What does "return" keyword mean in INFO_TABLE_RET declarations?

[Simon Marlow]

On 19 March 2018 at 00:53, Rahul Muttineni <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>
> 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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