Can I avoid "shift count is negative" warning? (involves template parameter)

Can I avoid "shift count is negative" warning? (involves template parameter)

[Phil Endecott]

Dear Experts,

template <int a>
int shift (int b)
{
   return (a>0) ? b>>a : b<<-a;
}

int f()
{
   return shift<1>(1);
   return shift<-1>(1);
}


$ g++ -W -c shiftwarn.cc
shiftwarn.cc: In function ‘int shift(int) [with int a = 1]’:
shiftwarn.cc:11:   instantiated from here
shiftwarn.cc:5: warning: left shift count is negative
shiftwarn.cc: In function ‘int shift(int) [with int a = -0x00000000000000001]’:
shiftwarn.cc:12:   instantiated from here
shiftwarn.cc:5: warning: right shift count is negative



Of course, the shift count is only negative in the branch of the the ?: 
that isn't used.

I get the same warning if I use if-then-else rather than ?:.  If 'a' is 
a regular function parameter rather than a template parameter, or if it 
is a constant, I don't get any warnings.

Is there any way to avoid this?  Do you consider it a bug?

This is g++ 4.1.2.

Many thanks for any suggestions.

Phil.

p.s. I've just noticed that it doesn't warn about having two return statements...

Re: Can I avoid "shift count is negative" warning? (involves template parameter)

[John Love-Jensen]

Hi Phil,

You can avoid it by using specialization.

For a 32-bit int, that would be 32 positive, and 32 negative
specializations.

Maybe a bit much, but it'd work.

HTH,
--Eljay

Re: Can I avoid "shift count is negative" warning? (involves template parameter)

[jlh]

Hello!

This might not apply to g++ 4.1.2, which is what you use, but I
don't have this version right here to test it.  It works fine for
gentoo's g++ 4.1.1 and produces no warnings:

template <int a>
int shift (int b)
{
	if (a > 0) {
		unsigned ua = a;
		return b >> ua;
	} else {
		unsigned ua = -a;
		return b << ua;
	}
}
template int shift<1>(int);
template int shift<-1>(int);

However, you will have to turn on optimization in order to get the
same efficient code than with your version (x86 here).

(I do get a warning for converting -1 to unsigned on g++ 3.4.6)

jlh

Re: Can I avoid "shift count is negative" warning? (involves template parameter)

[Phil Endecott]

jlh wrote:
> template <int a>
> int shift (int b)
> {
> 	if (a > 0) {
> 		unsigned ua = a;
> 		return b >> ua;
> 	} else {
> 		unsigned ua = -a;
> 		return b << ua;
> 	}
> }
> template int shift<1>(int);
> template int shift<-1>(int);


Thanks, that works!

Phil.

Re: Can I avoid "shift count is negative" warning? (involves template parameter)

[me22]

On 03/05/07, John Love-Jensen <eljay@adobe.com> wrote:
> Hi Phil,
>
> You can avoid it by using specialization.
>
> For a 32-bit int, that would be 32 positive, and 32 negative
> specializations.
>
> Maybe a bit much, but it'd work.
>
> HTH,
> --Eljay
>
I think something like this should work, without needing so many
specializations:

template <int a, bool n = a<0>
struct rshift_by;
template <int a>
struct rshift_by<a, true> {
    static int shift(int b) { return b << -a; }
}
template <int a>
struct rshift_by<a, false> {
    static int shift(int b) { return b >> a; }
}

template <int a>
int shift (int b)
{
  return rshift_by<a>::shift(b);
}

int f()
{
  return shift<1>(1);
  return shift<-1>(1);
}

It also avoids the if branch on the compile-time constant, though GCC
is quite good at optimizing those. (IIRC some other compilers (MSVC)
whine about it, though.)

~ Scott McMurray

P.S. I love how the template syntax gives a<0>, don't you?