switches in GCC JIT?

[Basile Starynkevitch]

On 06/22/2015 11:42 PM, David Malcolm wrote:
> On Mon, 2015-06-22 at 23:40 +0200, Basile Starynkevitch wrote:
>> Hello David & all
>>
>> I'm guessing that GCCJIT is able to emit switch like statements, e.g.
>> using GIMPLE_SWITCH statements
>> internally.
> No, it doesn't.
>
>> But I don't understand how is it possible. It looks like
>> gimple_build_switch does not occur in gcc/jit/
> Currently gcc/jit builds functions at the tree level and hands them off
> to the gimplifier, so you wouldn't see that in any case.  (in theory it
> could be ported to directly generate gimple).
>
>> Are switch statements omitted from GCCJIT?
> Yes.
>
>> If yes, why???
> I intentionally didn't implement them, to keep the API simpler.
>
> I've never run into a need for them when implementing jit-compilation,
> and in theory they could be implemented using conditionals (albeit
> without the nice optimizations that we have for lowering GIMPLE_SWITCH).
>
> There was some discussion about this here:
>   https://gcc.gnu.org/ml/jit/2014-q4/msg00116.html
>
>> David, do you intend to improve that?
> Do you have a use-case for them?  We can add them if we need them.

Any language (MELT, Ocaml, Haskell, ....) having some pattern matching 
would use a lot of switches.
Or most efficient implementations of Rete algorithm, or similar stuff 
when compiling Prolog-like or CLIPS-like rules.

Also, translation of most finite state automatons is done by a switch 
(often a quite big one, with one case per each state).

At last, any kind of "byte-code" interpreter uses switches.

And many languages have a switch like construct, that would be trivial 
to translate to a GIMPLE_SWITCH, but painful to translate otherwise.


All the bytecodes I know (e.g. JVM & Ocaml) have switch-like constructs, 
and it would be easy to translate them to a GIMPLE_SWITCH, and painful 
otherwise.

BTW, if we don't have switches, we should at least have indirect jumps, 
and the ability to retrieve, as a label, the starting address of any 
basic block. (i.e. the equivalent of goto *ptr; and of &&label in C). If 
that is possible today, we need more documentation about that (at least 
saying that switch statements could be translated that way)

And of course, leveraging on all the important optimizations done by GCC 
on GIMPLE_SWITCH is essential...

Actually, I'm surprised you are asking what is the use case for 
switches. I feel they are obvious and numerous... I would be annoyed, 
e.g. if I could not use switches in my C++ or C code. Replacing a switch 
with a sequence of if is an annoyance, and is probably a major 
performance loss (unless GCC optimizations are clever enough to replace 
them with a switch; which might sometimes be true, but not always).

Regards

-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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