compiler-rt/lib/builtins/ppc/fixunstfdi.c

   1 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   2 // See https://llvm.org/LICENSE.txt for license information.
   3 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   4
   5 // uint64_t __fixunstfdi(long double x);
   6 // This file implements the PowerPC 128-bit double-double -> uint64_t conversion
   7
   8 #include "DD.h"
   9
  10 uint64_t __fixunstfdi(long double input) {
  11   const DD x = {.ld = input};
  12   const doublebits hibits = {.d = x.s.hi};
  13
  14   const uint32_t highWordMinusOne =
  15       (uint32_t)(hibits.x >> 32) - UINT32_C(0x3ff00000);
  16
  17   // If (1.0 - tiny) <= input < 0x1.0p64:
  18   if (UINT32_C(0x04000000) > highWordMinusOne) {
  19     const int unbiasedHeadExponent = highWordMinusOne >> 20;
  20
  21     uint64_t result = hibits.x & UINT64_C(0x000fffffffffffff); // mantissa(hi)
  22     result |= UINT64_C(0x0010000000000000); // matissa(hi) with implicit bit
  23     result <<= 11; // mantissa(hi) left aligned in the int64 field.
  24
  25     // If the tail is non-zero, we need to patch in the tail bits.
  26     if (0.0 != x.s.lo) {
  27       const doublebits lobits = {.d = x.s.lo};
  28       int64_t tailMantissa = lobits.x & INT64_C(0x000fffffffffffff);
  29       tailMantissa |= INT64_C(0x0010000000000000);
  30
  31       // At this point we have the mantissa of |tail|
  32
  33       const int64_t negationMask = ((int64_t)(lobits.x)) >> 63;
  34       tailMantissa = (tailMantissa ^ negationMask) - negationMask;
  35
  36       // Now we have the mantissa of tail as a signed 2s-complement integer
  37
  38       const int biasedTailExponent = (int)(lobits.x >> 52) & 0x7ff;
  39
  40       // Shift the tail mantissa into the right position, accounting for the
  41       // bias of 11 that we shifted the head mantissa by.
  42       tailMantissa >>=
  43           (unbiasedHeadExponent - (biasedTailExponent - (1023 - 11)));
  44
  45       result += tailMantissa;
  46     }
  47
  48     result >>= (63 - unbiasedHeadExponent);
  49     return result;
  50   }
  51
  52   // Edge cases are handled here, with saturation.
  53   if (1.0 > x.s.hi)
  54     return UINT64_C(0);
  55   else
  56     return UINT64_MAX;
  57 }