Re: Calling conventions (particularly 32-bit ARM)

On Tue, 14 Jan 2025 21:48:19 +0000, Michael S wrote:

On Tue, 14 Jan 2025 19:18:27 +0100
Terje Mathisen <terje.mathisen@tmsw.no> wrote:
>

Stephen Fuld wrote:

On 1/12/2025 5:20 PM, Waldek Hebisch wrote:

You are implicitely assuming that passing large number of
arguments is expensive.

>
I guess.  I am actually assuming that passing arguments in memory
is more expensive than passing them in registers.  I don't think
that is controversial.

>
Usually true, except for recursive functions where you have to store
most stuff on the stack anyway, so going directly there can sometimes
generate more compact code.
>
Terje
>

>
I would think that for Fortran (==everything passed by reference)
memory would beat registers most of the time.

Pass by COMMON block was even faster.

                                              May be, except for
functions with 0-4 parameters.

Do common Fortarn compilers even bother with passing in register?

Fortran compilers are given an ABI (leaning towards C, C++) and
are required to "do something reasonable" in mapping Fortran
conventions into C conventions. C subroutines on the called
side, then, have to have a data structure identical to what
Fortran compiler would have produced (Dope Vector). C callers
will have to use those kinds of structures to successfully
call Fortran entry points.

It would require replacement of natural by-reference "pointer in
register points to value in memory" calling sequence to something like
copy-in/copy-out, right?

No, Fortran will pass dope vectors to called subroutines. The
called subroutine needs to understand the dope vector.