx)" (because iff ww_ctx is constant, then > the comparison to NULL is constant), which is actually much easier to > read, while carrying a totally different semantic meaning. Making > things worse, the comparison to NULL *may* be marked constant under > some very random situations (ie the compiler could turn a "taking an > address of a variable is never NULL" kind of knowledge and combining > it with other knowledge, and turn a complicated "ctx" expression into > a "I know this cannot be NULL" thing, and thus the "== NULL" is a > constant, even though ctx itself is some dynamic calculation). > > Whoever wrote the original should be shot. And this commit shouldn't > have been marked as being somehow about gcc-version dependence, but > about removing completely crap code. > Unfortunately gcc disagreed there, which was another compiler bug. __builtin_constant_p(ww_ctx) was NOT equal to __builtin_constant_p(ww_ctx == NULL), iirc. __builtin_constant_p(ww_ctx == NULL) is equal to __builtin_constant_p(ww_ctx != NULL), but the former is more readable, since it shows we expect ww_ctx to be null.
But yeah I guess it was too broken in gcc after all, so that's why it had to be killed altogether. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
