lib/mulosi4.c

   1 /*===-- mulosi4.c - Implement __mulosi4 -----------------------------------===
   2  *
   3  *                     The LLVM Compiler Infrastructure
   4  *
   5  * This file is dual licensed under the MIT and the University of Illinois Open
   6  * Source Licenses. See LICENSE.TXT for details.
   7  *
   8  * ===----------------------------------------------------------------------===
   9  *
  10  * This file implements __mulosi4 for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 /* Returns: a * b */
  18
  19 /* Effects: sets *overflow to 1  if a * b overflows */
  20
  21 si_int
  22 __mulosi4(si_int a, si_int b, int* overflow)
  23 {
  24     const int N = (int)(sizeof(si_int) * CHAR_BIT);
  25     const si_int MIN = (si_int)1 << (N-1);
  26     const si_int MAX = ~MIN;
  27     *overflow = 0;
  28     si_int result = a * b;
  29     if (a == MIN)
  30     {
  31         if (b != 0 && b != 1)
  32             *overflow = 1;
  33         return result;
  34     }
  35     if (b == MIN)
  36     {
  37         if (a != 0 && a != 1)
  38             *overflow = 1;
  39         return result;
  40     }
  41     si_int sa = a >> (N - 1);
  42     si_int abs_a = (a ^ sa) - sa;
  43     si_int sb = b >> (N - 1);
  44     si_int abs_b = (b ^ sb) - sb;
  45     if (abs_a < 2 || abs_b < 2)
  46         return result;
  47     if (sa == sb)
  48     {
  49         if (abs_a > MAX / abs_b)
  50             *overflow = 1;
  51     }
  52     else
  53     {
  54         if (abs_a > MIN / -abs_b)
  55             *overflow = 1;
  56     }
  57     return result;
  58 }