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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Calling conventions (particularly 32-bit ARM)
Date: Tue, 14 Jan 2025 23:27:22 +0000
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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.