Bitwise OR used for signature enhanced operand

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Hi,
I have this snippet where the .net compiler outputs the above mentioned warning.

C#

private static int BitwiseRotate(int x, int c)
{
	var num = (uint) x;
	return (int) (x << c | num >> 32 - c);
}

If possible I would correct this to get no warning. I do not want to place any compiler hint to ommit this warning.

I guess this warning comes because of the left- and right shifts, but I do not know if casting would loose data. So if anyone has some constructive advice it would be great.

Thanks in advance.

Posted 9-Jun-14 22:35pm

Florian Trück

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future2015 10-Jun-14 7:35am

BitwiseRotate is for what?

4 solutions

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Kornfeld Eliyahu Peter · Answer 1 · 2014-06-09T22:46:00

Solution 1

The problem is with the types of num and c...
c is a int (signed) where num is uint (unsigned) so num >> 32- c will force the compiler to extend num to int (unsigned) and that the reason for the warning...

Posted 9-Jun-14 22:46pm

Kornfeld Eliyahu Peter

Comments

Florian Trück 10-Jun-14 4:51am

So it would be ok, if I change all types to uint?

Kornfeld Eliyahu Peter 10-Jun-14 4:56am

Not sure as you have 32 - c, and that can be signed, so better try int...

phil.o · Answer 2 · 2014-06-10T01:57:00

Solution 3

What is the benefit of the use of the num variable?

Your code could be shortened to:

C#

private static int BitwiseRotate(int x, int c) {
   return (x << c) | (x >> (32 - c));
}

No need to cross cast between int and uint.

I think the problem you had in the first place was that you forgot some parenthesis:

C#

x >> 32 - c

should have been

C#

x >> (32 - c)

because the right-shift operator has precedence over subtraction; thus the compiler understood

C#

(x >> 32) - c

which it logically reported as potentially problematic.

Hope this makes sense.

Posted 10-Jun-14 1:57am

phil.o

Updated 10-Jun-14 20:02pm

v3

Comments

Kornfeld Eliyahu Peter 10-Jun-14 8:06am

You just right about the num variable and the needless casting, but in the case of precedence not! Additive operators (+/-) have higher precedence than bitwise shift (<</>>)...

phil.o 10-Jun-14 9:17am

Yes, I just checked, and +/- actually have precedence on << and >>.
I thought the contrary because I've always seen the (32 - c) part explicitly grouped.
So this is in fact just a problem of the initial cast to uint.
Thanks for pointing it out.

Kornfeld Eliyahu Peter 10-Jun-14 9:18am

It's maybe a good idea to remove that part from your solution - it will worth a 5 then... :-)

future2015 · Answer 3 · 2014-06-10T01:46:00

Solution 2

In C#, shift operators are not logical and if operand is negative, shifts 1.

private static int BitwiseRotate(int x, int c) // rotate left
{
var num = (uint) x;
return (int) ((num << c) | num >> (32 - c));
}

Posted 10-Jun-14 1:46am

future2015

Florian Trück · Answer 4 · 2014-06-23T01:00:00

Solution 4

What about this one?

C#

private static int BitwiseRotate(int value, int count)
{
    var val = (uint)value;
    return (int)((val << count) | (val >> (32 - count)));
}

It generates no compiler warnings.
I found it on the net.

Posted 23-Jun-14 1:00am

Florian Trück