Missing optimisation opportunity in code gen for always-saturated applications?

As a part of the work on moving some fragile analyses from Core to final phase before code gen (#9718 (closed)) I found an unused analysis in CoreToStg which basically assigns each binder this:

data StgBinderInfo
  = NoStgBinderInfo
  | SatCallsOnly        -- All occurrences are *saturated* *function* calls
                        -- This means we don't need to build an info table and
                        -- slow entry code for the thing
                        -- Thunks never get this value

The idea is , as mentioned in the comment, for SatCallsOnly binder we can avoid generating code for slow entry. However this is currently not used (see D5232), and this ticket is to decide whether to use or remove this.

I found these comments in StgCmmClosure that explains how this became unused and how it was used before:

-----------------------------------------------------------------------------
--                staticClosureRequired
-----------------------------------------------------------------------------

{-  staticClosureRequired is never called (hence commented out)

    SimonMar writes (Sept 07) It's an optimisation we used to apply at
    one time, I believe, but it got lost probably in the rewrite of
    the RTS/code generator.  I left that code there to remind me to
    look into whether it was worth doing sometime

{- Avoiding generating entries and info tables
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At present, for every function we generate all of the following,
just in case.  But they aren't always all needed, as noted below:

[NB1: all of this applies only to *functions*.  Thunks always
have closure, info table, and entry code.]

[NB2: All are needed if the function is *exported*, just to play safe.]

* Fast-entry code  ALWAYS NEEDED

* Slow-entry code
        Needed iff (a) we have any un-saturated calls to the function
        OR         (b) the function is passed as an arg
        OR         (c) we're in the parallel world and the function has free vars
                       [Reason: in parallel world, we always enter functions
                       with free vars via the closure.]

* The function closure
        Needed iff (a) we have any un-saturated calls to the function
        OR         (b) the function is passed as an arg
        OR         (c) if the function has free vars (ie not top level)

  Why case (a) here?  Because if the arg-satis check fails,
  UpdatePAP stuffs a pointer to the function closure in the PAP.
  [Could be changed; UpdatePAP could stuff in a code ptr instead,
   but doesn't seem worth it.]

  [NB: these conditions imply that we might need the closure
  without the slow-entry code.  Here's how.

        f x y = let g w = ...x..y..w...
                in
                ...(g t)...

  Here we need a closure for g which contains x and y,
  but since the calls are all saturated we just jump to the
  fast entry point for g, with R1 pointing to the closure for g.]


* Standard info table
        Needed iff (a) we have any un-saturated calls to the function
        OR         (b) the function is passed as an arg
        OR         (c) the function has free vars (ie not top level)

        NB.  In the sequential world, (c) is only required so that the function closure has
        an info table to point to, to keep the storage manager happy.
        If (c) alone is true we could fake up an info table by choosing
        one of a standard family of info tables, whose entry code just
        bombs out.

        [NB In the parallel world (c) is needed regardless because
        we enter functions with free vars via the closure.]

        If (c) is retained, then we'll sometimes generate an info table
        (for storage mgr purposes) without slow-entry code.  Then we need
        to use an error label in the info table to substitute for the absent
        slow entry code.
-}

staticClosureRequired
        :: Name
        -> StgBinderInfo
        -> LambdaFormInfo
        -> Bool
staticClosureRequired binder bndr_info
                      (LFReEntrant top_level _ _ _ _)        -- It's a function
  = ASSERT( isTopLevel top_level )
        -- Assumption: it's a top-level, no-free-var binding
        not (satCallsOnly bndr_info)

staticClosureRequired binder other_binder_info other_lf_info = True
-}

Trac metadata

Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar, simonpj
Operating system
Architecture