gmp_pi1_t

[Joerg Arndt]

* Niels Möller <nisse at lysator.liu.se> [Dec 14. 2009 09:02]:
> Torbjorn Granlund <tg at gmplib.org> writes:
> 
> > nisse at lysator.liu.se (Niels Möller) writes:
> 
> >   If I do that, should I eliminate the gmp_pi1_t type as well, and pass a
> >   single limb (the remaining inv32 member) instead of a gmp_pi1_t *?
> >   
> > I supoose there is no harm with keeping gmp_pi1_t, since it will be
> > easier to modify it.  Or do you see some harm?
> 
> Estetic considerations aside, the only obvious drawback is that the
> function call overhead will be a cycle or two less for passing the limb
> directly in a register, than for storing it in memory and passing a
> poiter to it. But I don't know if that difference is going to be
> measurable.

The 'inline' keyword is your friend.
Inlined functions are optimized across their
boundaries which is often more important than
just avoiding call overhead.

Note that (AMD64) a few args are passed in registers
and the call overhead can be neglected unless there
are very many calls.



> 
> (In recent years I've been under some influence of "extreme programming"
> thinking, so I'm suspicious to adding abstractings that do nothing
> useful today but are supposed to be useful in some future version).

The compiler should remove 'fall through' functions entirely.
IIRC there is some optimization switch regarding this issue.

> 
> > I expect more gmp_piX_t structures in the future, BTW.
> 
> Anyway, I think structures like these should be const declared in the
> prototypes, for both clarity and performance. So that code like
> 
>   gmp_pi1_t inv;
> 
>   ... inv->inv32 ...;
>   foo (..., &inv);
>   ... inv->inv32 ...;
> 
> can keep the value of inv32 in a register over the function call. Are
> gcc or other compilers clever enough to do that optimization?

With inlined functions _much_ more is done.
Inspect the generated machine code as follows:

# create assembler code:
c++ -S -fverbose-asm -g -O2 test.cc -o test.s
# create asm interlaced with source lines:
as -alhnd test.s > test.lst


> 
> Regards,
> /Niels
> 
> -- 
> Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26.
> Internet email is subject to wholesale government surveillance.
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