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

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

// uint64_t __fixunstfdi(long double x);
// This file implements the PowerPC 128-bit double-double -> uint64_t conversion

#include "DD.h"

uint64_t __fixunstfdi(long double input) {
  const DD x = {.ld = input};
  const doublebits hibits = {.d = x.s.hi};

  const uint32_t highWordMinusOne =
      (uint32_t)(hibits.x >> 32) - UINT32_C(0x3ff00000);

  // If (1.0 - tiny) <= input < 0x1.0p64:
  if (UINT32_C(0x04000000) > highWordMinusOne) {
    const int unbiasedHeadExponent = highWordMinusOne >> 20;

    uint64_t result = hibits.x & UINT64_C(0x000fffffffffffff); // mantissa(hi)
    result |= UINT64_C(0x0010000000000000); // matissa(hi) with implicit bit
    result <<= 11; // mantissa(hi) left aligned in the int64 field.

    // If the tail is non-zero, we need to patch in the tail bits.
    if (0.0 != x.s.lo) {
      const doublebits lobits = {.d = x.s.lo};
      int64_t tailMantissa = lobits.x & INT64_C(0x000fffffffffffff);
      tailMantissa |= INT64_C(0x0010000000000000);

      // At this point we have the mantissa of |tail|

      const int64_t negationMask = ((int64_t)(lobits.x)) >> 63;
      tailMantissa = (tailMantissa ^ negationMask) - negationMask;

      // Now we have the mantissa of tail as a signed 2s-complement integer

      const int biasedTailExponent = (int)(lobits.x >> 52) & 0x7ff;

      // Shift the tail mantissa into the right position, accounting for the
      // bias of 11 that we shifted the head mantissa by.
      tailMantissa >>=
          (unbiasedHeadExponent - (biasedTailExponent - (1023 - 11)));

      result += tailMantissa;
    }

    result >>= (63 - unbiasedHeadExponent);
    return result;
  }

  // Edge cases are handled here, with saturation.
  if (1.0 > x.s.hi)
    return UINT64_C(0);
  else
    return UINT64_MAX;
}