[PATCH 1/4] system_data_types.7: Add '__int128'

[Alejandro Colomar]

Hi Paul,

On 2020-10-02 21:54, Paul Eggert wrote:
 > On 10/2/20 12:01 PM, Alejandro Colomar wrote:
 >> If you propose not to document the stdint types either,
 >
 > This is not a stdint.h issue. __int128 is not in stdint.h and is not a
 > system data type in any real sense; it's purely a compiler issue.
 > Besides, do we start repeating the GCC manual too, while we're at it? At
 > some point we need to restrain ourselves and stay within the scope of
 > the man pages.

I know it's not in stdint,
but I mean that it behaves as any other stdint type.
So I see value in having them documented together in the same page.
And it's very useful in some (very specific) cases
where portability isn't in mind
(although many compilers are starting to provide this type).

 >
 > PS. Have you ever tried to use __int128 in real code? I have, to my
 > regret. It's a portability and bug minefield and should not be used
 > unless you really know what you're doing, which most people do not.

I have.
I used unsigned __int128, for operating on a big bit matrix.
This type helped me remove a whole abstraction for the columns:

unsigned __int128 mat[128];	// This is a 128x128 bit matrix.

This way I avoided either having to use a shorter type,
which would have been a bit weird:

uint64_t	mat[128][2];	// This is more complicated to see.

Or having to use GMP,
which would have also complicated unnecessarily my code.

And of course, I didn't care about portability,
because I just wanted it to work.

Thanks,

Alex