math: cleanup exp2.c exp2f.c and exp2l.c

* old code relied on sign extension on right shift
* exp2l ld64 wrapper was wrong
* use scalbn instead of bithacks

1 files changed, 22 insertions, 31 deletions

diff --git a/src/math/exp2.c b/src/math/exp2.c
index 08c21a66..8e252280 100644
--- a/src/math/exp2.c
+++ b/src/math/exp2.c
@@ -27,11 +27,9 @@
 
 #include "libm.h"
 
-#define TBLBITS 8
-#define TBLSIZE (1 << TBLBITS)
+#define TBLSIZE 256
 
 static const double
-huge  = 0x1p1000,
 redux = 0x1.8p52 / TBLSIZE,
 P1    = 0x1.62e42fefa39efp-1,
 P2    = 0x1.ebfbdff82c575p-3,
@@ -39,8 +37,6 @@ P3    = 0x1.c6b08d704a0a6p-5,
 P4    = 0x1.3b2ab88f70400p-7,
 P5    = 0x1.5d88003875c74p-10;
 
-static const volatile double twom1000 = 0x1p-1000;
-
 static const double tbl[TBLSIZE * 2] = {
 /*  exp2(z + eps)          eps     */
   0x1.6a09e667f3d5dp-1,  0x1.9880p-44,
@@ -334,25 +330,28 @@ static const double tbl[TBLSIZE * 2] = {
  */
 double exp2(double x)
 {
-	double r, t, twopk, twopkp1000, z;
-	uint32_t hx, ix, lx, i0;
-	int k;
+	double r, t, z;
+	uint32_t hx, ix, i0;
+	union {uint32_t u; int32_t i;} k;
 
 	/* Filter out exceptional cases. */
 	GET_HIGH_WORD(hx, x);
 	ix = hx & 0x7fffffff;
 	if (ix >= 0x40900000) {        /* |x| >= 1024 */
 		if (ix >= 0x7ff00000) {
-			GET_LOW_WORD(lx, x);
-			if (((ix & 0xfffff) | lx) != 0 || (hx & 0x80000000) == 0)
-				return x + x; /* x is NaN or +Inf */
-			else
-				return 0.0;   /* x is -Inf */
+			GET_LOW_WORD(ix, x);
+			if (hx == 0xfff00000 && ix == 0) /* -inf */
+				return 0;
+			return x;
+		}
+		if (x >= 1024) {
+			STRICT_ASSIGN(double, x, x * 0x1p1023);
+			return x;
+		}
+		if (x <= -1075) {
+			STRICT_ASSIGN(double, x, 0x1p-1000*0x1p-1000);
+			return x;
 		}
-		if (x >= 0x1.0p10)
-			return huge * huge; /* overflow */
-		if (x <= -0x1.0ccp10)
-			return twom1000 * twom1000; /* underflow */
 	} else if (ix < 0x3c900000) {  /* |x| < 0x1p-54 */
 		return 1.0 + x;
 	}
@@ -361,24 +360,16 @@ double exp2(double x)
 	STRICT_ASSIGN(double, t, x + redux);
 	GET_LOW_WORD(i0, t);
 	i0 += TBLSIZE / 2;
-	k = (i0 >> TBLBITS) << 20;
-	i0 = (i0 & (TBLSIZE - 1)) << 1;
+	k.u = i0 / TBLSIZE * TBLSIZE;
+	k.i /= TBLSIZE;
+	i0 %= TBLSIZE;
 	t -= redux;
 	z = x - t;
 
 	/* Compute r = exp2(y) = exp2t[i0] * p(z - eps[i]). */
-	t = tbl[i0];       /* exp2t[i0] */
-	z -= tbl[i0 + 1];  /* eps[i0]   */
-	if (k >= -1021 << 20)
-		INSERT_WORDS(twopk, 0x3ff00000 + k, 0);
-	else
-		INSERT_WORDS(twopkp1000, 0x3ff00000 + k + (1000 << 20), 0);
+	t = tbl[2*i0];       /* exp2t[i0] */
+	z -= tbl[2*i0 + 1];  /* eps[i0]   */
 	r = t + t * z * (P1 + z * (P2 + z * (P3 + z * (P4 + z * P5))));
 
-	/* Scale by 2**(k>>20). */
-	if (k < -1021 << 20)
-		return r * twopkp1000 * twom1000;
-	if (k == 1024 << 20)
-		return r * 2.0 * 0x1p1023;
-	return r * twopk;
+	return scalbn(r, k.i);
 }