simplify scalbn*.c implementations

The old scalbn.c was wrong and slow, the new one is just slow.
(scalbn(0x1p+1023,-2097) should give 0x1p-1074, but the old code gave 0)

1 files changed, 22 insertions, 49 deletions

diff --git a/src/math/scalbnl.c b/src/math/scalbnl.c
index 0ed5b7fd..a5d0adba 100644
--- a/src/math/scalbnl.c
+++ b/src/math/scalbnl.c
@@ -1,21 +1,3 @@
-/* origin: FreeBSD /usr/src/lib/msun/src/s_scalbnl.c */
-/*
- * ====================================================
- * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
- *
- * Developed at SunPro, a Sun Microsystems, Inc. business.
- * Permission to use, copy, modify, and distribute this
- * software is freely granted, provided that this notice
- * is preserved.
- * ====================================================
- */
-/*
- * scalbnl (long double x, int n)
- * scalbnl(x,n) returns x* 2**n  computed by  exponent
- * manipulation rather than by actually performing an
- * exponentiation or a multiplication.
- */
-
 #include "libm.h"
 
 #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
@@ -24,40 +6,31 @@ long double scalbnl(long double x, int n)
 	return scalbn(x, n);
 }
 #elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
-static const long double
-huge = 0x1p16000L,
-tiny = 0x1p-16000L;
-
 long double scalbnl(long double x, int n)
 {
-	union IEEEl2bits u;
-	int k;
+	union IEEEl2bits scale;
 
-	u.e = x;
-	k = u.bits.exp;                    /* extract exponent */
-	if (k == 0) {                      /* 0 or subnormal x */
-		if ((u.bits.manh|u.bits.manl) == 0)  /* +-0 */
-			return x;
-		u.e *= 0x1p128;
-		k = u.bits.exp - 128;
-		if (n < -50000)
-			return tiny*x;  /*underflow*/
-	}
-	if (k == 0x7fff)                   /* NaN or Inf */
-		return x + x;
-	k = k + n;
-	if (k >= 0x7fff)
-		return huge*copysignl(huge, x);  /* overflow  */
-	if (k > 0) {                       /* normal result */
-		u.bits.exp = k;
-		return u.e;
+	if (n > 16383) {
+		x *= 0x1p16383L;
+		n -= 16383;
+		if (n > 16383) {
+			x *= 0x1p16383L;
+			n -= 16383;
+			if (n > 16383)
+				return x * 0x1p16383L;
+		}
+	} else if (n < -16382) {
+		x *= 0x1p-16382L;
+		n += 16382;
+		if (n < -16382) {
+			x *= 0x1p-16382L;
+			n += 16382;
+			if (n < -16382)
+				return x * 0x1p-16382L;
+		}
 	}
-	if (k <= -128)
-		if (n > 50000)  /* in case integer overflow in n+k */
-			return huge*copysign(huge, x);  /*overflow*/
-		return tiny*copysign(tiny, x);  /*underflow*/
-	k += 128;                          /* subnormal result */
-	u.bits.exp = k;
-	return u.e*0x1p-128;
+	scale.e = 1.0L;
+	scale.bits.exp = 0x3fff + n;
+	return x * scale.e;
 }
 #endif