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, 48 deletions

diff --git a/src/math/scalbnf.c b/src/math/scalbnf.c
index 0a6168b0..243dafdf 100644
--- a/src/math/scalbnf.c
+++ b/src/math/scalbnf.c
@@ -1,54 +1,28 @@
-/* origin: FreeBSD /usr/src/lib/msun/src/s_scalbnf.c */
-/*
- * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
- */
-/*
- * ====================================================
- * 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.
- * ====================================================
- */
-
 #include "libm.h"
 
-static const float
-two25  = 3.355443200e+07,  /* 0x4c000000 */
-twom25 = 2.9802322388e-08, /* 0x33000000 */
-huge   = 1.0e+30,
-tiny   = 1.0e-30;
-
 float scalbnf(float x, int n)
 {
-	int32_t k, ix;
-	GET_FLOAT_WORD(ix, x);
-	k = (ix&0x7f800000)>>23;           /* extract exponent */
-	if (k == 0) {                      /* 0 or subnormal x */
-		if ((ix&0x7fffffff) == 0)  /* +-0 */
-			return x;
-		x *= two25;
-		GET_FLOAT_WORD(ix, x);
-		k = ((ix&0x7f800000)>>23) - 25;
-		if (n < -50000)
-			return tiny*x;  /*underflow*/
-	}
-	if (k == 0xff)                     /* NaN or Inf */
-		return x + x;
-	k = k + n;
-	if (k > 0xfe)
-		return huge*copysignf(huge, x);  /* overflow  */
-	if (k > 0) {                       /* normal result */
-		SET_FLOAT_WORD(x, (ix&0x807fffff)|(k<<23));
-		return x;
+	float scale;
+
+	if (n > 127) {
+		x *= 0x1p127f;
+		n -= 127;
+		if (n > 127) {
+			x *= 0x1p127f;
+			n -= 127;
+			if (n > 127)
+				return x * 0x1p127f;
+		}
+	} else if (n < -126) {
+		x *= 0x1p-126f;
+		n += 126;
+		if (n < -126) {
+			x *= 0x1p-126f;
+			n += 126;
+			if (n < -126)
+				return x * 0x1p-126f;
+		}
 	}
-	if (k <= -25)
-		if (n > 50000)  /* in case integer overflow in n+k */
-			return huge*copysignf(huge,x);  /*overflow*/
-		return tiny*copysignf(tiny, x);  /*underflow*/
-	k += 25;                           /* subnormal result */
-	SET_FLOAT_WORD(x, (ix&0x807fffff)|(k<<23));
-	return x*twom25;
+	SET_FLOAT_WORD(scale, (uint32_t)(0x7f+n)<<23);
+	return x * scale;
 }