switches in GCC JIT?

[Dibyendu Majumdar]

On 23 June 2015 at 01:25, David Malcolm <dmalcolm@redhat.com> wrote:
> As it happens, none of the bytecode languages I've implemented so far
> have switch-like constructs.
>

Often bytecodes in dynamic languages when translated do not exactly
map to C constructs, so this is not surprising I think.

> But I see now that the JVM has opcodes "lookupswitch" and "tableswitch";
> those alone make a compelling case for libgccjit supporting switches.
>
> I'm working on it now; the API I'm thinking of looks like this:
>
> extern void
> gcc_jit_block_end_with_switch (gcc_jit_block *block,
>                                   gcc_jit_location *loc,
>                                   gcc_jit_rvalue *expr,
>                                   gcc_jit_block *default_block,
>                                   int num_cases,
>                                   gcc_jit_rvalue **case_min_values,
>                                   gcc_jit_rvalue **case_max_values,
>                                   gcc_jit_block **case_blocks);
>
>
> thus supporting ranged cases, whilst also allowing individual values, by
> simply passing in the same table for both case_min_values and
> case_max_values.
>

The API looks ok to me but would it be more user friendly if you
created a struct to hold the case values and blocks together? That is:

struct case {
  gcc_jit_rvalue *min_value;
  gcc_jit_rvalue *max_value;
  gcc_jit_block *block;
};

Then the last three parameters could be replaced by 'struct case *'.

>> 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)
>>

It would also be useful to have the indirect jump capability - LLVM
offers this and I used it in Ravi to help with performance (although
in the particular use case I did not really see much benefit).

Regards
Dibyendu