We have: * wiki: https://gitlab.haskell.org/ghc/ghc/wikis/commentary/rts/storage/gc/cafs * a huge Note in CmmBuildInfoTables: https://gitlab.haskell.org/ghc/ghc/blob/master/compiler%2Fcmm%2FCmmBuildInfoTables.hs#L42
Maybe we need links to these from other places? Omer's question is referring specifically to the [FUN] optimisation described in the Note. Cheers Simon On Mon, 6 Jan 2020 at 17:50, Simon Peyton Jones <simo...@microsoft.com> wrote: > Omer, > > > > I think I’m not understanding all the details, but I have a clear “big > picture”. Simon can correct me if I’m wrong. > > > > · The *info table* for any *closure* (top-level or otherwise) has > a (possibly empty) Static Reference Table, *SRT*. > > · The SRT for an info table identifies the static top level > closures that the *code* for that info table mentions. (In principle > the garbage collector could parse the code! But it’s easier to find these > references if they in a dedicated table alongside the code.) > > · A top level closure is a *CAF* if it is born updatable. > > · A top level closure is *CAFFY* if it is a CAF, or mentions > another CAFFY closure. > > · An entry in the SRT can point > > o To a top-level updatable closure. This may now point into the dynamic > heap, and is what we want to keep alive. If the closure hasn’t been > updated, we should keep alive anything its SRT points to. > > o Directly to another SRT (or info table?) for a CAFFY top-level > closure, which is a bit faster if we know the thing is non-updatable. > > · If a function f calls a top-level function g, and g is CAFFY, > then f’s SRT should point to g’s closure or (if g is not a CAF) directly to > its SRT. > > · If f is top level, and calls itself, there is no need to include > a pointer to f’s closure in f’s own SRT. > > I think this last point is the one you are asking, but I’m not certain. > > All this should be written down somewhere, and perhaps is. But where? > > Simon > > > > *From:* ghc-devs <ghc-devs-boun...@haskell.org> *On Behalf Of *Simon > Marlow > *Sent:* 06 January 2020 08:17 > *To:* Ömer Sinan Ağacan <omeraga...@gmail.com> > *Cc:* ghc-devs <ghc-devs@haskell.org> > *Subject:* Re: Code generation/SRT question > > > > There's no need to set the srt field of f_info if f_closure is the SRT, > since any reference to f_info in the code will give rise to a reference to > f_closure in the SRT corresponding to that code fragment. Does that make > sense? > > > > The use of a closure as an SRT is really quite a nice optimisation > actually. > > > > Cheers > > Simon > > > > On Wed, 1 Jan 2020 at 09:35, Ömer Sinan Ağacan <omeraga...@gmail.com> > wrote: > > Hi Simon, > > In Cmm if I have a recursive group of functions f and g, and I'm using f's > closure as the SRT for this group, should f's entry block's info table have > f_closure as its SRT? > > In Cmm syntax > > f_entry() { > { info_tbls: [... > (c1vn, > label: ... > rep: ... > srt: ??????] > stack_info: ... > } > {offset > c1vn: > ... > } > } > > Here should I have `f_closure` in the srt field? > > I'd expect yes, but looking at the current SRT code, in > CmmBuildInfoTables.updInfoSRTs, we have this: > > (newInfo, srtEntries) = case mapLookup (g_entry g) funSRTEnv of > > Nothing -> > -- if we don't add SRT entries to this closure, then we > -- want to set the srt field in its info table as usual > (info_tbl { cit_srt = mapLookup (g_entry g) srt_env }, []) > > Just srtEntries -> srtTrace "maybeStaticFun" (ppr res) > (info_tbl { cit_rep = new_rep }, res) > where res = [ CmmLabel lbl | SRTEntry lbl <- srtEntries ] > > Here we only update SRT field of the block if we're not adding SRT entries > to > the function's closure, so in the example above, because we're using the > function as SRT (and adding SRT entries to its closure) SRT field of c1vn > won't > be updated. > > Am I missing anything? > > Thanks, > > Ömer > >
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