Mantissa size of double variable compile with GCC in Intel® Atom™ [duplicate]

I'am runing a C++ software with double variables. My platform is upboard with an Intel® Atom™ x5-z8350 (1.92 GHz). The OS is Ubuntu server 18.04 and I compile the software with GCC 7.5.0.I compile my .cpp only with the optimization flag -o2.

How can I find / what is the size of the mantissa for the double variable floating point representation? Is is the standard 64-bit format with 53 bits mantissa or the x86/x86-64 80-bit format with 64 bits mantissa?

Thank you very much.Gabriel