http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58359
--- Comment #11 from Marc Glisse <glisse at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #10) > That is still wrong, __builtin_unreachable is still very much useful even at > the RTL level (where we expand it as basic blocks without successors). > Perhaps if-conversion (for LOOP_VERSIONED loop only) can just drop the > __builtin_unreachable () from the to be vectorized loop? It isn't just vectorization, or even just loop optimizations that are hindered by spurious control flow. Most of the __builtin_unreachable I see are of the form: if(x<0)__builtin_unreachable(); (often hidden below a macro called ASSUME or a similar name) For those, it is tempting to say that replacing the GIMPLE_COND, leading to a block that contains only a call to __builtin_unreachable, with an ASSERT_EXPR, or range information on this SSA_NAME (now that we store it), or anything without control flow, wouldn't lose any information. In reality, it would still lose it sometimes, but I don't know how often/bad that is. Just thinking of lowering *some* of the calls earlier... (though it wouldn't help with comment #2, where tree-tailcall.c should probably be taught about __builtin_unreachable)