"Zack Weinberg" <[EMAIL PROTECTED]> writes: > Back in 2000 I wrote a really simple patch that caused gcc to treat an > ASM_OPERANDS that clobbered "pc" as a control flow barrier, exactly > for this problem. > > http://gcc.gnu.org/ml/gcc-patches/2000-01/msg00190.html > > I still think it was a good idea, but at the time it was received > unenthusiastically. I especially think "just use __builtin_trap()" > misses the mark - at least some variants of the Linux kernel's BUG() > macro, for instance, want to stick annotations in the assembly stream, > which you cannot do with __builtin_trap...
I think we should just introduce __builtin_unreachable(), as others have suggested. I think that is more consonant with our usual approach to these issues. I also think it would be good to have one option affecting it: turn __builtin_unreachable() into an abort(), or turn it into a "cannot be reached" marker. I think the former should be the default at -O0, the latter at -O1 and above. Note that a "cannot be reached" marker permits further optimization to delete code which leads up to it. Actually implementing that further optimization may call for adding appropriate warnings, probably controlled by -Wunreachable-code. Ian
