Transparently hooking into the STG stack to validate an escape analysis
Csaba Hruska
csaba.hruska at gmail.com
Wed Dec 15 15:16:27 UTC 2021
Hi,
IMO the Cmm STG machine implementation is just too complex for student
projects. It's not fun to work with at all.
Why did you choose this approach?
IMO the escape analysis development and validation would be much smoother
and fun when you'd use the external STG interpreter.
When you have a solid and working design of your analysis and
transformations then you could implement it in GHC's native backend if it
needs any changes at all.
What do you think?
Do you disagree?
Have you seen my presentation about the stg interpreter?
https://www.youtube.com/watch?v=Ey5OFPkxF_w
Cheers,
Csaba
On Wed, Dec 8, 2021 at 11:20 AM Sebastian Graf <sgraf1337 at gmail.com> wrote:
> Hi Devs,
>
> my master's student Sebastian and I (also Sebastian :)) are working on an
> escape analysis in STG, see
> https://gitlab.haskell.org/ghc/ghc/-/issues/16891#note_347903.
>
> We have a prototype for the escape analysis that we want to
> validate/exploit now.
> The original plan was to write the transformation that allocates
> non-escaping objects on the STG stack. But that is quite tricky for many
> reasons, one of them being treatment of the GC.
>
> This mail is rather lengthy, so I suggest you skip to "What we hope could
> work" and see if you can answer it without the context I provide below. If
> you can't, I would be very grateful if you were willing to suffer through
> the exposition.
>
> # Instrumentation
>
> So instead we thought about doing a (easily changed and thus versatile)
> instrumentation-based approach:
> Assign a sequence number to every instantiation (a term I will use to
> mean "allocation of g's closure") of things that we our analysis
> determines as escaping or non-escaping, such as STG's let bindings
> (focusing on non-let-no-escape functions for now).
> (One sequence number *per allocation* of the let binding's closure, not
> based on its syntactic identity.)
> Then, we set a bit in a (dynamically growing) global bit vector whenever
> the let "RHS is entered" and then unset it when we "leave the let-body".
> Example:
>
> f = \x y ->
> let {
> g = [y] \z -> y + z;
> } in g x
>
> Here, our analysis could see that no instantiation (which I say instead of
> "allocation of g's closure") of g will ever escape its scope within f.
> Our validation would give a fresh sequence number to the instantiation of
> g whenever f is called and store it in g's closure (which we arrange by
> piggy-backing on -prof and adding an additional field to the profiling
> header).
> Then, when g's RHS is entered, we set the bit in the global bit vector,
> indicating "this instantiation of g might escape".
> After leaving the RHS of g, we also leave the body of the defining let,
> which means we unset the bit in the bit vector, meaning "every use so far
> wasn't in an escaping scenario".
>
> So far so good. Modifying the code upon entering g takes a bit of
> tinkering but can be done by building on TickyTicky in StgToCmm.
> But what is not done so easily is inserting the continuation upon entering
> the let that will unset the bit!
>
> # What doesn't work: Modifying the Sequel
>
> At first, we tried to modify the sequel
> <https://gitlab.haskell.org/ghc/ghc/-/blob/4c6985cc91b9cf89b393f69c9a603e8df0aea9e0/compiler/GHC/StgToCmm/Monad.hs#L210-222> of
> the let-body to an `AssignTo`.
> That requires us to know the registers in which the let-body will return
> its results, which in turn means we have to know the representation of
> those results, so we have to write a function `stgExprPrimRep :: GenStgExpr
> p -> [PrimRep]`.
> Urgh! We were very surprised that there was no such function. And while we
> tested our function, we very soon knew why. Consider the following pattern
> synonym matcher:
>
> GHC.Natural.$mNatJ#
> :: forall {rep :: GHC.Types.RuntimeRep} {r :: TYPE rep}.
> GHC.Num.Natural.Natural
> -> (GHC.Num.BigNat.BigNat -> r) -> ((# #) -> r) -> r
> = {} \r [scrut_sBE cont_sBF fail_sBG]
> case scrut_sBE of {
> GHC.Num.Natural.NS _ -> fail_sBG GHC.Prim.(##);
> GHC.Num.Natural.NB ds_sBJ ->
> let {
> sat_sBK :: GHC.Num.BigNat.BigNat
> = CCCS GHC.Num.BigNat.BN#! [ds_sBJ];
> } in cont_sBF sat_sBK;
> };
>
> Note how its result is representation-polymorphic! It only works because
> our machine implementation allows tail-calls.
> It's obvious in hindsight that we could never write `stgExprPrimRep` in
> such a way that it will work on the expression `cont_sBF sat_sBK`.
> So the sequel approach seems infeasible.
>
> # What we hope could work: A special stack frame
>
> The other alternative would be to insert a special continuation frame on
> the stack when we enter the let-body (inspired by stg_restore_cccs).
> This continuation frame would simply push all registers (FP regs, GP regs,
> Vec regs, ...) to the C stack, do its work (unsetting the bit), then pop
> all registers again and jump to the topmost continuation on the STG stack.
> Example:
>
> f :: forall rep (r :: TYPE rep). Int# -> (Int# -> r) -> r
> f = \x g ->
> let {
> h = [x] \a -> x + a;
> } in
> case h x of b {
> __DEFAULT -> g b
> }
>
> We are only interested in unsetting the bit for h here. Consider the stack
> when entering the body of h.
>
> caller_of_f_cont_info <- Sp
>
> Now push our special continuation frame:
>
> caller_of_f_cont_info
> seq_h
> unset_bit_stk_info <- Sp
>
> E.g., the stack frame contains the info pointer and the sequence number.
> (Btw., I hope I got the stack layout about right and this is even possible)
> Then, after we entered the continuation of the __DEFAULT alt, we do a jump
> to g.
> Plot twist: g returns an unboxed 8-tuple of `Int#`s (as caller_of_f_cont_info
> knows, but f certainly doesn't!), so before it returns it will push two
> args on the stack (correct?):
>
> caller_of_f_cont_info
> seq_h
> unset_bit_stk_info
> unboxed tuple component 7
> unboxed tuple component 8 <- Sp
>
> And then `g` jumps directly to the entry code for `unset_bit_stk_info`
> (which does the register saving I mentioned), which absolutely can't figure
> out from Sp alone where seq_h is.
> Darn! I think Luite's recent work on the StgToByteCode had to work around
> similar issues, I found this wiki page
> <https://gitlab.haskell.org/ghc/ghc-wiki-mirror/-/blob/master/Unboxed-Tuples-and-Sums-in-GHCi-bytecode.md#returning-a-tuple>.
> But we aren't in a position where we know the representation of `r` *at
> all*!
>
> So our idea was to scan the stack, beginning from `Sp`, until I find
> `unset_bit_stk_info`, giving us the following situation:
>
> caller_of_f_cont_info
> seq_h
> unset_bit_stk_info <- Bp
> unboxed tuple component 7
> unboxed tuple component 8 <- Sp
>
> I suggestively named the register in which we store the result Bp in
> analogy to the traditional base pointer. This information would be enough
> to unset the bit at index seq_h
> and then copy the unboxed tuple components 7 and 8 up by two words:
>
> caller_of_f_cont_info
> unboxed tuple component 7
> unboxed tuple component 8 <- Sp
>
> Then jump to caller_of_f_cont_info, which knows what to make of the stack
> and the register state.
>
> The stack scanning is horrible and too brittle and slow for a production
> setting, as any of the unboxed tuple components could have the same bit
> pattern as `unset_bit_stk_info`.
> We simply hope that is never the case and it's fine for the purposes of a
> quick-and-dirty instrumentation.
>
> QUESTION 1: What do you think? Could this work? Can you anticipate pit
> falls of this approach?
>
> # What about Thunks and StgRhsCon?
>
> QUESTION 2: The instrumentation as described won't work for Thunks (which
> are only entered once) and constructor applications (like sat_sBK in the
> BigNat matcher above). Not sure how to change that without incurring huge
> performance hits (by deactivating memoisation). Ideas are welcome here.
>
> Thanks for reading this far. I hope you could follow and are now fizzing
> with excitement because you have a much better idea of how to do this and
> will let me know :)
>
> Sebastian
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