Cut out the gordian handling of subnormals by bit fiddling, and

instead use the FPU to convert subnormals to normals.  (NB: Further
simplification is possible, such as using the FPU for the rounding
step.)

This fixes a bug reported by stefanf where long double subnormals in
the Intel 80-bit format would be output with one fewer digit than
necessary when the default precision was used.

1 files changed, 15 insertions, 128 deletions

diff --git a/lib/libc/gdtoa/_hdtoa.c b/lib/libc/gdtoa/_hdtoa.c
index deb6582..ff7ed55 100644
--- a/lib/libc/gdtoa/_hdtoa.c
+++ b/lib/libc/gdtoa/_hdtoa.c
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2004 David Schultz <das@FreeBSD.ORG>
+ * Copyright (c) 2004, 2005 David Schultz <das@FreeBSD.ORG>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -28,10 +28,8 @@
 __FBSDID("$FreeBSD$");
 
 #include <float.h>
-#include <inttypes.h>
 #include <limits.h>
 #include <math.h>
-#include <stdlib.h>
 #include "fpmath.h"
 #include "gdtoaimp.h"
 
@@ -39,51 +37,8 @@ __FBSDID("$FreeBSD$");
 #define	INFSTR	"Infinity"
 #define	NANSTR	"NaN"
 
-#define	DBL_BIAS	(DBL_MAX_EXP - 1)
-#define	LDBL_BIAS	(LDBL_MAX_EXP - 1)
-
-#ifdef	LDBL_IMPLICIT_NBIT
-#define	LDBL_NBIT_ADJ	0
-#else
-#define	LDBL_NBIT_ADJ	1
-#endif
-
-/*
- * Efficiently compute the log2 of an integer.  Uses a combination of
- * arcane tricks found in fortune and arcane tricks not (yet) in
- * fortune.  This routine behaves similarly to fls(9).
- */
-static int
-log2_32(uint32_t n)
-{
-
-        n |= (n >> 1);
-        n |= (n >> 2);
-        n |= (n >> 4);
-        n |= (n >> 8);
-        n |= (n >> 16);
-
-	n = (n & 0x55555555) + ((n & 0xaaaaaaaa) >> 1);
-	n = (n & 0x33333333) + ((n & 0xcccccccc) >> 2);
-	n = (n & 0x0f0f0f0f) + ((n & 0xf0f0f0f0) >> 4);
-	n = (n & 0x00ff00ff) + ((n & 0xff00ff00) >> 8);
-	n = (n & 0x0000ffff) + ((n & 0xffff0000) >> 16);
-	return (n - 1);
-}
-
-#if (LDBL_MANH_SIZE > 32 || LDBL_MANL_SIZE > 32)
-
-static int
-log2_64(uint64_t n)
-{
-
-	if (n >> 32 != 0)
-		return (log2_32((uint32_t)(n >> 32)) + 32);
-	else
-		return (log2_32((uint32_t)n));
-}
-
-#endif	/* (LDBL_MANH_SIZE > 32 || LDBL_MANL_SIZE > 32) */
+#define	DBL_ADJ		(DBL_MAX_EXP - 2 + ((DBL_MANT_DIG - 1) % 4))
+#define	LDBL_ADJ	(LDBL_MAX_EXP - 2 + ((LDBL_MANT_DIG - 1) % 4))
 
 /*
  * Round up the given digit string.  If the digit string is fff...f,
@@ -168,46 +123,24 @@ char *
 __hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign,
     char **rve)
 {
+	static const int sigfigs = (DBL_MANT_DIG + 3) / 4;
 	union IEEEd2bits u;
 	char *s, *s0;
 	int bufsize;
-	int impnbit;	/* implicit normalization bit */
-	int pos;
-	int shift;	/* for subnormals, # of shifts required to normalize */
-	int sigfigs;	/* number of significant hex figures in result */
 
 	u.d = d;
 	*sign = u.bits.sign;
 
 	switch (fpclassify(d)) {
 	case FP_NORMAL:
-		sigfigs = (DBL_MANT_DIG + 3) / 4;
-		impnbit = 1 << ((DBL_MANT_DIG - 1) % 4);
-		*decpt = u.bits.exp - DBL_BIAS + 1 -
-		    ((DBL_MANT_DIG - 1) % 4);
+		*decpt = u.bits.exp - DBL_ADJ;
 		break;
 	case FP_ZERO:
 		*decpt = 1;
 		return (nrv_alloc("0", rve, 1));
 	case FP_SUBNORMAL:
-		/*
-		 * The position of the highest-order bit tells us by
-		 * how much to adjust the exponent (decpt).  The
-		 * adjustment is raised to the next nibble boundary
-		 * since we will later choose the leftmost hexadecimal
-		 * digit so that all subsequent digits align on nibble
-		 * boundaries.
-		 */
-		if (u.bits.manh != 0) {
-			pos = log2_32(u.bits.manh);
-			shift = DBL_MANH_SIZE - pos;
-		} else {
-			pos = log2_32(u.bits.manl);
-			shift = DBL_MANH_SIZE + DBL_MANL_SIZE - pos;
-		}
-		sigfigs = (3 + DBL_MANT_DIG - shift) / 4;
-		impnbit = 0;
-		*decpt = DBL_MIN_EXP - ((shift + 3) & ~(4 - 1));
+		u.d *= 0x1p514;
+		*decpt = u.bits.exp - (514 + DBL_ADJ);
 		break;
 	case FP_INFINITE:
 		*decpt = INT_MAX;
@@ -254,11 +187,9 @@ __hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign,
 	 * At this point, we have snarfed all the bits in the
 	 * mantissa, with the possible exception of the highest-order
 	 * (partial) nibble, which is dealt with by the next
-	 * statement.  That nibble is usually in manh, but it could be
-	 * in manl instead for small subnormals.  We also tack on the
-	 * implicit normalization bit if appropriate.
+	 * statement.  We also tack on the implicit normalization bit.
 	 */
-	*s = u.bits.manh | u.bits.manl | impnbit;
+	*s = u.bits.manh | (1U << ((DBL_MANT_DIG - 1) % 4));
 
 	/* If ndigits < 0, we are expected to auto-size the precision. */
 	if (ndigits < 0) {
@@ -283,71 +214,29 @@ __hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign,
 
 /*
  * This is the long double version of __hdtoa().
- *
- * On architectures that have an explicit integer bit, unnormals and
- * pseudo-denormals cause problems in the conversion routine, so they
- * are ``fixed'' by effectively toggling the integer bit.  Although
- * this is not correct behavior, the hardware will not produce these
- * formats externally.
  */
 char *
 __hldtoa(long double e, const char *xdigs, int ndigits, int *decpt, int *sign,
     char **rve)
 {
+	static const int sigfigs = (LDBL_MANT_DIG + 3) / 4;
 	union IEEEl2bits u;
 	char *s, *s0;
 	int bufsize;
-	int impnbit;	/* implicit normalization bit */
-	int pos;
-	int shift;	/* for subnormals, # of shifts required to normalize */
-	int sigfigs;	/* number of significant hex figures in result */
 
 	u.e = e;
 	*sign = u.bits.sign;
 
 	switch (fpclassify(e)) {
 	case FP_NORMAL:
-		sigfigs = (LDBL_MANT_DIG + 3) / 4;
-		impnbit = 1 << ((LDBL_MANT_DIG - 1) % 4);
-		*decpt = u.bits.exp - LDBL_BIAS + 1 -
-		    ((LDBL_MANT_DIG - 1) % 4);
+		*decpt = u.bits.exp - LDBL_ADJ;
 		break;
 	case FP_ZERO:
 		*decpt = 1;
 		return (nrv_alloc("0", rve, 1));
 	case FP_SUBNORMAL:
-		/*
-		 * The position of the highest-order bit tells us by
-		 * how much to adjust the exponent (decpt).  The
-		 * adjustment is raised to the next nibble boundary
-		 * since we will later choose the leftmost hexadecimal
-		 * digit so that all subsequent digits align on nibble
-		 * boundaries.
-		 */
-#ifdef	LDBL_IMPLICIT_NBIT
-		/* Don't trust the normalization bit to be off. */
-		u.bits.manh &= ~(~0ULL << (LDBL_MANH_SIZE - 1));
-#endif
-		if (u.bits.manh != 0) {
-#if LDBL_MANH_SIZE > 32
-			pos = log2_64(u.bits.manh);
-#else
-			pos = log2_32(u.bits.manh);
-#endif
-			shift = LDBL_MANH_SIZE - LDBL_NBIT_ADJ - pos;
-		} else {
-#if LDBL_MANL_SIZE > 32
-			pos = log2_64(u.bits.manl);
-#else
-			pos = log2_32(u.bits.manl);
-#endif
-			shift = LDBL_MANH_SIZE + LDBL_MANL_SIZE -
-			    LDBL_NBIT_ADJ - pos;
-		}
-		sigfigs = (3 + LDBL_MANT_DIG - LDBL_NBIT_ADJ - shift) / 4;
-		*decpt = LDBL_MIN_EXP + LDBL_NBIT_ADJ -
-		    ((shift + 3) & ~(4 - 1));
-		impnbit = 0;
+		u.e *= 0x1p514L;
+		*decpt = u.bits.exp - (514 + LDBL_ADJ);
 		break;
 	case FP_INFINITE:
 		*decpt = INT_MAX;
@@ -394,11 +283,9 @@ __hldtoa(long double e, const char *xdigs, int ndigits, int *decpt, int *sign,
 	 * At this point, we have snarfed all the bits in the
 	 * mantissa, with the possible exception of the highest-order
 	 * (partial) nibble, which is dealt with by the next
-	 * statement.  That nibble is usually in manh, but it could be
-	 * in manl instead for small subnormals.  We also tack on the
-	 * implicit normalization bit if appropriate.
+	 * statement.  We also tack on the implicit normalization bit.
 	 */
-	*s = u.bits.manh | u.bits.manl | impnbit;
+	*s = u.bits.manh | (1U << ((LDBL_MANT_DIG - 1) % 4));
 
 	/* If ndigits < 0, we are expected to auto-size the precision. */
 	if (ndigits < 0) {