math: rewrite rounding functions (ceil, floor, trunc, round, rint)

* faster, smaller, cleaner implementation than the bit hacks of fdlibm
* use arithmetics like y=(double)(x+0x1p52)-0x1p52, which is an integer
neighbor of x in all rounding modes (0<=x<0x1p52) and only use bithacks
when that's faster and smaller (for float it usually is)
* the code assumes standard excess precision handling for casts
* long double code supports both ld80 and ld128
* nearbyint is not changed (it is a wrapper around rint)

1 files changed, 13 insertions, 57 deletions

diff --git a/src/math/trunc.c b/src/math/trunc.c
index 44b04ecc..d13711b5 100644
--- a/src/math/trunc.c
+++ b/src/math/trunc.c
@@ -1,63 +1,19 @@
-/* origin: FreeBSD /usr/src/lib/msun/src/s_trunc.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.
- * ====================================================
- */
-/*
- * trunc(x)
- * Return x rounded toward 0 to integral value
- * Method:
- *      Bit twiddling.
- * Exception:
- *      Inexact flag raised if x not equal to trunc(x).
- */
-
 #include "libm.h"
 
-static const double huge = 1.0e300;
-
 double trunc(double x)
 {
-	int32_t i0,i1,j0;
-	uint32_t i;
+	union {double f; uint64_t i;} u = {x};
+	int e = (int)(u.i >> 52 & 0x7ff) - 0x3ff + 12;
+	uint64_t m;
 
-	EXTRACT_WORDS(i0, i1, x);
-	j0 = ((i0>>20)&0x7ff) - 0x3ff;
-	if (j0 < 20) {
-		if (j0 < 0) { /* |x|<1, return 0*sign(x) */
-			/* raise inexact if x != 0 */
-			if (huge+x > 0.0) {
-				i0 &= 0x80000000U;
-				i1 = 0;
-			}
-		} else {
-			i = 0x000fffff>>j0;
-			if (((i0&i)|i1) == 0)
-				return x; /* x is integral */
-			/* raise inexact */
-			if (huge+x > 0.0) {
-				i0 &= ~i;
-				i1 = 0;
-			}
-		}
-	} else if (j0 > 51) {
-		if (j0 == 0x400)
-			return x + x;  /* inf or NaN */
-		return x;              /* x is integral */
-	} else {
-		i = (uint32_t)0xffffffff>>(j0-20);
-		if ((i1&i) == 0)
-			return x;      /* x is integral */
-		/* raise inexact */
-		if (huge+x > 0.0)
-			i1 &= ~i;
-	}
-	INSERT_WORDS(x, i0, i1);
-	return x;
+	if (e >= 52 + 12)
+		return x;
+	if (e < 12)
+		e = 1;
+	m = -1ULL >> e;
+	if ((u.i & m) == 0)
+		return x;
+	FORCE_EVAL(x + 0x1p120f);
+	u.i &= ~m;
+	return u.f;
 }