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Diffstat (limited to 'lib/floattidf.c')
-rw-r--r-- | lib/floattidf.c | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/lib/floattidf.c b/lib/floattidf.c new file mode 100644 index 000000000..0f72b33f3 --- /dev/null +++ b/lib/floattidf.c @@ -0,0 +1,83 @@ +//===-- floattidf.c - Implement __floattidf -------------------------------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file implements __floattidf for the compiler_rt library. +// +//===----------------------------------------------------------------------===// + +#if __x86_64 + +#include "int_lib.h" +#include <float.h> + +// Returns: convert a to a double, rounding toward even. + +// Assumption: double is a IEEE 64 bit floating point type +// ti_int is a 128 bit integral type + +// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm + +si_int __clzti2(ti_int a); + +double +__floattidf(ti_int a) +{ + if (a == 0) + return 0.0; + const unsigned N = sizeof(ti_int) * CHAR_BIT; + const ti_int s = a >> (N-1); + a = (a ^ s) - s; + int sd = N - __clzti2(a); // number of significant digits + int e = sd - 1; // exponent + if (sd > DBL_MANT_DIG) + { + // start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx + // finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR + // 12345678901234567890123456 + // 1 = msb 1 bit + // P = bit DBL_MANT_DIG-1 bits to the right of 1 + // Q = bit DBL_MANT_DIG bits to the right of 1 + // R = "or" of all bits to the right of Q + switch (sd) + { + case DBL_MANT_DIG + 1: + a <<= 1; + break; + case DBL_MANT_DIG + 2: + break; + default: + a = ((tu_int)a >> (sd - (DBL_MANT_DIG+2))) | + ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0); + }; + // finish: + a |= (a & 4) != 0; // Or P into R + ++a; // round - this step may add a significant bit + a >>= 2; // dump Q and R + // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits + if (a & ((tu_int)1 << DBL_MANT_DIG)) + { + a >>= 1; + ++e; + } + // a is now rounded to DBL_MANT_DIG bits + } + else + { + a <<= (DBL_MANT_DIG - sd); + // a is now rounded to DBL_MANT_DIG bits + } + double_bits fb; + fb.u.high = ((su_int)s & 0x80000000) | // sign + ((e + 1023) << 20) | // exponent + ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high + fb.u.low = (su_int)a; // mantissa-low + return fb.f; +} + +#endif |