diff options
| -rw-r--r-- | src/math/sqrtl.c | 254 |
1 files changed, 253 insertions, 1 deletions
diff --git a/src/math/sqrtl.c b/src/math/sqrtl.c index 83a8f80c..1b9f19c7 100644 --- a/src/math/sqrtl.c +++ b/src/math/sqrtl.c @@ -1,7 +1,259 @@ +#include <stdint.h> #include <math.h> +#include <float.h> +#include "libm.h" +#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024 long double sqrtl(long double x) { - /* FIXME: implement in C, this is for LDBL_MANT_DIG == 64 only */ return sqrt(x); } +#elif (LDBL_MANT_DIG == 113 || LDBL_MANT_DIG == 64) && LDBL_MAX_EXP == 16384 +#include "sqrt_data.h" + +#define FENV_SUPPORT 1 + +typedef struct { + uint64_t hi; + uint64_t lo; +} u128; + +/* top: 16 bit sign+exponent, x: significand. */ +static inline long double mkldbl(uint64_t top, u128 x) +{ + union ldshape u; +#if LDBL_MANT_DIG == 113 + u.i2.hi = x.hi; + u.i2.lo = x.lo; + u.i2.hi &= 0x0000ffffffffffff; + u.i2.hi |= top << 48; +#elif LDBL_MANT_DIG == 64 + u.i.se = top; + u.i.m = x.lo; + /* force the top bit on non-zero (and non-subnormal) results. */ + if (top & 0x7fff) + u.i.m |= 0x8000000000000000; +#endif + return u.f; +} + +/* return: top 16 bit is sign+exp and following bits are the significand. */ +static inline u128 asu128(long double x) +{ + union ldshape u = {.f=x}; + u128 r; +#if LDBL_MANT_DIG == 113 + r.hi = u.i2.hi; + r.lo = u.i2.lo; +#elif LDBL_MANT_DIG == 64 + r.lo = u.i.m<<49; + /* ignore the top bit: pseudo numbers are not handled. */ + r.hi = u.i.m>>15; + r.hi &= 0x0000ffffffffffff; + r.hi |= (uint64_t)u.i.se << 48; +#endif + return r; +} + +/* returns a*b*2^-32 - e, with error 0 <= e < 1. */ +static inline uint32_t mul32(uint32_t a, uint32_t b) +{ + return (uint64_t)a*b >> 32; +} + +/* returns a*b*2^-64 - e, with error 0 <= e < 3. */ +static inline uint64_t mul64(uint64_t a, uint64_t b) +{ + uint64_t ahi = a>>32; + uint64_t alo = a&0xffffffff; + uint64_t bhi = b>>32; + uint64_t blo = b&0xffffffff; + return ahi*bhi + (ahi*blo >> 32) + (alo*bhi >> 32); +} + +static inline u128 add64(u128 a, uint64_t b) +{ + u128 r; + r.lo = a.lo + b; + r.hi = a.hi; + if (r.lo < a.lo) + r.hi++; + return r; +} + +static inline u128 add128(u128 a, u128 b) +{ + u128 r; + r.lo = a.lo + b.lo; + r.hi = a.hi + b.hi; + if (r.lo < a.lo) + r.hi++; + return r; +} + +static inline u128 sub64(u128 a, uint64_t b) +{ + u128 r; + r.lo = a.lo - b; + r.hi = a.hi; + if (a.lo < b) + r.hi--; + return r; +} + +static inline u128 sub128(u128 a, u128 b) +{ + u128 r; + r.lo = a.lo - b.lo; + r.hi = a.hi - b.hi; + if (a.lo < b.lo) + r.hi--; + return r; +} + +/* a<<n, 0 <= n <= 127 */ +static inline u128 lsh(u128 a, int n) +{ + if (n == 0) + return a; + if (n >= 64) { + a.hi = a.lo<<(n-64); + a.lo = 0; + } else { + a.hi = (a.hi<<n) | (a.lo>>(64-n)); + a.lo = a.lo<<n; + } + return a; +} + +/* a>>n, 0 <= n <= 127 */ +static inline u128 rsh(u128 a, int n) +{ + if (n == 0) + return a; + if (n >= 64) { + a.lo = a.hi>>(n-64); + a.hi = 0; + } else { + a.lo = (a.lo>>n) | (a.hi<<(64-n)); + a.hi = a.hi>>n; + } + return a; +} + +/* returns a*b exactly. */ +static inline u128 mul64_128(uint64_t a, uint64_t b) +{ + u128 r; + uint64_t ahi = a>>32; + uint64_t alo = a&0xffffffff; + uint64_t bhi = b>>32; + uint64_t blo = b&0xffffffff; + uint64_t lo1 = ((ahi*blo)&0xffffffff) + ((alo*bhi)&0xffffffff) + (alo*blo>>32); + uint64_t lo2 = (alo*blo)&0xffffffff; + r.hi = ahi*bhi + (ahi*blo>>32) + (alo*bhi>>32) + (lo1>>32); + r.lo = (lo1<<32) + lo2; + return r; +} + +/* returns a*b*2^-128 - e, with error 0 <= e < 7. */ +static inline u128 mul128(u128 a, u128 b) +{ + u128 hi = mul64_128(a.hi, b.hi); + uint64_t m1 = mul64(a.hi, b.lo); + uint64_t m2 = mul64(a.lo, b.hi); + return add64(add64(hi, m1), m2); +} + +/* returns a*b % 2^128. */ +static inline u128 mul128_tail(u128 a, u128 b) +{ + u128 lo = mul64_128(a.lo, b.lo); + lo.hi += a.hi*b.lo + a.lo*b.hi; + return lo; +} + + +/* see sqrt.c for detailed comments. */ + +long double sqrtl(long double x) +{ + u128 ix, ml; + uint64_t top; + + ix = asu128(x); + top = ix.hi >> 48; + if (predict_false(top - 0x0001 >= 0x7fff - 0x0001)) { + /* x < 0x1p-16382 or inf or nan. */ + if (2*ix.hi == 0 && ix.lo == 0) + return x; + if (ix.hi == 0x7fff000000000000 && ix.lo == 0) + return x; + if (top >= 0x7fff) + return __math_invalidl(x); + /* x is subnormal, normalize it. */ + ix = asu128(x * 0x1p112); + top = ix.hi >> 48; + top -= 112; + } + + /* x = 4^e m; with int e and m in [1, 4) */ + int even = top & 1; + ml = lsh(ix, 15); + ml.hi |= 0x8000000000000000; + if (even) ml = rsh(ml, 1); + top = (top + 0x3fff) >> 1; + + /* r ~ 1/sqrt(m) */ + static const uint64_t three = 0xc0000000; + uint64_t r, s, d, u, i; + i = (ix.hi >> 42) % 128; + r = (uint32_t)__rsqrt_tab[i] << 16; + /* |r sqrt(m) - 1| < 0x1p-8 */ + s = mul32(ml.hi>>32, r); + d = mul32(s, r); + u = three - d; + r = mul32(u, r) << 1; + /* |r sqrt(m) - 1| < 0x1.7bp-16, switch to 64bit */ + r = r<<32; + s = mul64(ml.hi, r); + d = mul64(s, r); + u = (three<<32) - d; + r = mul64(u, r) << 1; + /* |r sqrt(m) - 1| < 0x1.a5p-31 */ + s = mul64(u, s) << 1; + d = mul64(s, r); + u = (three<<32) - d; + r = mul64(u, r) << 1; + /* |r sqrt(m) - 1| < 0x1.c001p-59, switch to 128bit */ + + static const u128 threel = {.hi=three<<32, .lo=0}; + u128 rl, sl, dl, ul; + rl.hi = r; + rl.lo = 0; + sl = mul128(ml, rl); + dl = mul128(sl, rl); + ul = sub128(threel, dl); + sl = mul128(ul, sl); /* repr: 3.125 */ + /* -0x1p-116 < s - sqrt(m) < 0x3.8001p-125 */ + sl = rsh(sub64(sl, 4), 125-(LDBL_MANT_DIG-1)); + /* s < sqrt(m) < s + 1 ULP + tiny */ + + long double y; + u128 d2, d1, d0; + d0 = sub128(lsh(ml, 2*(LDBL_MANT_DIG-1)-126), mul128_tail(sl,sl)); + d1 = sub128(sl, d0); + d2 = add128(add64(sl, 1), d1); + sl = add64(sl, d1.hi >> 63); + y = mkldbl(top, sl); + if (FENV_SUPPORT) { + /* handle rounding modes and inexact exception. */ + top = predict_false((d2.hi|d2.lo)==0) ? 0 : 1; + top |= ((d1.hi^d2.hi)&0x8000000000000000) >> 48; + y += mkldbl(top, (u128){0}); + } + return y; +} +#else +#error unsupported long double format +#endif |