compiler-rt/lib/builtins/mulosi4.c

//===-- mulosi4.c - Implement __mulosi4 -----------------------------------===//
//
// 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 __mulosi4 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: a * b

// Effects: sets *overflow to 1  if a * b overflows

COMPILER_RT_ABI si_int __mulosi4(si_int a, si_int b, int *overflow) {
  const int N = (int)(sizeof(si_int) * CHAR_BIT);
  const si_int MIN = (si_int)1 << (N - 1);
  const si_int MAX = ~MIN;
  *overflow = 0;
  si_int result = a * b;
  if (a == MIN) {
    if (b != 0 && b != 1)
      *overflow = 1;
    return result;
  }
  if (b == MIN) {
    if (a != 0 && a != 1)
      *overflow = 1;
    return result;
  }
  si_int sa = a >> (N - 1);
  si_int abs_a = (a ^ sa) - sa;
  si_int sb = b >> (N - 1);
  si_int abs_b = (b ^ sb) - sb;
  if (abs_a < 2 || abs_b < 2)
    return result;
  if (sa == sb) {
    if (abs_a > MAX / abs_b)
      *overflow = 1;
  } else {
    if (abs_a > MIN / -abs_b)
      *overflow = 1;
  }
  return result;
}