% (c) The University of Glasgow 2006

\section[CprAnalyse]{Identify functions that always return a
constructed product result}

\begin{code}
#ifndef OLD_STRICTNESS
module CprAnalyse ( ) where

#else

module CprAnalyse ( cprAnalyse ) where

#include "HsVersions.h"

import DynFlags
import CoreLint
import CoreSyn
import CoreUtils
import Id
import IdInfo
import Demand
import VarEnv
import Util
import Outputable

import Maybe
\end{code}

This module performs an analysis of a set of Core Bindings for the
Constructed Product Result (CPR) transformation.

It detects functions that always explicitly (manifestly?) construct a
result value with a product type.  A product type is a type which has
only one constructor. For example, tuples and boxed primitive values
have product type.

We must also ensure that the function's body starts with sufficient
manifest lambdas otherwise loss of sharing can occur.  See the comment
in @StrictAnal.lhs@.

The transformation of bindings to worker/wrapper pairs is done by the
worker-wrapper pass.  The worker-wrapper pass splits bindings on the
basis of both strictness and CPR info.  If an id has both then it can
combine the transformations so that only one pair is produced.

The analysis here detects nested CPR information.  For example, if a
function returns a constructed pair, the first element of which is a
constructed int, then the analysis will detect nested CPR information
for the int as well.  Unfortunately, the current transformations can't
take advantage of the nested CPR information.  They have (broken now,
I think) code which will flatten out nested CPR components and rebuild
them in the wrapper, but enabling this would lose laziness.  It is
possible to make use of the nested info: if we knew that a caller was
strict in that position then we could create a specialized version of
the function which flattened/reconstructed that position.

It is not known whether this optimisation would be worthwhile.

So we generate and carry round nested CPR information, but before
using this info to guide the creation of workers and wrappers we map
all components of a CPRInfo to NoCprInfo.


Data types
~~~~~~~~~~

Within this module Id's CPR information is represented by
``AbsVal''. When adding this information to the Id's pragma info field
we convert the ``Absval'' to a ``CprInfo'' value.

Abstract domains consist of a `no information' value (Top), a function
value (Fun) which when applied to an argument returns a new AbsVal
(note the argument is not used in any way), , for product types, a
corresponding length tuple (Tuple) of abstract values.  And finally,
Bot.  Bot is not a proper abstract value but a generic bottom is
useful for calculating fixpoints and representing divergent
computations.  Note that we equate Bot and Fun^n Bot (n > 0), and
likewise for Top.  This saves a lot of delving in types to keep
everything exactly correct.

Since functions abstract to constant functions we could just
represent them by the abstract value of their result.  However,  it
turns out (I know - I tried!) that this requires a lot of type
manipulation and the code is more straightforward if we represent
functions by an abstract constant function.

\begin{code}
data AbsVal = Top                -- Not a constructed product

            | Fun AbsVal         -- A function that takes an argument
                                 -- and gives AbsVal as result.

            | Tuple              -- A constructed product of values

            | Bot                -- Bot'tom included for convenience
                                 -- we could use appropriate Tuple Vals
     deriving (Eq,Show)

-- For pretty debugging
instance Outputable AbsVal where
  ppr Top       = ptext (sLit "Top")
  ppr (Fun r)   = ptext (sLit "Fun->") <> (parens.ppr) r
  ppr Tuple     = ptext (sLit "Tuple ")
  ppr Bot       = ptext (sLit "Bot")


-- lub takes the lowest upper bound of two abstract values, standard.
lub :: AbsVal -> AbsVal -> AbsVal
lub Bot a = a
lub a Bot = a
lub Top a = Top
lub a Top = Top
lub Tuple Tuple         = Tuple
lub (Fun l) (Fun r)     = Fun (lub l r)
lub l r = panic "CPR Analysis tried to take the lub of a function and a tuple"


\end{code}

The environment maps Ids to their abstract CPR value.

\begin{code}

type CPREnv = VarEnv AbsVal

initCPREnv = emptyVarEnv

\end{code}

Programs
~~~~~~~~

Take a list of core bindings and return a new list with CPR function
ids decorated with their CprInfo pragmas.

\begin{code}

cprAnalyse :: DynFlags -> [CoreBind] -> IO [CoreBind]
cprAnalyse dflags binds
  = do {
        showPass dflags "Constructed Product analysis" ;
        let { binds_plus_cpr = do_prog binds } ;
        endPass dflags "Constructed Product analysis"
                Opt_D_dump_cpranal binds_plus_cpr
    }
  where
    do_prog :: [CoreBind] -> [CoreBind]
    do_prog binds = snd $ mapAccumL cprAnalBind initCPREnv binds
\end{code}

The cprAnal functions take binds/expressions and an environment which
gives CPR info for visible ids and returns a new bind/expression
with ids decorated with their CPR info.

\begin{code}
-- Return environment extended with info from this binding
cprAnalBind :: CPREnv -> CoreBind -> (CPREnv, CoreBind)
cprAnalBind rho (NonRec b e)
  | isImplicitId b      -- Don't touch the CPR info on constructors, selectors etc
  = (rho, NonRec b e)
  | otherwise
  = (extendVarEnv rho b absval, NonRec b' e')
  where
    (e', absval) = cprAnalExpr rho e
    b' = addIdCprInfo b e' absval

cprAnalBind rho (Rec prs)
  = (final_rho, Rec (map do_pr prs))
  where
    do_pr (b,e) = (b', e')
                where
                  b'           = addIdCprInfo b e' absval
                  (e', absval) = cprAnalExpr final_rho e

        -- When analyzing mutually recursive bindings the iterations to find
        -- a fixpoint is bounded by the number of bindings in the group.
        -- for simplicity we just iterate that number of times.
    final_rho = nTimes (length prs) do_one_pass init_rho
    init_rho  = rho `extendVarEnvList` [(b,Bot) | (b,e) <- prs]

    do_one_pass :: CPREnv -> CPREnv
    do_one_pass rho = foldl (\ rho (b,e) -> extendVarEnv rho b (snd (cprAnalExpr rho e)))
                            rho prs


cprAnalExpr :: CPREnv -> CoreExpr -> (CoreExpr, AbsVal)

-- If Id will always diverge when given sufficient arguments then
-- we can just set its abs val to Bot.  Any other CPR info
-- from other paths will then dominate,  which is what we want.
-- Check in rho,  if not there it must be imported, so check
-- the var's idinfo.
cprAnalExpr rho e@(Var v)
    | isBottomingId v = (e, Bot)
    | otherwise       = (e, case lookupVarEnv rho v of
                             Just a_val -> a_val
                             Nothing    -> getCprAbsVal v)

-- Literals are unboxed
cprAnalExpr rho (Lit l) = (Lit l, Top)

-- For apps we don't care about the argument's abs val.  This
-- app will return a constructed product if the function does. We strip
-- a Fun from the functions abs val, unless the argument is a type argument
-- or it is already Top or Bot.
cprAnalExpr rho (App fun arg@(Type _))
    = (App fun_cpr arg, fun_res)
    where
      (fun_cpr, fun_res)  = cprAnalExpr rho fun

cprAnalExpr rho (App fun arg)
    = (App fun_cpr arg_cpr, res_res)
    where
      (fun_cpr, fun_res)  = cprAnalExpr rho fun
      (arg_cpr, _)        = cprAnalExpr rho arg
      res_res             = case fun_res of
                                Fun res_res -> res_res
                                Top         -> Top
                                Bot         -> Bot
                                Tuple       -> WARN( True, ppr (App fun arg) ) Top
                                                -- This really should not happen!


-- Map arguments to Top (we aren't constructing them)
-- Return the abstract value of the body, since functions
-- are represented by the CPR value of their result, and
-- add a Fun for this lambda..
cprAnalExpr rho (Lam b body) | isTyVar b = (Lam b body_cpr, body_aval)
                             | otherwise = (Lam b body_cpr, Fun body_aval)
      where
      (body_cpr, body_aval) = cprAnalExpr (extendVarEnv rho b Top) body

cprAnalExpr rho (Let bind body)
    = (Let bind' body', body_aval)
    where
      (rho', bind') = cprAnalBind rho bind
      (body', body_aval) = cprAnalExpr rho' body

cprAnalExpr rho (Case scrut bndr alts)
    = (Case scrut_cpr bndr alts_cpr, alts_aval)
      where
      (scrut_cpr, scrut_aval) = cprAnalExpr rho scrut
      (alts_cpr, alts_aval) = cprAnalCaseAlts (extendVarEnv rho bndr scrut_aval) alts

cprAnalExpr rho (Note n exp)
    = (Note n exp_cpr, expr_aval)
      where
      (exp_cpr, expr_aval) = cprAnalExpr rho exp

cprAnalExpr rho (Type t)
    = (Type t, Top)

cprAnalCaseAlts :: CPREnv -> [CoreAlt] -> ([CoreAlt], AbsVal)
cprAnalCaseAlts rho alts
    = foldr anal_alt ([], Bot) alts
      where
      anal_alt :: CoreAlt -> ([CoreAlt], AbsVal) -> ([CoreAlt], AbsVal)
      anal_alt (con, binds, exp)  (done, aval)
          = ((con,binds,exp_cpr) : done, exp_aval `lub` aval)
            where (exp_cpr, exp_aval) = cprAnalExpr rho' exp
                  rho' = rho `extendVarEnvList` (zip binds (repeat Top))


addIdCprInfo :: Id -> CoreExpr -> AbsVal -> Id
addIdCprInfo bndr rhs absval
  | useful_info && ok_to_add = setIdCprInfo bndr cpr_info
  | otherwise                = bndr
  where
    cpr_info    = absToCprInfo absval
    useful_info = case cpr_info of { ReturnsCPR -> True; NoCPRInfo -> False }

    ok_to_add = case absval of
                  Fun _ -> idArity bndr >= n_fun_tys absval
                      -- Enough visible lambdas

                  Tuple  -> exprIsHNF rhs || isStrict (idDemandInfo bndr)
                        -- If the rhs is a value, and returns a constructed product,
                        -- it will be inlined at usage sites, so we give it a Tuple absval
                        -- If it isn't a value, we won't inline it (code/work dup worries), so
                        -- we discard its absval.
                        --
                        -- Also, if the strictness analyser has figured out that it's strict,
                        -- the let-to-case transformation will happen, so again it's good.
                        -- (CPR analysis runs before the simplifier has had a chance to do
                        --  the let-to-case transform.)
                        -- This made a big difference to PrelBase.modInt, which had something like
                        --      modInt = \ x -> let r = ... -> I# v in
                        --                      ...body strict in r...
                        -- r's RHS isn't a value yet; but modInt returns r in various branches, so
                        -- if r doesn't have the CPR property then neither does modInt

                  _ -> False

    n_fun_tys :: AbsVal -> Int
    n_fun_tys (Fun av) = 1 + n_fun_tys av
    n_fun_tys other    = 0


absToCprInfo :: AbsVal -> CprInfo
absToCprInfo Tuple   = ReturnsCPR
absToCprInfo (Fun r) = absToCprInfo r
absToCprInfo _       = NoCPRInfo


-- Cpr Info doesn't store the number of arguments a function has,  so the caller
-- must take care to add the appropriate number of Funs.
getCprAbsVal v = case idCprInfo v of
                        NoCPRInfo -> Top
                        ReturnsCPR -> nTimes arity Fun Tuple
               where
                 arity = idArity v
        -- Imported (non-nullary) constructors will have the CPR property
        -- in their IdInfo, so no need to look at their unfolding
#endif /* OLD_STRICTNESS */
\end{code}