What does "return" keyword mean in INFO_TABLE_RET declarations?

[Rahul Muttineni]

Hi Omer,

An INFO_TABLE_RET is a frame that "can be returned to" and the return
keyword allows you to provide a name for the value(s) that was(were)
returned to this frame and do something with it if you wish. If you didn't
have this keyword, you would have to do low-level stack manipulations
yourself to get a handle on the return value and it's easy to mess up.

You can think of INFO_TABLE_RET as a traditional stack frame in languages
like C, except it's powerful because you can specify custom logic on how
you deal with the returned value. In some cases, like stg_atomically_frame,
you may not even return the value further down into the stack until certain
conditions are met (the transaction is valid).

Hope that helps,
Rahul

On Sun, Mar 18, 2018 at 8:18 PM, Ömer Sinan Ağacan <omeragacan at gmail.com>
wrote:

> Hi,
>
> I'm trying to understand what a "return" list in INFO_TABLE_RET declaration
> line specifies. As far as I understand a "return" in the declaration line
> is
> something different than a "return" in the body. For example, in this
> definition: (in HeapStackCheck.cmm)
>
>     INFO_TABLE_RET ( stg_ret_p, RET_SMALL, W_ info_ptr, P_ ptr )
>         return (/* no return values */)
>     {
>         return (ptr);
>     }
>
> The return list is empty and it even says "no return values" explicitly,
> yet it
> returns something.
>
> My guess is that the "return" list in the header is actually for
> arguments. I
> found this info table which has an argument: (in StgMiscClosures.cmm)
>
>     INFO_TABLE_RET (stg_restore_cccs_eval, RET_SMALL, W_ info_ptr, W_ cccs)
>         return (P_ ret)
>     {
>         unwind Sp = Sp + WDS(2);
>     #if defined(PROFILING)
>         CCCS = cccs;
>     #endif
>         jump stg_ap_0_fast(ret);
>     }
>
> This is the use site: (in Interpreter.c)
>
>     #if defined(PROFILING)
>         // restore the CCCS after evaluating the closure
>         Sp_subW(2);
>         SpW(1) = (W_)cap->r.rCCCS;
>         SpW(0) = (W_)&stg_restore_cccs_eval_info;
>     #endif
>         Sp_subW(2);
>         SpW(1) = (W_)tagged_obj;
>         SpW(0) = (W_)&stg_enter_info;
>         RETURN_TO_SCHEDULER_NO_PAUSE(ThreadRunGHC, ThreadYielding);
>
> If I understand this correctly, the "tagged_obj" code will put the return
> value
> in R1, pop the stack (which will have stg_restore_ccs_eval_info at the
> bottom)
> and jump to this the info table code shown above. So `P_ ret` is the value
> of
> `tagged_obj`, and the "return" list is actually for parameters.
>
> Did I get this right? If I did, I'm curious why it's called "return" and
> not
> "args" or something like that.
>
> Thanks,
>
> Ömer
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-- 
Rahul Muttineni
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