lib/builtins/ppc/fixtfdi.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

// int64_t __fixunstfdi(long double x);
// This file implements the PowerPC 128-bit double-double -> int64_t conversion

#include "../int_math.h"
#include "DD.h"

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

  const uint32_t absHighWord =
      (uint32_t)(hibits.x >> 32) & UINT32_C(0x7fffffff);
  const uint32_t absHighWordMinusOne = absHighWord - UINT32_C(0x3ff00000);

  // If (1.0 - tiny) <= input < 0x1.0p63:
  if (UINT32_C(0x03f00000) > absHighWordMinusOne) {
    // Do an unsigned conversion of the absolute value, then restore the sign.
    const int unbiasedHeadExponent = absHighWordMinusOne >> 20;

    int64_t result = hibits.x & INT64_C(0x000fffffffffffff); // mantissa(hi)
    result |= INT64_C(0x0010000000000000); // matissa(hi) with implicit bit
    result <<= 10; // mantissa(hi) with one zero preceding bit.

    const int64_t hiNegationMask = ((int64_t)(hibits.x)) >> 63;

    // 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|
      // We need to negate it if head and tail have different signs.
      const int64_t loNegationMask = ((int64_t)(lobits.x)) >> 63;
      const int64_t negationMask = loNegationMask ^ hiNegationMask;
      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 10 that we shifted the head mantissa by.
      tailMantissa >>=
          (unbiasedHeadExponent - (biasedTailExponent - (1023 - 10)));

      result += tailMantissa;
    }

    result >>= (62 - unbiasedHeadExponent);

    // Restore the sign of the result and return
    result = (result ^ hiNegationMask) - hiNegationMask;
    return result;
  }

  // Edge cases handled here:

  // |x| < 1, result is zero.
  if (1.0 > crt_fabs(x.s.hi))
    return INT64_C(0);

  // x very close to INT64_MIN, care must be taken to see which side we are on.
  if (x.s.hi == -0x1.0p63) {

    int64_t result = INT64_MIN;

    if (0.0 < x.s.lo) {
      // If the tail is positive, the correct result is something other than
      // INT64_MIN. we'll need to figure out what it is.

      const doublebits lobits = {.d = x.s.lo};
      int64_t tailMantissa = lobits.x & INT64_C(0x000fffffffffffff);
      tailMantissa |= INT64_C(0x0010000000000000);

      // Now we negate the tailMantissa
      tailMantissa = (tailMantissa ^ INT64_C(-1)) + INT64_C(1);

      // And shift it by the appropriate amount
      const int biasedTailExponent = (int)(lobits.x >> 52) & 0x7ff;
      tailMantissa >>= 1075 - biasedTailExponent;

      result -= tailMantissa;
    }

    return result;
  }

  // Signed overflows, infinities, and NaNs
  if (x.s.hi > 0.0)
    return INT64_MAX;
  else
    return INT64_MIN;
}