-- Filters superfluous location instructions from the byte code
--
-- We keep branch targets and the location before a jump instruction

MODULE {InstrLocFilter} {instrLocFilter} {}

INCLUDE "ByteCodeAst.ag"
INCLUDE "ByteCodeLocationInfo.ag"

import
{
import Data.ByteString.Lazy(ByteString,pack)
import ByteCode
import Data.Word
import Data.Bits
import qualified Data.IntSet as IntSet
import Data.IntSet(IntSet)
import Data.Monoid
}

WRAPPER BodyInfo

{
-- | Computes the minimum length of an instruction (in bytes)
instrLocFilter :: BodyInfo -> BodyInfo
instrLocFilter body = body' where
  inh   = Inh_BodyInfo {}
  sem   = sem_BodyInfo body
  syn   = wrap_BodyInfo sem inh
  body' = output_Syn_BodyInfo syn
}

ATTR BodyInfo Instructions Instruction CaseOffsets DebugType
     Exceptions Exception AllTraits
     [ | | output : SELF ]

SEM BodyInfo | Info
  instructions.isBranch    = True
  instructions.revIsBranch = False

SEM Instructions | Cons
  lhs.output = if @hd.skip then @tl.output else @hd.output : @tl.output


ATTR Instructions Instruction [ retain : IntSet | | ]
SEM BodyInfo | Info
  instructions.retain = @instructions.locs `mappend` @exceptions.locs

ATTR Instruction [ | | skip : Bool ]

SEM Instruction | * - Location
  lhs.skip = False

SEM Instruction | Location
  lhs.skip = not (IntSet.member @index @lhs.retain)


-- Is the predecessor a branching instruction (except LookupSwitch)
ATTR Instructions Instruction [ | isBranch : Bool revIsBranch : Bool | ]
SEM Instruction
  | * - IfEq IfFalse IfGe IfGt IfLe IfLt IfNGe IfNGt IfNLe IfNLt IfNe IfStrictEq IfStrictNe IfTrue Jump LookupSwitch
      lhs.isBranch    = False
      lhs.revIsBranch = False
  | IfEq IfFalse IfGe IfGt IfLe IfLt IfNGe IfNGt IfNLe IfNLt IfNe IfStrictEq IfStrictNe IfTrue Jump LookupSwitch
      lhs.isBranch    = True
      lhs.revIsBranch = True

SEM Instructions | Cons  -- reverse flow
  tl.revIsBranch  = @lhs.revIsBranch
  hd.revIsBranch  = @tl.revIsBranch
  lhs.revIsBranch = @hd.revIsBranch

-- Compute the set of location to retain
ATTR Exceptions Exception Instructions Instruction CaseOffsets [ | | locs USE {`mappend`} {mempty} : IntSet ]

SEM Exception | Info
  lhs.locs = IntSet.fromList $ map fromIntegral $ [@from,@to,@target]

-- Location following a branch target
SEM Instruction | Location
  +locs = if @lhs.isBranch || @lhs.revIsBranch then (IntSet.insert @index) else id
  
SEM Instruction
  | LookupSwitch IfEq IfFalse IfGe IfGt IfLe IfLt IfNGe IfNGt IfNLe IfNLt IfNe IfStrictEq IfStrictNe IfTrue Jump
      +locs = IntSet.insert @loc.target

SEM CaseOffsets | Cons
  +locs = IntSet.insert @loc.target


-- Slight hacks to deal with AG pecularities
SEM BodyInfo | Info
  instructions.isBranch < instructions.revLocation
  instructions.isBranch < instructions.revIsBranch