error: unknown constraint 'x' (x86_64)

[叶芝秋]

I tried to use tcc run the following code on x86_64, it threw the following error.
gcc done it well.

double glibc_sqrt(double x)
{
    double res;

    asm("sqrtsd %1, %0" : "=x" (res) : "xm" (x));

    return res;
}

error: unknown constraint 'x'