compiler-rt/lib/builtins/arm/dnan2.c

//===-- dnan2.c - Handle double-precision NaN inputs to binary operation --===//
//
// 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 helper function is available for use by double-precision float
// arithmetic implementations to handle propagating NaNs from the input
// operands to the output, in a way that matches Arm hardware FP.
//
// On input, a and b are floating-point numbers in IEEE 754 encoding, and at
// least one of them must be a NaN. The return value is the correct output NaN.
//
// A signalling NaN in the input (with bit 51 clear) takes priority over any
// quiet NaN, and is adjusted on return by setting bit 51 to make it quiet. If
// both inputs are the same type of NaN then the first input takes priority:
// the input a is used instead of b.
//
//===----------------------------------------------------------------------===//

#include <stdint.h>

uint64_t __compiler_rt_dnan2(uint64_t a, uint64_t b) {
  // Make shifted-left copies of a and b to discard the sign bit. Then add 1 at
  // the bit position where the quiet vs signalling bit ended up. This squashes
  // all the signalling NaNs to the top of the range of 64-bit values, from
  // 0xfff0000000000001 to 0xffffffffffffffff inclusive; meanwhile, all the
  // quiet NaN values wrap round to the bottom, from 0 to 0x000fffffffffffff
  // inclusive. So we can detect a signalling NaN by asking if it's greater
  // than 0xfff0000000000000, and a quiet one by asking if it's less than
  // 0x0010000000000000.
  uint64_t aadj = (a << 1) + 0x0010000000000000;
  uint64_t badj = (b << 1) + 0x0010000000000000;

  if (aadj > 0xfff0000000000000)   // a is a signalling NaN?
    return a | 0x0008000000000000; //   if so, return it with the quiet bit set
  if (badj > 0xfff0000000000000)   // b is a signalling NaN?
    return b | 0x0008000000000000; //   if so, return it with the quiet bit set
  if (aadj < 0x0010000000000000)   // a is a quiet NaN?
    return a;                      // if so, return it
  return b;                        // otherwise we expect b must be a quiet NaN
}