long double vs double

Question

I know that size of various data types can change depending on which system I am on. I use XP 32bits, and using the sizeof() operator in C++, it seems like long double is 12 bytes, and double is 8.

However, most major sources states that long double is 8 bytes, and the range is therefore the same as a double.

How come I have 12 bytes? If long double is indeed 12 bytes, doesn't this extends the range of value also? Or the long signature is only used (the compiler figures) when the value exceed the range of a double, and thus, extends beyond 8 bytes?

Thank you.

Answer 1

Quoting from Wikipedia:

On the x86 architecture, most compilers implement long double as the 80-bit extended precision type supported by that hardware (sometimes stored as 12 or 16 bytes to maintain data structure .

and

Compilers may also use long double for a 128-bit quadruple precision format, which is currently implemented in software.

In other words, yes, a long double may be able to store a larger range of values than a double. But it's completely up to the compiler.

Answer 2

For modern compilers on x64, Clang and GCC uses 16-byte double for long double while VC++ uses 8-byte double. In other words, with Clang and GCC you get higher precision double but for VC++ long double is same as double. The modern x86 CPUs do support these 16-byte doubles so I think Clang and GCC are doing the right thing and allows you to access lower level hardware capability using higher level language primitives.

Answer 3

The standard byte sizes for numbers are the guaranteed minimum sizes across all platforms. They may be larger on some systems, but they will never be smaller.

Answer 4

As far as my programming newbie experience prompts:

• Use periodicaly normalized float [-1.0,+1.0]

• Hold normalizing value separately with double, or long double

• Normalizing introduces noise=small errors=high frequencies to variables values

• From time to time it is useful to normalize with median value hold separately and keep data sorted (original data order could be saved as permutation vector)