path: root/compiler/prelude/PrimOp.lhs

%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[PrimOp]{Primitive operations (machine-level)}

\begin{code}
{-# OPTIONS -w #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
-- for details

module PrimOp (
	PrimOp(..), allThePrimOps,
	primOpType, primOpSig,
	primOpTag, maxPrimOpTag, primOpOcc,

	tagToEnumKey,

	primOpOutOfLine, primOpNeedsWrapper, 
	primOpOkForSpeculation, primOpIsCheap, primOpIsDupable,

	getPrimOpResultInfo,  PrimOpResultInfo(..)
    ) where

#include "HsVersions.h"

import TysPrim
import TysWiredIn

import NewDemand
import Var		( TyVar )
import OccName		( OccName, pprOccName, mkVarOccFS )
import TyCon		( TyCon, isPrimTyCon, tyConPrimRep, PrimRep(..) )
import Type		( Type, mkForAllTys, mkFunTy, mkFunTys, tyConAppTyCon,
			  typePrimRep )
import BasicTypes	( Arity, Boxity(..) )
import Unique		( Unique, mkPrimOpIdUnique )
import Outputable
import FastTypes
\end{code}

%************************************************************************
%*									*
\subsection[PrimOp-datatype]{Datatype for @PrimOp@ (an enumeration)}
%*									*
%************************************************************************

These are in \tr{state-interface.verb} order.

\begin{code}

-- supplies: 
-- data PrimOp = ...
#include "primop-data-decl.hs-incl"
\end{code}

Used for the Ord instance

\begin{code}
primOpTag :: PrimOp -> Int
primOpTag op = iBox (tagOf_PrimOp op)

-- supplies   
-- tagOf_PrimOp :: PrimOp -> FastInt
#include "primop-tag.hs-incl"


instance Eq PrimOp where
    op1 == op2 = tagOf_PrimOp op1 ==# tagOf_PrimOp op2

instance Ord PrimOp where
    op1 <  op2 =  tagOf_PrimOp op1 <# tagOf_PrimOp op2
    op1 <= op2 =  tagOf_PrimOp op1 <=# tagOf_PrimOp op2
    op1 >= op2 =  tagOf_PrimOp op1 >=# tagOf_PrimOp op2
    op1 >  op2 =  tagOf_PrimOp op1 ># tagOf_PrimOp op2
    op1 `compare` op2 | op1 < op2  = LT
		      | op1 == op2 = EQ
		      | otherwise  = GT

instance Outputable PrimOp where
    ppr op = pprPrimOp op

instance Show PrimOp where
    showsPrec p op = showsPrecSDoc p (pprPrimOp op)
\end{code}

An @Enum@-derived list would be better; meanwhile... (ToDo)

\begin{code}
allThePrimOps :: [PrimOp]
allThePrimOps =
#include "primop-list.hs-incl"
\end{code}

\begin{code}
tagToEnumKey :: Unique
tagToEnumKey = mkPrimOpIdUnique (primOpTag TagToEnumOp)
\end{code}



%************************************************************************
%*									*
\subsection[PrimOp-info]{The essential info about each @PrimOp@}
%*									*
%************************************************************************

The @String@ in the @PrimOpInfos@ is the ``base name'' by which the user may
refer to the primitive operation.  The conventional \tr{#}-for-
unboxed ops is added on later.

The reason for the funny characters in the names is so we do not
interfere with the programmer's Haskell name spaces.

We use @PrimKinds@ for the ``type'' information, because they're
(slightly) more convenient to use than @TyCons@.
\begin{code}
data PrimOpInfo
  = Dyadic	OccName		-- string :: T -> T -> T
		Type
  | Monadic	OccName		-- string :: T -> T
		Type
  | Compare	OccName		-- string :: T -> T -> Bool
		Type

  | GenPrimOp   OccName  	-- string :: \/a1..an . T1 -> .. -> Tk -> T
		[TyVar] 
		[Type] 
		Type 

mkDyadic str  ty = Dyadic  (mkVarOccFS str) ty
mkMonadic str ty = Monadic (mkVarOccFS str) ty
mkCompare str ty = Compare (mkVarOccFS str) ty
mkGenPrimOp str tvs tys ty = GenPrimOp (mkVarOccFS str) tvs tys ty
\end{code}

%************************************************************************
%*									*
\subsubsection{Strictness}
%*									*
%************************************************************************

Not all primops are strict!

\begin{code}
primOpStrictness :: PrimOp -> Arity -> StrictSig
	-- See Demand.StrictnessInfo for discussion of what the results
	-- The arity should be the arity of the primop; that's why
	-- this function isn't exported.
#include "primop-strictness.hs-incl"
\end{code}

%************************************************************************
%*									*
\subsubsection[PrimOp-comparison]{PrimOpInfo basic comparison ops}
%*									*
%************************************************************************

@primOpInfo@ gives all essential information (from which everything
else, notably a type, can be constructed) for each @PrimOp@.

\begin{code}
primOpInfo :: PrimOp -> PrimOpInfo
#include "primop-primop-info.hs-incl"
\end{code}

Here are a load of comments from the old primOp info:

A @Word#@ is an unsigned @Int#@.

@decodeFloat#@ is given w/ Integer-stuff (it's similar).

@decodeDouble#@ is given w/ Integer-stuff (it's similar).

Decoding of floating-point numbers is sorta Integer-related.  Encoding
is done with plain ccalls now (see PrelNumExtra.lhs).

A @Weak@ Pointer is created by the @mkWeak#@ primitive:

	mkWeak# :: k -> v -> f -> State# RealWorld 
			-> (# State# RealWorld, Weak# v #)

In practice, you'll use the higher-level

	data Weak v = Weak# v
	mkWeak :: k -> v -> IO () -> IO (Weak v)

The following operation dereferences a weak pointer.  The weak pointer
may have been finalized, so the operation returns a result code which
must be inspected before looking at the dereferenced value.

	deRefWeak# :: Weak# v -> State# RealWorld ->
			(# State# RealWorld, v, Int# #)

Only look at v if the Int# returned is /= 0 !!

The higher-level op is

	deRefWeak :: Weak v -> IO (Maybe v)

Weak pointers can be finalized early by using the finalize# operation:
	
	finalizeWeak# :: Weak# v -> State# RealWorld -> 
	   		   (# State# RealWorld, Int#, IO () #)

The Int# returned is either

	0 if the weak pointer has already been finalized, or it has no
	  finalizer (the third component is then invalid).

	1 if the weak pointer is still alive, with the finalizer returned
	  as the third component.

A {\em stable name/pointer} is an index into a table of stable name
entries.  Since the garbage collector is told about stable pointers,
it is safe to pass a stable pointer to external systems such as C
routines.

\begin{verbatim}
makeStablePtr#  :: a -> State# RealWorld -> (# State# RealWorld, StablePtr# a #)
freeStablePtr   :: StablePtr# a -> State# RealWorld -> State# RealWorld
deRefStablePtr# :: StablePtr# a -> State# RealWorld -> (# State# RealWorld, a #)
eqStablePtr#    :: StablePtr# a -> StablePtr# a -> Int#
\end{verbatim}

It may seem a bit surprising that @makeStablePtr#@ is a @IO@
operation since it doesn't (directly) involve IO operations.  The
reason is that if some optimisation pass decided to duplicate calls to
@makeStablePtr#@ and we only pass one of the stable pointers over, a
massive space leak can result.  Putting it into the IO monad
prevents this.  (Another reason for putting them in a monad is to
ensure correct sequencing wrt the side-effecting @freeStablePtr@
operation.)

An important property of stable pointers is that if you call
makeStablePtr# twice on the same object you get the same stable
pointer back.

Note that we can implement @freeStablePtr#@ using @_ccall_@ (and,
besides, it's not likely to be used from Haskell) so it's not a
primop.

Question: Why @RealWorld@ - won't any instance of @_ST@ do the job? [ADR]

Stable Names
~~~~~~~~~~~~

A stable name is like a stable pointer, but with three important differences:

	(a) You can't deRef one to get back to the original object.
	(b) You can convert one to an Int.
	(c) You don't need to 'freeStableName'

The existence of a stable name doesn't guarantee to keep the object it
points to alive (unlike a stable pointer), hence (a).

Invariants:
	
	(a) makeStableName always returns the same value for a given
	    object (same as stable pointers).

	(b) if two stable names are equal, it implies that the objects
	    from which they were created were the same.

	(c) stableNameToInt always returns the same Int for a given
	    stable name.


-- HWL: The first 4 Int# in all par... annotations denote:
--   name, granularity info, size of result, degree of parallelism
--      Same  structure as _seq_ i.e. returns Int#
-- KSW: v, the second arg in parAt# and parAtForNow#, is used only to determine
--   `the processor containing the expression v'; it is not evaluated

These primops are pretty wierd.

	dataToTag# :: a -> Int    (arg must be an evaluated data type)
	tagToEnum# :: Int -> a    (result type must be an enumerated type)

The constraints aren't currently checked by the front end, but the
code generator will fall over if they aren't satisfied.

\begin{code}
#ifdef DEBUG
primOpInfo op = pprPanic "primOpInfo:" (ppr op)
#endif
\end{code}

%************************************************************************
%*									*
\subsubsection[PrimOp-ool]{Which PrimOps are out-of-line}
%*									*
%************************************************************************

Some PrimOps need to be called out-of-line because they either need to
perform a heap check or they block.


\begin{code}
primOpOutOfLine :: PrimOp -> Bool
#include "primop-out-of-line.hs-incl"
\end{code}


primOpOkForSpeculation
~~~~~~~~~~~~~~~~~~~~~~
Sometimes we may choose to execute a PrimOp even though it isn't
certain that its result will be required; ie execute them
``speculatively''.  The same thing as ``cheap eagerness.'' Usually
this is OK, because PrimOps are usually cheap, but it isn't OK for
(a)~expensive PrimOps and (b)~PrimOps which can fail.

PrimOps that have side effects also should not be executed speculatively.

Ok-for-speculation also means that it's ok *not* to execute the
primop. For example
	case op a b of
	  r -> 3
Here the result is not used, so we can discard the primop.  Anything
that has side effects mustn't be dicarded in this way, of course!

See also @primOpIsCheap@ (below).


\begin{code}
primOpOkForSpeculation :: PrimOp -> Bool
	-- See comments with CoreUtils.exprOkForSpeculation
primOpOkForSpeculation op 
  = not (primOpHasSideEffects op || primOpOutOfLine op || primOpCanFail op)
\end{code}


primOpIsCheap
~~~~~~~~~~~~~
@primOpIsCheap@, as used in \tr{SimplUtils.lhs}.  For now (HACK
WARNING), we just borrow some other predicates for a
what-should-be-good-enough test.  "Cheap" means willing to call it more
than once, and/or push it inside a lambda.  The latter could change the
behaviour of 'seq' for primops that can fail, so we don't treat them as cheap.

\begin{code}
primOpIsCheap :: PrimOp -> Bool
primOpIsCheap op = primOpOkForSpeculation op
-- In March 2001, we changed this to 
--	primOpIsCheap op = False
-- thereby making *no* primops seem cheap.  But this killed eta
-- expansion on case (x ==# y) of True -> \s -> ... 
-- which is bad.  In particular a loop like
--	doLoop n = loop 0
--     where
--         loop i | i == n    = return ()
--                | otherwise = bar i >> loop (i+1)
-- allocated a closure every time round because it doesn't eta expand.
-- 
-- The problem that originally gave rise to the change was
--	let x = a +# b *# c in x +# x
-- were we don't want to inline x. But primopIsCheap doesn't control
-- that (it's exprIsDupable that does) so the problem doesn't occur
-- even if primOpIsCheap sometimes says 'True'.
\end{code}

primOpIsDupable
~~~~~~~~~~~~~~~
primOpIsDupable means that the use of the primop is small enough to
duplicate into different case branches.  See CoreUtils.exprIsDupable.

\begin{code}
primOpIsDupable :: PrimOp -> Bool
	-- See comments with CoreUtils.exprIsDupable
	-- We say it's dupable it isn't implemented by a C call with a wrapper
primOpIsDupable op = not (primOpNeedsWrapper op)
\end{code}


\begin{code}
primOpCanFail :: PrimOp -> Bool
#include "primop-can-fail.hs-incl"
\end{code}

And some primops have side-effects and so, for example, must not be
duplicated.

\begin{code}
primOpHasSideEffects :: PrimOp -> Bool
#include "primop-has-side-effects.hs-incl"
\end{code}

Inline primitive operations that perform calls need wrappers to save
any live variables that are stored in caller-saves registers.

\begin{code}
primOpNeedsWrapper :: PrimOp -> Bool
#include "primop-needs-wrapper.hs-incl"
\end{code}

\begin{code}
primOpType :: PrimOp -> Type  -- you may want to use primOpSig instead
primOpType op
  = case (primOpInfo op) of
      Dyadic occ ty ->	    dyadic_fun_ty ty
      Monadic occ ty ->	    monadic_fun_ty ty
      Compare occ ty ->	    compare_fun_ty ty

      GenPrimOp occ tyvars arg_tys res_ty -> 
	mkForAllTys tyvars (mkFunTys arg_tys res_ty)

primOpOcc :: PrimOp -> OccName
primOpOcc op = case (primOpInfo op) of
		Dyadic    occ _	    -> occ
		Monadic   occ _	    -> occ
		Compare   occ _	    -> occ
		GenPrimOp occ _ _ _ -> occ

-- primOpSig is like primOpType but gives the result split apart:
-- (type variables, argument types, result type)
-- It also gives arity, strictness info

primOpSig :: PrimOp -> ([TyVar], [Type], Type, Arity, StrictSig)
primOpSig op
  = (tyvars, arg_tys, res_ty, arity, primOpStrictness op arity)
  where
    arity = length arg_tys
    (tyvars, arg_tys, res_ty)
      = case (primOpInfo op) of
	  Monadic   occ ty -> ([],     [ty],    ty    )
	  Dyadic    occ ty -> ([],     [ty,ty], ty    )
	  Compare   occ ty -> ([],     [ty,ty], boolTy)
	  GenPrimOp occ tyvars arg_tys res_ty
                           -> (tyvars, arg_tys, res_ty)
\end{code}

\begin{code}
data PrimOpResultInfo
  = ReturnsPrim	    PrimRep
  | ReturnsAlg	    TyCon

-- Some PrimOps need not return a manifest primitive or algebraic value
-- (i.e. they might return a polymorphic value).  These PrimOps *must*
-- be out of line, or the code generator won't work.

getPrimOpResultInfo :: PrimOp -> PrimOpResultInfo
getPrimOpResultInfo op
  = case (primOpInfo op) of
      Dyadic  _ ty		 	  -> ReturnsPrim (typePrimRep ty)
      Monadic _ ty		 	  -> ReturnsPrim (typePrimRep ty)
      Compare _ ty		 	  -> ReturnsAlg boolTyCon
      GenPrimOp _ _ _ ty | isPrimTyCon tc -> ReturnsPrim (tyConPrimRep tc)
			 | otherwise      -> ReturnsAlg tc
			 where
			   tc = tyConAppTyCon ty
			-- All primops return a tycon-app result
			-- The tycon can be an unboxed tuple, though, which
			-- gives rise to a ReturnAlg
\end{code}

The commutable ops are those for which we will try to move constants
to the right hand side for strength reduction.

\begin{code}
commutableOp :: PrimOp -> Bool
#include "primop-commutable.hs-incl"
\end{code}

Utils:
\begin{code}
dyadic_fun_ty  ty = mkFunTys [ty, ty] ty
monadic_fun_ty ty = mkFunTy  ty ty
compare_fun_ty ty = mkFunTys [ty, ty] boolTy
\end{code}

Output stuff:
\begin{code}
pprPrimOp  :: PrimOp -> SDoc
pprPrimOp other_op = pprOccName (primOpOcc other_op)
\end{code}