From: Chris Hanson <org/chris-hanson/cph>
Date: Sun, 17 Oct 2004 21:35:40 +0000 (+0000)
Subject: In double_to_bignum(), use the length of the significand to limit how
X-Git-Tag: 20090517-FFI~1528
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=c32715797b7d89ff6edd6445bfc735ab3dfe328f;p=mit-scheme.git

In double_to_bignum(), use the length of the significand to limit how
much information we extract from it.  Previously, this code generated
garbage bits for the LSBs when the exponent was larger than the length
of the significand; now it generates zero bits instead.
---

diff --git a/v7/src/microcode/bignum.c b/v7/src/microcode/bignum.c
index f76215630..9b1cb361a 100644
--- a/v7/src/microcode/bignum.c
+++ b/v7/src/microcode/bignum.c
@@ -1,8 +1,10 @@
 /* -*-C-*-
 
-$Id: bignum.c,v 9.51 2003/02/14 18:28:15 cph Exp $
+$Id: bignum.c,v 9.52 2004/10/17 21:35:40 cph Exp $
 
-Copyright (c) 1989-2000 Massachusetts Institute of Technology
+Copyright 1986,1987,1988,1989,1990,1991 Massachusetts Institute of Technology
+Copyright 1992,1993,1994,1996,1997,2000 Massachusetts Institute of Technology
+Copyright 2004 Massachusetts Institute of Technology
 
 This file is part of MIT/GNU Scheme.
 
@@ -34,6 +36,7 @@ USA.
 
 #include "bignmint.h"
 #include "limits.h"
+#include "float.h"
 #include <math.h>
 
 #ifndef MIT_SCHEME
@@ -557,20 +560,20 @@ DEFUN (bignum_to_ulong, (bignum), bignum_type bignum)
 
 #endif /* not BIGNUM_NO_ULONG */
 
-#define DTB_WRITE_DIGIT(factor)						\
+#define DTB_WRITE_DIGIT(n_bits) do					\
 {									\
-  significand *= (factor);						\
+  significand *= (1L << (n_bits));					\
   digit = ((bignum_digit_type) significand);				\
   (*--scan) = digit;							\
   significand -= ((double) digit);					\
-}
+  n_valid_bits -= (n_bits);						\
+} while (0)
 
 bignum_type
 DEFUN (double_to_bignum, (x), double x)
 {
-  extern double frexp ();
   int exponent;
-  fast double significand = (frexp (x, (&exponent)));
+  double significand = (frexp (x, (&exponent)));
   if (exponent <= 0) return (BIGNUM_ZERO ());
   if (exponent == 1) return (BIGNUM_ONE (x < 0));
   if (significand < 0) significand = (-significand);
@@ -578,20 +581,35 @@ DEFUN (double_to_bignum, (x), double x)
     bignum_length_type length = (BIGNUM_BITS_TO_DIGITS (exponent));
     bignum_type result = (bignum_allocate (length, (x < 0)));
     bignum_digit_type * start = (BIGNUM_START_PTR (result));
-    fast bignum_digit_type * scan = (start + length);
-    fast bignum_digit_type digit;
-    int odd_bits = (exponent % BIGNUM_DIGIT_LENGTH);
-    if (odd_bits > 0)
-      DTB_WRITE_DIGIT (1L << odd_bits);
+    bignum_digit_type * scan = (start + length);
+    unsigned int n_valid_bits = DBL_MANT_DIG;
+    bignum_digit_type digit;
+    {
+      int odd_bits = (exponent % BIGNUM_DIGIT_LENGTH);
+      if (odd_bits > 0)
+	DTB_WRITE_DIGIT (odd_bits);
+    }
     while (start < scan)
       {
-	if (significand == 0)
+	if ((significand == 0)  || (n_valid_bits <= 0))
 	  {
 	    while (start < scan)
 	      (*--scan) = 0;
 	    break;
 	  }
-	DTB_WRITE_DIGIT (BIGNUM_RADIX);
+	if (n_valid_bits >= BIGNUM_DIGIT_LENGTH)
+	  DTB_WRITE_DIGIT (BIGNUM_DIGIT_LENGTH);
+	else
+	  {
+	    significand *= (1L << BIGNUM_DIGIT_LENGTH);
+	    digit = ((bignum_digit_type) significand);
+	    (*--scan)
+	      = (digit
+		 & (((1L << n_valid_bits) - 1)
+		    << (BIGNUM_DIGIT_LENGTH - n_valid_bits)));
+	    significand = 0.0;
+	    n_valid_bits = 0;
+	  }
       }
     return (result);
   }