Re: Error in returned value of sizeof(long double) in amd64?

[Michael Cheponis <michael.cheponis%gmail.com@localhost>]

Thank you.

We, then, in effect, are carrying around 'dead bytes' on amd64, for, reason?

That is, what is the 'win' in doing this ?  

This is of no consequence for a handful of long doubles, but big arrays of long double....

(This is just curiosity; I have found exactly one scientific calculation I needed to do whose precision improved using long doubles, a satellite orbital prediction over very long times)

-Mike

On Fri, Jul 5, 2024 at 9:35 PM Martin Husemann <martin%duskware.de@localhost> wrote:

On Fri, Jul 05, 2024 at 07:13:05PM -0700, Michael Cheponis wrote:
> sizeof(long double) reports 16 in both amd64 and arm64.  I think that amd64
> uses 80-bit long double floats, and arm64 uses 128-bit long double floats.
> See enclosed code.

The storage size of a long double is one thing, but the exact format
used in calculations may vary.

On amd64:

> cc -dM -E - < /dev/null | fgrep LDBL_MANT
#define __LDBL_MANT_DIG__ 64


On aarch64:

> cc -dM -E - < /dev/null | fgrep LDBL_MAN
#define __LDBL_MANT_DIG__ 113

(use just LDBL in the fgrep to see more exact limits of the format in use)

Martin