switches in GCC JIT?

[David Malcolm]

On Mon, 2015-06-22 at 20:25 -0400, David Malcolm wrote:
> On Tue, 2015-06-23 at 00:02 +0200, Basile Starynkevitch wrote:
> > 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.
> 
> As it happens, none of the bytecode languages I've implemented so far
> have switch-like constructs.
> 
> But I see now that the JVM has opcodes "lookupswitch" and "tableswitch";
> those alone make a compelling case for libgccjit supporting switches.
"those by themselves", I should say.