diff fmgen/e_expf.c @ 0:c55ea9478c80

Hello Gensokyo!
author Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
date Tue, 21 May 2013 10:29:21 +0200
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+/* e_expf.c -- float version of e_exp.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 <stdint.h>
+
+static const float
+one	= 1.0,
+halF[2]	= {0.5,-0.5,},
+ln2HI[2]   ={ 6.9313812256e-01,		/* 0x3f317180 */
+	     -6.9313812256e-01,},	/* 0xbf317180 */
+ln2LO[2]   ={ 9.0580006145e-06,  	/* 0x3717f7d1 */
+	     -9.0580006145e-06,},	/* 0xb717f7d1 */
+invln2 =  1.4426950216e+00, 		/* 0x3fb8aa3b */
+P1   =  1.6666667163e-01, /* 0x3e2aaaab */
+P2   = -2.7777778450e-03, /* 0xbb360b61 */
+P3   =  6.6137559770e-05, /* 0x388ab355 */
+P4   = -1.6533901999e-06, /* 0xb5ddea0e */
+P5   =  4.1381369442e-08; /* 0x3331bb4c */
+
+typedef union {
+  float value;
+  uint32_t word;
+} _f32;
+
+/* Get a 32 bit int from a float.  */
+#define GET_FLOAT_WORD(i,d)                                     \
+do {                                                            \
+  _f32 gf_u;                                   \
+  gf_u.value = (d);                                             \
+  (i) = gf_u.word;                                              \
+} while (0)
+
+/* Set a float from a 32 bit int.  */
+#define SET_FLOAT_WORD(d,i)                                     \
+do {                                                            \
+  _f32 sf_u;                                   \
+  sf_u.word = (i);                                              \
+  (d) = sf_u.value;                                             \
+} while (0)
+
+float expf(float x)	/* default IEEE double exp */
+{
+	float y,hi=0,lo=0,c,t;
+	int32_t k,xsb;
+	uint32_t hx;
+
+	GET_FLOAT_WORD(hx,x);
+	xsb = (hx>>31)&1;		/* sign bit of x */
+	hx &= 0x7fffffff;		/* high word of |x| */
+
+    /* filter out non-finite argument */
+	if(hx >= 0x42b17218) {			/* if |x|>=88.721... */
+	    if(hx>0x7f800000)
+            return x+x;	 		/* NaN */
+        if(hx==0x7f800000)
+		    return (xsb==0)? x:0.0f;    /* exp(+-inf)={inf,0} */
+	}
+
+    /* argument reduction */
+	if(hx > 0x3eb17218) {		/* if  |x| > 0.5 ln2 */ 
+	    if(hx < 0x3F851592) {	/* and |x| < 1.5 ln2 */
+		    hi = x-ln2HI[xsb]; lo=ln2LO[xsb]; k = 1-xsb-xsb;
+	    } else {
+		    k  = invln2*x+halF[xsb];
+		    t  = k;
+		    hi = x - t*ln2HI[0];	/* t*ln2HI is exact here */
+		    lo = t*ln2LO[0];
+	    }
+	    x  = hi - lo;
+	} 
+	else k = 0;
+
+    /* x is now in primary range */
+	t  = x*x;
+	c  = x - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
+	if(k==0) 	return one-((x*c)/(c-(float)2.0)-x); 
+	else 		y = one-((lo-(x*c)/((float)2.0-c))-hi);
+	{
+	    uint32_t hy;
+	    GET_FLOAT_WORD(hy,y);
+	    SET_FLOAT_WORD(y,hy+(k<<23));	/* add k to y's exponent */
+	    return y;
+	}
+}
+