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@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
_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

Rahul Muttineni