AVR Freaks

Although I do not really plan on using double, I would like to understand why this is a problem.

#include <stdint.h>
#include <avr/io.h>
#include <util/delay.h>

double y[1] = {2.0}; // doesn't work
//static double y[1] = {2.0}; // works
//uint8_t y[1] = {2}; // works
int main(void)
{
    DDRB |= 1 << DDB1;
    while(1)
    {
        if( y[0] > 0)
        {
            PORTB ^= 1 << PORTB1;
        }
        _delay_ms(1000);
    }
    return 0;
}

The code above does not compile for the ATtiny4. I try to compile with avr-gcc and options -mmcu=attiny4, -Os, and -DF_CPU=1000000UL. The compiler balks with the following message
/usr/local/Cellar/avr-gcc/6.2.0/lib/gcc/avr/6.2.0/../../../../avr/lib/avrtiny/libm.a(gesf2.o): In function `__gesf2':
(.text.avr-libc.fplib+0x0): undefined reference to `__fp_cmp'
collect2: error: ld returned 1 exit status 

The optimization level does not seem to have any affect on it. Changing the declaration of the array y to one of the following fixes things.
 

static double y[1] = {2.0};

or 

uint8_t y[1] = {2};

It also works if y is passed to a function.

#include <stdint.h>
#include <avr/io.h>
#include <util/delay.h>

double y[1] = {2.0};
void update(double *y)
{
    y[0] = 2.0;
}
int main(void)
{
    DDRB |= 1 << DDB1;
    while( 1)
    {
        update(y);
        if( y[0] > 0)
        {
            PORTB ^= 1 << PORTB1;
        }
        _delay_ms(1000);
    }
    return 0;
}

What accounts for these behaviors? Why are things so? Thanks.