"Godmar Back" <godmar@xxxxxxxxx> writes: > I have a question regarding the compilation of switch statements for > i386 targets. What heuristics or algorithm does gcc use to decide > whether to use a binary search or a jump table? I'm specifically > interested in switch statements for a dense range of values and in > which the default: branch is unreachable. Look at expand_case in gcc/stmt.c. > I've tried a number of approaches in order to obtain a jump table, but > it seems gcc 4.x always ends up creating binary searches. For > instance, I've tried casting the switch value to a limited range enum > and placing a gcc_unreachable() in the default: case. A cursory > inspection of the source code also didn't reveal any documentation of > the strategy used. Does gcc have reason to believe that a binary > search is always faster? No, it uses a heuristic to choose. Probably the most relevant one here is that if the difference between the maximum and minimum case labels is more than 10 * the number of case labels, gcc will use comparisons rather than a table. > If so, is this true for all variants of i386? I believe that all i386 variants are handled in the same way. > Would it depend on the number of case arms? Yes. Ian
