Re: RFA: Support infinity, NaN, and denormalized numbers in floatformat.c

[Andreas Schwab <schwab at linux-m68k dot org>]

Ian Lance Taylor <ian@wasabisystems.com> writes:

> @@ -306,8 +359,18 @@ floatformat_to_double (fmt, from, to)
>        mant = get_field (ufrom, fmt->byteorder, fmt->totalsize,
>  			 mant_off, mant_bits);
>  
> -      dto += ldexp ((double)mant, exponent - mant_bits);
> -      exponent -= mant_bits;
> +      /* Handle denormalized numbers.  FIXME: What should we do for
> +	 non-IEEE formats?  */
> +      if (exponent == 0 && mant != 0)
> +	dto += ldexp ((double)mant,
> +		      (- fmt->exp_bias
> +		       - mant_bits
> +		       - (mant_off - fmt->man_start)
> +		       + 1));
> +      else
> +	dto += ldexp ((double)mant, exponent - mant_bits);
> +      if (exponent != 0)
> +	exponent -= mant_bits;

That mishandles numbers between 1 and 2 (ie. where the unbiased exponent
is zero) with floating-point formats with more than 32 mantissa bits.
Also, the handling of denormal numbers can be simplified by just setting
exponent appropriately.

Andreas.

	* floatformat.c (floatformat_to_double): Correctly handle numbers
	between 1 and 2.  Simplify handling of denormal number.
	(main): Test with 1.1.
---
 libiberty/floatformat.c | 38 ++++++++++++++------------------------
 1 file changed, 14 insertions(+), 24 deletions(-)

diff --git a/libiberty/floatformat.c b/libiberty/floatformat.c
index 1116c63..c58ab01 100644
--- a/libiberty/floatformat.c
+++ b/libiberty/floatformat.c
@@ -1,5 +1,5 @@
 /* IEEE floating point support routines, for GDB, the GNU Debugger.
-   Copyright 1991, 1994, 1999, 2000, 2003, 2005, 2006, 2010
+   Copyright 1991, 1994, 1999, 2000, 2003, 2005, 2006, 2010, 2012
    Free Software Foundation, Inc.
 
 This file is part of GDB.
@@ -463,7 +463,6 @@ floatformat_to_double (const struct floatformat *fmt,
   unsigned long mant;
   unsigned int mant_bits, mant_off;
   int mant_bits_left;
-  int special_exponent;		/* It's a NaN, denorm or zero */
 
   /* Split values are not handled specially, since the top half has
      the correctly rounded double value (in the only supported case of
@@ -503,20 +502,20 @@ floatformat_to_double (const struct floatformat *fmt,
   mant_off = fmt->man_start;
   dto = 0.0;
 
-  special_exponent = exponent == 0 || (unsigned long) exponent == fmt->exp_nan;
-
-  /* Don't bias zero's, denorms or NaNs.  */
-  if (!special_exponent)
-    exponent -= fmt->exp_bias;
-
   /* Build the result algebraically.  Might go infinite, underflow, etc;
      who cares. */
 
-  /* If this format uses a hidden bit, explicitly add it in now.  Otherwise,
-     increment the exponent by one to account for the integer bit.  */
-
-  if (!special_exponent)
+  /* For denorms use minimum exponent.  */
+  if (exponent == 0)
+    exponent = 1 - fmt->exp_bias;
+  else
     {
+      exponent -= fmt->exp_bias;
+
+      /* If this format uses a hidden bit, explicitly add it in now.
+	 Otherwise, increment the exponent by one to account for the
+	 integer bit.  */
+
       if (fmt->intbit == floatformat_intbit_no)
 	dto = ldexp (1.0, exponent);
       else
@@ -530,18 +529,8 @@ floatformat_to_double (const struct floatformat *fmt,
       mant = get_field (ufrom, fmt->byteorder, fmt->totalsize,
 			 mant_off, mant_bits);
 
-      /* Handle denormalized numbers.  FIXME: What should we do for
-	 non-IEEE formats?  */
-      if (special_exponent && exponent == 0 && mant != 0)
-	dto += ldexp ((double)mant,
-		      (- fmt->exp_bias
-		       - mant_bits
-		       - (mant_off - fmt->man_start)
-		       + 1));
-      else
-	dto += ldexp ((double)mant, exponent - mant_bits);
-      if (exponent != 0)
-	exponent -= mant_bits;
+      dto += ldexp ((double) mant, exponent - mant_bits);
+      exponent -= mant_bits;
       mant_off += mant_bits;
       mant_bits_left -= mant_bits;
     }
@@ -756,6 +745,7 @@ main (void)
 {
   ieee_test (0.0);
   ieee_test (0.5);
+  ieee_test (1.1);
   ieee_test (256.0);
   ieee_test (0.12345);
   ieee_test (234235.78907234);
-- 
1.7.11.5


-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."