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

//===-- lib/builtins/ppc/floattitf.c - Convert int128->long double -*-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
//
//===----------------------------------------------------------------------===//
//
// This file implements converting a signed 128 bit integer to a 128bit IBM /
// PowerPC long double (double-double) value.
//
//===----------------------------------------------------------------------===//

#include <stdint.h>

// Conversions from signed and unsigned 64-bit int to long double.
long double __floatditf(int64_t);
long double __floatunditf(uint64_t);

// Convert a signed 128-bit integer to long double.
// This uses the following property:  Let hi and lo be 64-bits each,
// and let signed_val_k() and unsigned_val_k() be the value of the
// argument interpreted as a signed or unsigned k-bit integer. Then,
//
// signed_val_128(hi,lo) = signed_val_64(hi) * 2^64 + unsigned_val_64(lo)
// = (long double)hi * 2^64 + (long double)lo,
//
// where (long double)hi and (long double)lo are signed and
// unsigned 64-bit integer to long double conversions, respectively.
long double __floattitf(__int128_t arg) {
  // Split the int128 argument into 64-bit high and low int64 parts.
  int64_t ArgHiPart = (int64_t)(arg >> 64);
  uint64_t ArgLoPart = (uint64_t)arg;

  // Convert each 64-bit part into long double. The high part
  // must be a signed conversion and the low part an unsigned conversion
  // to ensure the correct result.
  long double ConvertedHiPart = __floatditf(ArgHiPart);
  long double ConvertedLoPart = __floatunditf(ArgLoPart);

  // The low bit of ArgHiPart corresponds to the 2^64 bit in arg.
  // Multiply the high part by 2^64 to undo the right shift by 64-bits
  // done in the splitting. Then, add to the low part to obtain the
  // final result.
  return ((ConvertedHiPart * 0x1.0p64) + ConvertedLoPart);
}