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<p class="MsoNormal"><span style="font-size:12.0pt">It’s fine where it is, <b>provided</b> it takes the form of<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> Note [Stack frames]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 19 March 2018 18:50<br>
<b>To:</b> Rahul Muttineni <rahulmutt@gmail.com><br>
<b>Cc:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: What does "return" keyword mean in INFO_TABLE_RET declarations?<o:p></o:p></span></p>
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On 19 March 2018 at 00:53, Rahul Muttineni <<a href="mailto:rahulmutt@gmail.com" target="_blank">rahulmutt@gmail.com</a>> wrote:<o:p></o:p></p>
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Hi Omer,<o:p></o:p></p>
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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.
<o:p></o:p></p>
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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).<o:p></o:p></p>
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This is correct. The "documentation" for this is in the CmmParse.y module: <a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/cmm/CmmParse.y;b3b394b44e42f19ab7c23668a4008e4f728b51ba$151-165">
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/cmm/CmmParse.y;b3b394b44e42f19ab7c23668a4008e4f728b51ba$151-165</a><o:p></o:p></p>
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It wouldn't hurt to move all that to the wiki and leave a link behind, if anyone wants to do that.<o:p></o:p></p>
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Cheers<o:p></o:p></p>
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Simon<br>
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Hope that helps,<o:p></o:p></p>
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Rahul<o:p></o:p></p>
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On Sun, Mar 18, 2018 at 8:18 PM, Ömer Sinan Ağacan <<a href="mailto:omeragacan@gmail.com" target="_blank">omeragacan@gmail.com</a>> wrote:<o:p></o:p></p>
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Hi,<br>
<br>
I'm trying to understand what a "return" list in INFO_TABLE_RET declaration<br>
line specifies. As far as I understand a "return" in the declaration line is<br>
something different than a "return" in the body. For example, in this<br>
definition: (in HeapStackCheck.cmm)<br>
<br>
INFO_TABLE_RET ( stg_ret_p, RET_SMALL, W_ info_ptr, P_ ptr )<br>
return (/* no return values */)<br>
{<br>
return (ptr);<br>
}<br>
<br>
The return list is empty and it even says "no return values" explicitly, yet it<br>
returns something.<br>
<br>
My guess is that the "return" list in the header is actually for arguments. I<br>
found this info table which has an argument: (in StgMiscClosures.cmm)<br>
<br>
INFO_TABLE_RET (stg_restore_cccs_eval, RET_SMALL, W_ info_ptr, W_ cccs)<br>
return (P_ ret)<br>
{<br>
unwind Sp = Sp + WDS(2);<br>
#if defined(PROFILING)<br>
CCCS = cccs;<br>
#endif<br>
jump stg_ap_0_fast(ret);<br>
}<br>
<br>
This is the use site: (in Interpreter.c)<br>
<br>
#if defined(PROFILING)<br>
// restore the CCCS after evaluating the closure<br>
Sp_subW(2);<br>
SpW(1) = (W_)cap->r.rCCCS;<br>
SpW(0) = (W_)&stg_restore_cccs_eval_info;<br>
#endif<br>
Sp_subW(2);<br>
SpW(1) = (W_)tagged_obj;<br>
SpW(0) = (W_)&stg_enter_info;<br>
RETURN_TO_SCHEDULER_NO_PAUSE(ThreadRunGHC, ThreadYielding);<br>
<br>
If I understand this correctly, the "tagged_obj" code will put the return value<br>
in R1, pop the stack (which will have stg_restore_ccs_eval_info at the bottom)<br>
and jump to this the info table code shown above. So `P_ ret` is the value of<br>
`tagged_obj`, and the "return" list is actually for parameters.<br>
<br>
Did I get this right? If I did, I'm curious why it's called "return" and not<br>
"args" or something like that.<br>
<br>
Thanks,<br>
<br>
Ömer<br>
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<br>
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<o:p></o:p></p>
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<span class="gmail-hoenzb"><span style="color:#888888">-- <o:p></o:p></span></span></p>
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<span style="color:#888888">Rahul Muttineni</span><o:p></o:p></p>
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<br>
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<o:p> </o:p></p>
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