What does "return" keyword mean in INFO_TABLE_RET declarations?

[Simon Peyton Jones]

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 <ghc-devs-bounces at haskell.org> On Behalf Of Simon Marlow
Sent: 19 March 2018 18:50
To: Rahul Muttineni <rahulmutt at gmail.com>
Cc: ghc-devs <ghc-devs at haskell.org>
Subject: Re: What does "return" keyword mean in INFO_TABLE_RET declarations?

On 19 March 2018 at 00:53, Rahul Muttineni <rahulmutt at gmail.com<mailto:rahulmutt at gmail.com>> 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/CmmParse.y;b3b394b44e42f19ab7c23668a4008e4f728b51ba$151-165
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 <omeragacan at gmail.com<mailto: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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