zig/lib/std/special/compiler_rt/mulo.zig

const builtin = @import("builtin");

// mulo - multiplication overflow
// - muloXi4_generic for unoptimized version

// return a*b.
// return if a*b overflows => 1 else => 0
// see https://stackoverflow.com/a/26320664 for possible implementations

inline fn muloXi4_generic(comptime ST: type, a: ST, b: ST, overflow: *c_int) ST {
    @setRuntimeSafety(builtin.is_test);
    const BSIZE = @bitSizeOf(ST);
    comptime var UT = switch (ST) {
        i32 => u32,
        i64 => u64,
        i128 => u128,
        else => unreachable,
    };
    const min = @bitCast(ST, @as(UT, 1 << (BSIZE - 1)));
    const max = ~min;
    overflow.* = 0;
    const result = a *% b;

    // edge cases
    if (a == min) {
        if (b != 0 and b != 1) overflow.* = 1;
        return result;
    }
    if (b == min) {
        if (a != 0 and a != 1) overflow.* = 1;
        return result;
    }

    // take sign of x sx
    const sa = a >> (BSIZE - 1);
    const sb = b >> (BSIZE - 1);
    // take absolute value of a and b via
    // abs(x) = (x^sx)) - sx
    const abs_a = (a ^ sa) -% sa;
    const abs_b = (b ^ sb) -% sb;

    // unitary magnitude, cannot have overflow
    if (abs_a < 2 or abs_b < 2) return result;

    // compare the signs of operands
    if ((a ^ b) >> (BSIZE - 1) != 0) {
        if (abs_a > @divTrunc(max, abs_b)) overflow.* = 1;
    } else {
        if (abs_a > @divTrunc(min, -abs_b)) overflow.* = 1;
    }

    return result;
}

pub fn __mulosi4(a: i32, b: i32, overflow: *c_int) callconv(.C) i32 {
    return muloXi4_generic(i32, a, b, overflow);
}

pub fn __mulodi4(a: i64, b: i64, overflow: *c_int) callconv(.C) i64 {
    return muloXi4_generic(i64, a, b, overflow);
}

pub fn __muloti4(a: i128, b: i128, overflow: *c_int) callconv(.C) i128 {
    return muloXi4_generic(i128, a, b, overflow);
}

test {
    _ = @import("mulosi4_test.zig");
    _ = @import("mulodi4_test.zig");
    _ = @import("muloti4_test.zig");
}