What does "return" keyword mean in INFO_TABLE_RET declarations?
Simon Peyton Jones
simonpj at microsoft.com
Tue Mar 20 13:13:13 UTC 2018
It might help to
a) make the reference more specific, et
See Note [Stack frames] in CmmParse.y
b) put that citation close to the relevant definitions,
rather than at the head of the file.
It's not easy for authors to anticipate the path that others will follow later. But /you/ now know what you didn't understand, and what would have helped you.
Just add the info, in the places that would have meant you found it instantly. That'll save time for others.
Simon
| -----Original Message-----
| From: Ömer Sinan Ağacan <omeragacan at gmail.com>
| Sent: 20 March 2018 13:09
| To: Simon Peyton Jones <simonpj at microsoft.com>
| Cc: Simon Marlow <marlowsd at gmail.com>; Rahul Muttineni
| <rahulmutt at gmail.com>; ghc-devs <ghc-devs at haskell.org>
| Subject: Re: What does "return" keyword mean in INFO_TABLE_RET
| declarations?
|
| 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
| <ghc-devs at haskell.org>:
| > 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>
| 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/c
| > mm/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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