Re: "A diagram of C23 basic types"

On 2025-06-26, Richard Heathfield <rjh@cpax.org.uk> wrote:

On 26/06/2025 10:01, Lawrence D'Oliveiro wrote:

C
no longer offers a great performance advantage over a higher-level
language, so you might as well use the higher-level language.

>
Nothing is stopping you, but then comp.lang.c no longer offers
you the facility to discuss your chosen language, so you might as
well use the higher-level language's group.

Even a broken clock is right once or twice in a 24h period.

He did say that this advantage was in the manipulation
of multi-precision integers, like big decimals.

Indeed, most of the time is spent int the math routines themselves, not in
what dispatches them, Calculations written in C, using a certain bignum
libarary won't be much faster than the same calculations in a higher level
language, using the same bignum library.

A higher level language may also have a compiler which does
optimizations on the bignum code, such as CSE and constant folding,
basically treating it the same like fixnum integers.

C code consisting of calls into a bignum library will not be
aggressively optimized. If you wastefully perform a calculation
with constants that could be done at compile time, it almost
certainly won't be.

Example:

1> (compile-toplevel '(expt 2 150))
#<sys:vm-desc: a103620>
2> (disassemble *1)
data:
    0: 1427247692705959881058285969449495136382746624
syms:
code:
    0: 10000400 end d0
instruction count:
    1
#<sys:vm-desc: a103620>

The compiled code just retrieves the bignum integer result from static data
register d0. This is just from the compiler finding "expt" to be in a list of
functions that are reducible at compile time over constant inputs; no special
reasoning about large integers.

But if you were to write the C code to initialize a bignum from 5, and one from
150, and then call the bignum exponentiation routine, I doubt you'd get the
compiler to optimize all that away.

Maybe with a sufficiently advanced link-time optimization ...

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