[MLton-devel] cvs commit: Const.SmallIntInf and IntInf.rep

[Stephen Weeks]

sweeks      03/07/02 08:08:16

  Modified:    mlton/atoms const.fun const.sig prim.fun
  Log:
  Fixed bug that Henry saw (and Matthew found) with small negative
  IntInf constants.  The problem was int Const.SmallIntInf.t, which
  treated the word corresponding to a small IntInf inconsistently.
  SmallIntInf.toWord shifted and added the tag bit, whit
  SmallIntInf.fromWord assumed the tag bit wasn't there.  As a
  consequence, the SSA simplifier treated small negative IntInf's as
  large ones, i.e. as word vectors, and they got propagated to the x86
  codegen which tried to treat an constant integer as a base address.
  
  Unfortunately, it's not a good fix to use MLton.IntInf.rep, for two
  reasons.  First, MLton still needs work when compiled using
  mlton-stubs, so this just shifts the duplicated code from
  Const.SmallIntInf to the stubs.  Second, even worse,
  MLton.IntInf.Small w leaves the word untagged instead of tagged.  So
  there wasn't much to be gained by moving the code to the stubs, since
  we still need a slightly different treatment.  It might be worth
  moving to the stubs if we change the meaning of MLton.IntInf.rep.
  Anyone have any thoughts one way or the other on that?  I prefer
  leaving it as is for two reasons.  One, that's the way it is now and I
  assume at least PolySpace is using it.  Two, the shift and tag is
  really part of the low level implementation and it's not clear it
  should be exposed.
  
  Hmmm.  It occurs to me that the right thing to do is to change rep
  from:
         datatype rep =
   	  Small of word
   	| Big of word vector
  to:
         datatype rep =
   	  Small of int
   	| Big of word vector
  
  That has nothing to do with the bug at hand, but it does clarify the
  issue with the tag, since it makes clear that we're dealing with an
  int and not some low level representation.  It also solves the problem
  with legacy code, since it will cause type errors for those who use
  it.

Revision  Changes    Path
1.9       +2 -4      mlton/mlton/atoms/const.fun

Index: const.fun
===================================================================
RCS file: /cvsroot/mlton/mlton/mlton/atoms/const.fun,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -r1.8 -r1.9
--- const.fun	23 Jun 2003 04:58:55 -0000	1.8
+++ const.fun	2 Jul 2003 15:08:16 -0000	1.9
@@ -53,10 +53,8 @@
 					  0w1)))
 	 else NONE
 
-      fun fromWord (w: word): IntInf.t option =
-	 if w < 0wx80000000
-	    then SOME (IntInf.fromInt (Word.toIntX (Word.~>> (w, 0w1))))
-	 else NONE
+      fun fromWord (w: word): IntInf.t =
+	 IntInf.fromInt (Word.toIntX (Word.~>> (w, 0w1)))
    end
 
 datatype t =



1.8       +1 -1      mlton/mlton/atoms/const.sig

Index: const.sig
===================================================================
RCS file: /cvsroot/mlton/mlton/mlton/atoms/const.sig,v
retrieving revision 1.7
retrieving revision 1.8
diff -u -r1.7 -r1.8
--- const.sig	23 Jun 2003 04:58:55 -0000	1.7
+++ const.sig	2 Jul 2003 15:08:16 -0000	1.8
@@ -22,9 +22,9 @@
 
       structure SmallIntInf:
 	 sig
+	    val fromWord: word -> IntInf.t
 	    val isSmall: IntInf.t -> bool
 	    val toWord: IntInf.t -> word option
-	    val fromWord: word -> IntInf.t option
 	 end
 
       datatype t =



1.54      +1 -3      mlton/mlton/atoms/prim.fun

Index: prim.fun
===================================================================
RCS file: /cvsroot/mlton/mlton/mlton/atoms/prim.fun,v
retrieving revision 1.53
retrieving revision 1.54
diff -u -r1.53 -r1.54
--- prim.fun	25 Jun 2003 23:15:31 -0000	1.53
+++ prim.fun	2 Jul 2003 15:08:16 -0000	1.54
@@ -929,9 +929,7 @@
 	   | (Word_toInt (_, s), [Word w]) =>
 		int (IntX.make (WordX.toIntInf w, s))
 	   | (Word_toIntInf, [Word w]) =>
-		(case SmallIntInf.fromWord (WordX.toWord w) of
-		    NONE => ApplyResult.Unknown
-		  | SOME i => intInf i)
+		intInf (SmallIntInf.fromWord (WordX.toWord w))
 	   | (Word_toIntX (_, s), [Word w]) =>
 		int (IntX.make (WordX.toIntInfX w, s))
 	   | (Word_toWord (_, s), [Word w]) => word (WordX.resize (w, s))





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