It'd be good to document this clearly somewhere; and explain how it is used.
So that the next time someone wonders, they don't have to reproduce Omer's
journey
Simon
| -----Original Message-----
| From: ghc-devs <ghc-devs-boun...@haskell.org> On Behalf Of Ömer Sinan
| Agacan
| Sent: 19 March 2018 10:03
| To: Rahul Muttineni <rahulm...@gmail.com>
| Cc: ghc-devs <ghc-devs@haskell.org>
| Subject: Re: What does "return" keyword mean in INFO_TABLE_RET
| declarations?
|
| Hi Rahul,
|
| Thanks, that is really helpful.
|
| So my intuition was correct. I think the naming here is a bit
| unfortunate because unless you're already familiar with Cmm, when you
| see this:
|
| INFO_TABLE_RET ( stg_ret_p, RET_SMALL, W_ info_ptr, P_ ptr )
| return (/* no return values */)
| {
| return (ptr);
| }
|
| you will be _very_ confused.
|
| Ömer
|
| 2018-03-19 3:53 GMT+03:00 Rahul Muttineni <rahulm...@gmail.com>:
| > 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).
| >
| > Hope that helps,
| > Rahul
| >
| > On Sun, Mar 18, 2018 at 8:18 PM, Ömer Sinan Ağacan
| > <omeraga...@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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