diff options
Diffstat (limited to 'lib/msun/src/e_log2f.c')
| -rw-r--r-- | lib/msun/src/e_log2f.c | 51 |
1 files changed, 37 insertions, 14 deletions
diff --git a/lib/msun/src/e_log2f.c b/lib/msun/src/e_log2f.c index bb308d3..7166346 100644 --- a/lib/msun/src/e_log2f.c +++ b/lib/msun/src/e_log2f.c @@ -13,7 +13,7 @@ __FBSDID("$FreeBSD$"); /* - * Return the base 2 logarithm of x. See k_log.c for details on the algorithm. + * Float version of e_log2.c. See the latter for most comments. */ #include "math.h" @@ -30,29 +30,52 @@ static const float zero = 0.0; float __ieee754_log2f(float x) { - float f,hi,lo; + float f,hfsq,hi,lo,r,y; int32_t i,k,hx; GET_FLOAT_WORD(hx,x); - k=0; - if (hx < 0x00800000) { /* x < 2**-126 */ - if ((hx&0x7fffffff)==0) - return -two25/zero; /* log(+-0)=-inf */ - if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ - k -= 25; x *= two25; /* subnormal number, scale up x */ + k=0; + if (hx < 0x00800000) { /* x < 2**-126 */ + if ((hx&0x7fffffff)==0) + return -two25/zero; /* log(+-0)=-inf */ + if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ + k -= 25; x *= two25; /* subnormal number, scale up x */ GET_FLOAT_WORD(hx,x); - } + } if (hx >= 0x7f800000) return x+x; + if (hx == 0x3f800000) + return zero; /* log(1) = +0 */ k += (hx>>23)-127; hx &= 0x007fffff; i = (hx+(0x4afb0d))&0x800000; SET_FLOAT_WORD(x,hx|(i^0x3f800000)); /* normalize x or x/2 */ k += (i>>23); - f = __kernel_logf(x); - x = x - (float)1.0; - GET_FLOAT_WORD(hx,x); + y = (float)k; + f = x - (float)1.0; + hfsq = (float)0.5*f*f; + r = k_log1pf(f); + + /* + * We no longer need to avoid falling into the multi-precision + * calculations due to compiler bugs breaking Dekker's theorem. + * Keep avoiding this as an optimization. See e_log2.c for more + * details (some details are here only because the optimization + * is not yet available in double precision). + * + * Another compiler bug turned up. With gcc on i386, + * (ivln2lo + ivln2hi) would be evaluated in float precision + * despite runtime evaluations using double precision. So we + * must cast one of its terms to float_t. This makes the whole + * expression have type float_t, so return is forced to waste + * time clobbering its extra precision. + */ + if (sizeof(float_t) > sizeof(float)) + return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y; + + hi = f - hfsq; + GET_FLOAT_WORD(hx,hi); SET_FLOAT_WORD(hi,hx&0xfffff000); - lo = x - hi; - return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k; + lo = (f - hi) - hfsq + r; + return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y; } |
