Re: What does "return" keyword mean in INFO_TABLE_RET declarations?

20 Mar 2018

      I think this may be my bad. Both StgMiscClosures.cmm and Updates.cmm have this
line in the header:

    This file is written in a subset of C--, extended with various
    features specific to GHC.  It is compiled by GHC directly.  For the
    syntax of .cmm files, see the parser in ghc/compiler/cmm/CmmParse.y.

and CmmParse.y explains INFO_TABLE_RET:

    Stack Frames
    ------------

    A stack frame is written like this:

    INFO_TABLE_RET ( label, FRAME_TYPE, info_ptr, field1, ..., fieldN )
                   return ( arg1, ..., argM )
    {
      ... code ...
    }

    where field1 ... fieldN are the fields of the stack frame (with types)
    arg1...argN are the values returned to the stack frame (with types).
    The return values are assumed to be passed according to the
    NativeReturn convention.

    ...

It's just that sometimes it's not easy to find your way in a 880kloc code base.

Sorry for the noise,

Ömer

2018-03-20 12:57 GMT+03:00 Simon Peyton Jones via ghc-devs
:
...
It’s fine where it is, provided it takes the form of
Note [Stack frames]
and that Note is referred to from relevant places elsewhere.  E.g. Omer
didn’t find it.   One plausible place to point to it is the very definition
site of INFO_TABLE_RET, wherever that is.
Simon
From: ghc-devs  On Behalf Of Simon Marlow
Sent: 19 March 2018 18:50
To: Rahul Muttineni 
Cc: ghc-devs 
Subject: Re: What does "return" keyword mean in INFO_TABLE_RET declarations?
On 19 March 2018 at 00:53, Rahul Muttineni  wrote:
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).
This is correct.  The "documentation" for this is in the CmmParse.y module:
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/cmm/Cmm...
It wouldn't hurt to move all that to the wiki and leave a link behind, if
anyone wants to do that.
Cheers
Simon
Hope that helps,
Rahul
On Sun, Mar 18, 2018 at 8:18 PM, Ömer Sinan Ağacan 
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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