The CPU doesn't make that decision, the compiler does. And AFAIK, GCC doesn't use the 80-bit 8087 floating point registers in 64-bit code (it sticks to the 64-

The CPU doesn't make that decision, the compiler does. And AFAIK, GCC doesn't use the 80-bit 8087 floating point registers in 64-bit code (it sticks to the 64-bit SSE2 registers.) Even if 80-bit registers were used, they are still exponent+mantissa format, which represents small integers exactly.