diff options
Diffstat (limited to 'libc-top-half/musl/src/math/powf.c')
| -rw-r--r-- | libc-top-half/musl/src/math/powf.c | 185 |
1 files changed, 185 insertions, 0 deletions
diff --git a/libc-top-half/musl/src/math/powf.c b/libc-top-half/musl/src/math/powf.c new file mode 100644 index 0000000..de8fab5 --- /dev/null +++ b/libc-top-half/musl/src/math/powf.c @@ -0,0 +1,185 @@ +/* + * Copyright (c) 2017-2018, Arm Limited. + * SPDX-License-Identifier: MIT + */ + +#include <math.h> +#include <stdint.h> +#include "libm.h" +#include "exp2f_data.h" +#include "powf_data.h" + +/* +POWF_LOG2_POLY_ORDER = 5 +EXP2F_TABLE_BITS = 5 + +ULP error: 0.82 (~ 0.5 + relerr*2^24) +relerr: 1.27 * 2^-26 (Relative error ~= 128*Ln2*relerr_log2 + relerr_exp2) +relerr_log2: 1.83 * 2^-33 (Relative error of logx.) +relerr_exp2: 1.69 * 2^-34 (Relative error of exp2(ylogx).) +*/ + +#define N (1 << POWF_LOG2_TABLE_BITS) +#define T __powf_log2_data.tab +#define A __powf_log2_data.poly +#define OFF 0x3f330000 + +/* Subnormal input is normalized so ix has negative biased exponent. + Output is multiplied by N (POWF_SCALE) if TOINT_INTRINICS is set. */ +static inline double_t log2_inline(uint32_t ix) +{ + double_t z, r, r2, r4, p, q, y, y0, invc, logc; + uint32_t iz, top, tmp; + int k, i; + + /* x = 2^k z; where z is in range [OFF,2*OFF] and exact. + The range is split into N subintervals. + The ith subinterval contains z and c is near its center. */ + tmp = ix - OFF; + i = (tmp >> (23 - POWF_LOG2_TABLE_BITS)) % N; + top = tmp & 0xff800000; + iz = ix - top; + k = (int32_t)top >> (23 - POWF_SCALE_BITS); /* arithmetic shift */ + invc = T[i].invc; + logc = T[i].logc; + z = (double_t)asfloat(iz); + + /* log2(x) = log1p(z/c-1)/ln2 + log2(c) + k */ + r = z * invc - 1; + y0 = logc + (double_t)k; + + /* Pipelined polynomial evaluation to approximate log1p(r)/ln2. */ + r2 = r * r; + y = A[0] * r + A[1]; + p = A[2] * r + A[3]; + r4 = r2 * r2; + q = A[4] * r + y0; + q = p * r2 + q; + y = y * r4 + q; + return y; +} + +#undef N +#undef T +#define N (1 << EXP2F_TABLE_BITS) +#define T __exp2f_data.tab +#define SIGN_BIAS (1 << (EXP2F_TABLE_BITS + 11)) + +/* The output of log2 and thus the input of exp2 is either scaled by N + (in case of fast toint intrinsics) or not. The unscaled xd must be + in [-1021,1023], sign_bias sets the sign of the result. */ +static inline float exp2_inline(double_t xd, uint32_t sign_bias) +{ + uint64_t ki, ski, t; + double_t kd, z, r, r2, y, s; + +#if TOINT_INTRINSICS +#define C __exp2f_data.poly_scaled + /* N*x = k + r with r in [-1/2, 1/2] */ + kd = roundtoint(xd); /* k */ + ki = converttoint(xd); +#else +#define C __exp2f_data.poly +#define SHIFT __exp2f_data.shift_scaled + /* x = k/N + r with r in [-1/(2N), 1/(2N)] */ + kd = eval_as_double(xd + SHIFT); + ki = asuint64(kd); + kd -= SHIFT; /* k/N */ +#endif + r = xd - kd; + + /* exp2(x) = 2^(k/N) * 2^r ~= s * (C0*r^3 + C1*r^2 + C2*r + 1) */ + t = T[ki % N]; + ski = ki + sign_bias; + t += ski << (52 - EXP2F_TABLE_BITS); + s = asdouble(t); + z = C[0] * r + C[1]; + r2 = r * r; + y = C[2] * r + 1; + y = z * r2 + y; + y = y * s; + return eval_as_float(y); +} + +/* Returns 0 if not int, 1 if odd int, 2 if even int. The argument is + the bit representation of a non-zero finite floating-point value. */ +static inline int checkint(uint32_t iy) +{ + int e = iy >> 23 & 0xff; + if (e < 0x7f) + return 0; + if (e > 0x7f + 23) + return 2; + if (iy & ((1 << (0x7f + 23 - e)) - 1)) + return 0; + if (iy & (1 << (0x7f + 23 - e))) + return 1; + return 2; +} + +static inline int zeroinfnan(uint32_t ix) +{ + return 2 * ix - 1 >= 2u * 0x7f800000 - 1; +} + +float powf(float x, float y) +{ + uint32_t sign_bias = 0; + uint32_t ix, iy; + + ix = asuint(x); + iy = asuint(y); + if (predict_false(ix - 0x00800000 >= 0x7f800000 - 0x00800000 || + zeroinfnan(iy))) { + /* Either (x < 0x1p-126 or inf or nan) or (y is 0 or inf or nan). */ + if (predict_false(zeroinfnan(iy))) { + if (2 * iy == 0) + return issignalingf_inline(x) ? x + y : 1.0f; + if (ix == 0x3f800000) + return issignalingf_inline(y) ? x + y : 1.0f; + if (2 * ix > 2u * 0x7f800000 || + 2 * iy > 2u * 0x7f800000) + return x + y; + if (2 * ix == 2 * 0x3f800000) + return 1.0f; + if ((2 * ix < 2 * 0x3f800000) == !(iy & 0x80000000)) + return 0.0f; /* |x|<1 && y==inf or |x|>1 && y==-inf. */ + return y * y; + } + if (predict_false(zeroinfnan(ix))) { + float_t x2 = x * x; + if (ix & 0x80000000 && checkint(iy) == 1) + x2 = -x2; + /* Without the barrier some versions of clang hoist the 1/x2 and + thus division by zero exception can be signaled spuriously. */ + return iy & 0x80000000 ? fp_barrierf(1 / x2) : x2; + } + /* x and y are non-zero finite. */ + if (ix & 0x80000000) { + /* Finite x < 0. */ + int yint = checkint(iy); + if (yint == 0) + return __math_invalidf(x); + if (yint == 1) + sign_bias = SIGN_BIAS; + ix &= 0x7fffffff; + } + if (ix < 0x00800000) { + /* Normalize subnormal x so exponent becomes negative. */ + ix = asuint(x * 0x1p23f); + ix &= 0x7fffffff; + ix -= 23 << 23; + } + } + double_t logx = log2_inline(ix); + double_t ylogx = y * logx; /* cannot overflow, y is single prec. */ + if (predict_false((asuint64(ylogx) >> 47 & 0xffff) >= + asuint64(126.0 * POWF_SCALE) >> 47)) { + /* |y*log(x)| >= 126. */ + if (ylogx > 0x1.fffffffd1d571p+6 * POWF_SCALE) + return __math_oflowf(sign_bias); + if (ylogx <= -150.0 * POWF_SCALE) + return __math_uflowf(sign_bias); + } + return exp2_inline(ylogx, sign_bias); +} |