On 08 Feb 2008 18:04:23 -0800, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: > > Look at expand_case in gcc/stmt.c. > Thanks. So I did that, and that explains why my example with 4 cases does not use a table. gcc/expr.c says: /* If the machine does not have a case insn that compares the bounds, this means extra overhead for dispatch tables, which raises the threshold for using them. */ #ifndef CASE_VALUES_THRESHOLD #define CASE_VALUES_THRESHOLD (HAVE_casesi ? 4 : 5) #endif /* CASE_VALUES_THRESHOLD */ and a binary search is used if #arms < CASE_VALUES_THRESHOLD. This leads to my next question. I increased the number of arms to 5, and retained the "default: gcc_unreachable()". Now gcc generates a bounds check, followed by a table jump. Good. Now how do I get rid of the bounds checks? Is there a way to inform gcc that the default branch is never taken? (Something along the lines of MSVC's __assert(0)? I had thought gcc_unreachable() would do that - but I may be misinformed here.) Thanks, - Godmar
