compiler-rt/lib/builtins/floatuntisf.c

   1 //===-- floatuntisf.c - Implement __floatuntisf ---------------------------===//
   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 __floatuntisf 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 float, rounding toward even.
  18
  19 // Assumption: float is a IEEE 32 bit floating point type
  20 //             tu_int is a 128 bit integral type
  21
  22 // seee eeee emmm mmmm mmmm mmmm mmmm mmmm
  23
  24 COMPILER_RT_ABI float __floatuntisf(tu_int a) {
  25   if (a == 0)
  26     return 0.0F;
  27   const unsigned N = sizeof(tu_int) * CHAR_BIT;
  28   int sd = N - __clzti2(a); // number of significant digits
  29   int e = sd - 1;           // exponent
  30   if (sd > FLT_MANT_DIG) {
  31     //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  32     //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  33     //                                                12345678901234567890123456
  34     //  1 = msb 1 bit
  35     //  P = bit FLT_MANT_DIG-1 bits to the right of 1
  36     //  Q = bit FLT_MANT_DIG bits to the right of 1
  37     //  R = "or" of all bits to the right of Q
  38     switch (sd) {
  39     case FLT_MANT_DIG + 1:
  40       a <<= 1;
  41       break;
  42     case FLT_MANT_DIG + 2:
  43       break;
  44     default:
  45       a = (a >> (sd - (FLT_MANT_DIG + 2))) |
  46           ((a & ((tu_int)(-1) >> ((N + FLT_MANT_DIG + 2) - sd))) != 0);
  47     };
  48     // finish:
  49     a |= (a & 4) != 0; // Or P into R
  50     ++a;               // round - this step may add a significant bit
  51     a >>= 2;           // dump Q and R
  52     // a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits
  53     if (a & ((tu_int)1 << FLT_MANT_DIG)) {
  54       a >>= 1;
  55       ++e;
  56     }
  57     // a is now rounded to FLT_MANT_DIG bits
  58   } else {
  59     a <<= (FLT_MANT_DIG - sd);
  60     // a is now rounded to FLT_MANT_DIG bits
  61   }
  62   float_bits fb;
  63   fb.u = ((e + 127) << 23) |       // exponent
  64          ((su_int)a & 0x007FFFFF); // mantissa
  65   return fb.f;
  66 }
  67
  68 #endif // CRT_HAS_128BIT