compiler-rt/lib/builtins/floattixf.c

   1 //===-- floattixf.c - Implement __floattixf -------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements __floattixf for the compiler_rt library.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "int_lib.h"
  14
  15 #ifdef CRT_HAS_128BIT
  16
  17 // Returns: convert a to a long double, rounding toward even.
  18
  19 // Assumption: long double is a IEEE 80 bit floating point type padded to 128
  20 // bits ti_int is a 128 bit integral type
  21
  22 // gggg gggg gggg gggg gggg gggg gggg gggg | gggg gggg gggg gggg seee eeee eeee
  23 // eeee | 1mmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm
  24 // mmmm mmmm mmmm
  25
  26 COMPILER_RT_ABI long double __floattixf(ti_int a) {
  27   if (a == 0)
  28     return 0.0;
  29   const unsigned N = sizeof(ti_int) * CHAR_BIT;
  30   const ti_int s = a >> (N - 1);
  31   a = (a ^ s) - s;
  32   int sd = N - __clzti2(a); // number of significant digits
  33   int e = sd - 1;           // exponent
  34   if (sd > LDBL_MANT_DIG) {
  35     //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  36     //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  37     //                                                12345678901234567890123456
  38     //  1 = msb 1 bit
  39     //  P = bit LDBL_MANT_DIG-1 bits to the right of 1
  40     //  Q = bit LDBL_MANT_DIG bits to the right of 1
  41     //  R = "or" of all bits to the right of Q
  42     switch (sd) {
  43     case LDBL_MANT_DIG + 1:
  44       a <<= 1;
  45       break;
  46     case LDBL_MANT_DIG + 2:
  47       break;
  48     default:
  49       a = ((tu_int)a >> (sd - (LDBL_MANT_DIG + 2))) |
  50           ((a & ((tu_int)(-1) >> ((N + LDBL_MANT_DIG + 2) - sd))) != 0);
  51     };
  52     // finish:
  53     a |= (a & 4) != 0; // Or P into R
  54     ++a;               // round - this step may add a significant bit
  55     a >>= 2;           // dump Q and R
  56     // a is now rounded to LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits
  57     if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
  58       a >>= 1;
  59       ++e;
  60     }
  61     // a is now rounded to LDBL_MANT_DIG bits
  62   } else {
  63     a <<= (LDBL_MANT_DIG - sd);
  64     // a is now rounded to LDBL_MANT_DIG bits
  65   }
  66   long_double_bits fb;
  67   fb.u.high.s.low = ((su_int)s & 0x8000) | // sign
  68                     (e + 16383);           // exponent
  69   fb.u.low.all = (du_int)a;                // mantissa
  70   return fb.f;
  71 }
  72
  73 #endif // CRT_HAS_128BIT