compiler-rt/lib/builtins/floattisf.c

   1 //===-- floattisf.c - Implement __floattisf -------------------------------===//
   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 __floattisf 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 //             ti_int is a 128 bit integral type
  21
  22 // seee eeee emmm mmmm mmmm mmmm mmmm mmmm
  23
  24 COMPILER_RT_ABI float __floattisf(ti_int a) {
  25   if (a == 0)
  26     return 0.0F;
  27   const unsigned N = sizeof(ti_int) * CHAR_BIT;
  28   const ti_int s = a >> (N - 1);
  29   a = (a ^ s) - s;
  30   int sd = N - __clzti2(a); // number of significant digits
  31   si_int e = sd - 1;        // exponent
  32   if (sd > FLT_MANT_DIG) {
  33     //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  34     // finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  35     //                                                12345678901234567890123456
  36     //  1 = msb 1 bit
  37     //  P = bit FLT_MANT_DIG-1 bits to the right of 1
  38     //  Q = bit FLT_MANT_DIG bits to the right of 1
  39     //  R = "or" of all bits to the right of Q
  40     switch (sd) {
  41     case FLT_MANT_DIG + 1:
  42       a <<= 1;
  43       break;
  44     case FLT_MANT_DIG + 2:
  45       break;
  46     default:
  47       a = ((tu_int)a >> (sd - (FLT_MANT_DIG + 2))) |
  48           ((a & ((tu_int)(-1) >> ((N + FLT_MANT_DIG + 2) - sd))) != 0);
  49     };
  50     // finish:
  51     a |= (a & 4) != 0; // Or P into R
  52     ++a;               // round - this step may add a significant bit
  53     a >>= 2;           // dump Q and R
  54     // a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits
  55     if (a & ((tu_int)1 << FLT_MANT_DIG)) {
  56       a >>= 1;
  57       ++e;
  58     }
  59     // a is now rounded to FLT_MANT_DIG bits
  60   } else {
  61     a <<= (FLT_MANT_DIG - sd);
  62     // a is now rounded to FLT_MANT_DIG bits
  63   }
  64   float_bits fb;
  65   fb.u = ((su_int)s & 0x80000000) | // sign
  66          ((e + 127) << 23) |        // exponent
  67          ((su_int)a & 0x007FFFFF);  // mantissa
  68   return fb.f;
  69 }
  70
  71 #endif // CRT_HAS_128BIT