Home My Page Projects Code Snippets Project Openings diderot
Summary Activity Tracker Tasks SCM

SCM Repository

[diderot] View of /trunk/src/compiler/common/float-lit.sml
ViewVC logotype

View of /trunk/src/compiler/common/float-lit.sml

Parent Directory Parent Directory | Revision Log Revision Log


Revision 234 - (download) (annotate)
Thu Aug 5 21:34:29 2010 UTC (9 years, 3 months ago) by jhr
File size: 7143 byte(s)
  Working on static evalution
(* float-lit.sml
 *
 * COPYRIGHT (c) 2010 The Diderot Project (http://diderot.cs.uchicago.edu)
 * All rights reserved.
 *
 * Internal representation of floating-point literals with limited
 * support for arithmetic.
 *)

structure FloatLit :> sig

    type float

    val isZero : float -> bool

  (* return the representation of +/-0.0 *)
    val zero : bool -> float

  (* plus and minus one *)
    val one : float
    val m_one : float

  (* negate a float *)
    val negate : float -> float

  (* equality, comparisons, and hashing functions *)
    val same : (float * float) -> bool
    val compare : (float * float) -> order (* not ordering on reals *)
    val hash : float -> word

  (* special floats *)
    val nan : float		(* some quiet NaN *)
    val posInf : float		(* positive infinity *)
    val negInf : float		(* negative infinity *)

  (* create a float from pieces: isNeg is true if the number is negative, whole
   * is the whole-number part, frac is the fractional part, and exp is the
   * exponent.  This function may raise Overflow, when the exponent of the
   * normalized representation is too small or too large.
   *)
    val float : {isNeg : bool, whole : string, frac : string, exp : int} -> float
    val toString : float -> string
    val toReal : float -> real

  (* external representation (for pickling) *)
    val toBytes : float -> Word8Vector.vector
    val fromBytes : Word8Vector.vector -> float

  end = struct

    structure SS = Substring
    structure W = Word
    structure W8V = Word8Vector

  (* The value {isNeg, digits=[d0, ..., dn], exp} represents the number
   *
   *	[+/-] 0.d0...dn * 10^exp
   *
   * where the sign is negative if isNeg is true.
   *)
    datatype float
      = PosInf		(* positive infinity *)
      | NegInf		(* negative infinity *)
      | NaN		(* some quiet NaN *)
      | Flt of {isNeg : bool, digits : int list, exp : int}

  (* special floats *)
    val nan = NaN
    val posInf = PosInf
    val negInf = NegInf

    fun isZero (Flt{isNeg, digits=[0], exp}) = true
      | isZero _ = false

    fun zero isNeg = Flt{isNeg = isNeg, digits = [0], exp = 0}

    val one = Flt{isNeg = false, digits = [1], exp = 1}
    val m_one = Flt{isNeg = true, digits = [1], exp = 1}

  (* negate a float *)
    fun negate PosInf = NegInf
      | negate NegInf = PosInf
      | negate NaN = raise Fail "negate nan"
      | negate (Flt{isNeg, digits, exp}) =
	  Flt{isNeg = not isNeg, digits = digits, exp = exp}

  (* equality, comparisons, and hashing functions *)
    fun same (NegInf, NegInf) = true
      | same (PosInf, PosInf) = true
      | same (NaN, NaN) = true
      | same (Flt f1, Flt f2) =
	  (#isNeg f1 = #isNeg f2) andalso (#exp f1 = #exp f2)
	  andalso (#digits f1 = #digits f2)
      | same _ = false

    fun compare (NegInf, NegInf) = EQUAL
      | compare (NegInf, _) = LESS
      | compare (_, NegInf) = GREATER
      | compare (PosInf, PosInf) = EQUAL
      | compare (PosInf, _) = LESS
      | compare (_, PosInf) = GREATER
      | compare (NaN, NaN) = EQUAL
      | compare (NaN, _) = LESS
      | compare (_, NaN) = GREATER
      | compare (Flt f1, Flt f2) = (case (#isNeg f1, #isNeg f2)
	   of (false, true) => GREATER
	    | (true, false) => LESS
	    | _ => (case Int.compare(#exp f1, #exp f2)
		 of EQUAL => let
		      fun cmp ([], []) = EQUAL
			| cmp ([], _) = LESS
			| cmp (_, []) = GREATER
			| cmp (d1::r1, d2::r2) = (case Int.compare(d1, d2)
			     of EQUAL => cmp(r1, r2)
			      | order => order
			    (* end case *))
		      in
			cmp (#digits f1, #digits f2)
		      end
		  | order => order
		(* end case *))
	  (* end case *))

    fun hash PosInf = 0w1
      | hash NegInf = 0w3
      | hash NaN = 0w5
      | hash (Flt{isNeg, digits, exp}) = let
	  fun hashDigits ([], h, _) = h
	    | hashDigits (d::r, h, i) =
		hashDigits (r, W.<<(W.fromInt d, i+0w4), W.andb(i+0w1, 0wxf))
	  in
	    hashDigits(digits, W.fromInt exp, 0w0)
	  end

    fun float {isNeg, whole, frac, exp} = let
	  fun cvtDigit (c, l) = (Char.ord c - Char.ord #"0") :: l
	  fun isZero #"0" = true | isZero _ = false
	(* whole digits with leading zeros removed *)
	  val whole = SS.dropl isZero (SS.full whole)
	(* fractional digits with trailing zeros removed *)
	  val frac = SS.dropr isZero (SS.full frac)
	(* normalize by stripping leading zero digits *)
	  fun normalize {isNeg, digits=[], exp} = zero isNeg
	    | normalize {isNeg, digits=0::r, exp} =
		normalize {isNeg=isNeg, digits=r, exp=exp-1}
	    | normalize flt = Flt flt
	  in
	    case SS.foldr cvtDigit (SS.foldr cvtDigit [] frac) whole
	     of [] => zero isNeg
	      | digits => normalize {
		    isNeg = isNeg,
		    digits = digits,
		    exp = exp + SS.size whole
		  }
	    (* end case *)
	  end

    fun toString PosInf = "+inf"
      | toString NegInf = "-inf"
      | toString NaN = "nan"
      | toString (Flt{isNeg, digits, exp}) = let
	  val s = if isNeg then "-0." else "0."
	  val e = if exp < 0
		then ["e-", Int.toString(~exp)]
		else ["e", Int.toString exp]
	  in
	    concat(s :: List.foldr (fn (d, ds) => Int.toString d :: ds) e digits)
	  end

    fun toReal PosInf = Real.posInf
      | toReal NegInf = Real.negInf
      | toReal NaN = 0.0 / 0.0
      | toReal x = valOf(Real.fromString(toString x))  (* FIXME *)


  (***** external representation (for pickling) *****
   *
   * The representation we use is a sequence of bytes:
   *
   *    [sign, d0, ..., dn, exp0, ..., exp3]
   *
   * where
   *    sign	== 0 or 1
   *    di      == ith digit
   *    expi    == ith byte of exponent (exp0 is lsb, exp3 is msb).
   *
   * we encode Infs and NaNs using the sign byte:
   *
   *	2	== PosInf
   *	3	== NegInf
   *	4	== NaN
   *
   * NOTE: we could pack the sign and digits into 4-bit nibbles, but we are keeping
   * things simple for now.
   *)

    fun toBytes PosInf = Word8Vector.fromList [0w2]
      | toBytes NegInf = Word8Vector.fromList [0w3]
      | toBytes NaN = Word8Vector.fromList [0w4]
      | toBytes (Flt{isNeg, digits, exp}) = let
	  val sign = if isNeg then 0w1 else 0w0
	  val digits = List.map Word8.fromInt digits
	  val exp' = W.fromInt exp
	  fun byte i = Word8.fromLargeWord(W.toLargeWord((W.>>(exp', 0w8*i))))
	  val exp = [byte 0w0, byte 0w1, byte 0w2, byte 0w3]
	  in
	    Word8Vector.fromList(sign :: (digits @ exp))
	  end

    fun fromBytes v = let
	  fun error () = raise Fail "Bogus float pickle"
	  val len = W8V.length v
	  in
	    if (len = 1)
	      then (case W8V.sub(v, 0) (* special float value *)
		 of 0w2 => PosInf
		  | 0w3 => NegInf
		  | 0w4 => NaN
		  | _ => error()
		(* end case *))
	      else let
		val ndigits = W8V.length v - 5
		val _ = if (ndigits < 1) then error() else ()
		val isNeg = (case W8V.sub(v, 0)
		       of 0w0 => false
			| 0w1 => true
			| _ => error()
		      (* end case *))
		fun digit i = Word8.toInt(W8V.sub(v, i+1))
		fun byte i = W.<<(
		      W.fromLargeWord(Word8.toLargeWord(W8V.sub(v, ndigits+1+i))),
		      W.fromInt(8*i))
		val exp = W.toIntX(W.orb(byte 3, W.orb(byte 2, W.orb(byte 1, byte 0))))
		in
		  Flt{isNeg = isNeg, digits = List.tabulate(ndigits, digit), exp = exp}
		end
	  end

  end


root@smlnj-gforge.cs.uchicago.edu
ViewVC Help
Powered by ViewVC 1.0.0