Author: James Robertson
Date: 17:59:48 01/14/02
Go up one level in this thread
On January 14, 2002 at 18:48:32, Ed Schröder wrote:
>On January 14, 2002 at 17:28:25, James Robertson wrote:
>
>>On January 14, 2002 at 13:31:39, Ed Schröder wrote:
>>
>>>On January 14, 2002 at 12:11:52, James Robertson wrote:
>>>
>>>>Here are several questions I've been wondering about:
>>>>
>>>>1) How does one insert inline assembler statements into your code such that it
>>>>can be compiled by gcc (obviously __asm doesn't work)?
>>>
>>>James, welcome back.
>>
>>Thanks. =) I hope to post here periodically as my school schedule allows.
>>
>>>
>>>
>>>>2) How is switch() code generated? For instance, if the compiler encounters:
>>>>switch (var) {
>>>>case 0: do_stuff0(); break;
>>>>case 1: do_stuff1(); break;
>>>>case 2: do_stuff2(); break;
>>>}
>>>
>>>A good compiler will make use of the "indirect call", that is it will generate a
>>>table, store the memory address of the functions, and call the right routine
>>>from the generated table. The advantage, it takes 1 instruction.
>>>
>>>
>>>>will more efficient code be generated than if I write:
>>>>switch (var) {
>>>>case 297: do_stuff287; break;
>>>>case 0: do_stuff0; break;
>>>>case 9000: do_stuff9000; break;
>>>>}
>>>>
>>>>If so, what does the compiler do to take advantage of the fact that 0, 1, and 2
>>>>are consecutive numbers?
>>>
>>>See above.
>>>
>>>If you are using values that are in a reasonable range then create the table
>>>yourself. I recently posted an example of that for generating moves, you will
>>>have to search for that in the archive.
>>>
>>>Ed
>>
>>Ok, this looks interesting. Assuming I create this table that contains locations
>>of other pieces of code (not other functions), then how would I be able to jump
>>to these other locations in the code based on some sort of array reference? I
>>don't think I'm familiar with the syntax. I'll see what I can do about searching
>>for stuff in the archive, but that might take more time than I can spare.
>>
>>Thanks for you help. =)
>
>It works like this, instead of doing a "switch / case",
>
>switch (var) {
>case 0: do_stuff0(); break;
>case 1: do_stuff1(); break;
>case 2: do_stuff2(); break;
>}
>
>you do this,
>
>void (*jump[])() = { do_stuff0, do_stuff1, do_stuff2 };
>
> (jump[var])();
>
>That is actually what any good compiler will do in this case.
>
>Ed
Cool. Ok, now suppose that instead of calling funcions, the different cases
merely contain code? E.g.
switch (var) {
case 0: int x = 0; break;
case 1: do_stuff1(); break;
case 2: if (something) do_something_else(); break;
}
James
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