sizeof(long double) vs. std::numeric_limits and x86-64 vs x86

[Ian Lance Taylor]

Michael Veksler <mveksler@tx.technion.ac.il> writes:

> I wanted to check how many bits long double takes. So I checked
> sizeof(long double) which, naturally, may account for padding. On
> x86-64 it takes 16 bytes and only 12 on x86, a difference of 4
> bytes. This supported my hope that on x86-64 we would have "long
> double"= |__float128|.

Alas, no.  On x86_64 long double is an 80-bit floating point number,
just as on 32-bit x86.  The size is 16 bytes because the x86_64 prefers
to align large values to 8-byte boundaries.

gcc works this way because the 80-bit floating point format is supported
directly in hardware, unlike the 128-bit format.

Ian