llvm-project/compiler-rt/lib/builtins/mulvsi3.c

/* ===-- mulvsi3.c - Implement __mulvsi3 -----------------------------------===
 *
 * 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 __mulvsi3 for the compiler_rt library.
 *
 * ===----------------------------------------------------------------------===
 */

#include "int_lib.h"

/* Returns: a * b */

/* Effects: aborts if a * b overflows */

COMPILER_RT_ABI si_int
__mulvsi3(si_int a, si_int b)
{
    const int N = (int)(sizeof(si_int) * CHAR_BIT);
    const si_int MIN = (si_int)1 << (N-1);
    const si_int MAX = ~MIN;
    if (a == MIN)
    {
        if (b == 0 || b == 1)
            return a * b;
        compilerrt_abort();
    }
    if (b == MIN)
    {
        if (a == 0 || a == 1)
            return a * b;
        compilerrt_abort();
    }
    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 a * b;
    if (sa == sb)
    {
        if (abs_a > MAX / abs_b)
            compilerrt_abort();
    }
    else
    {
        if (abs_a > MIN / -abs_b)
            compilerrt_abort();
    }
    return a * b;
}