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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: comp.arch
Subject: Re: Calling conventions (particularly 32-bit ARM)
Date: Wed, 08 Jan 2025 18:20:43 -0500
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> ABI calling conventions tend to be designed to support at least C,
> including varargs and often also tolerant of differences between the
> number of arguments in the caller and callee.

I can agree that it's important to support those use-cases (varargs
obviously, mismatched arg numbers less so), but I think the focus of
optimization of the ABI should be calls to functions known to take the
exact same number of arguments (after all, even in C we normally know
the prototype of the called function; only sloppy ancient C calls
functions without proper declarations), even if it comes at the cost of
using different calling conventions for the two cases.

>>But in any case, I suspect there are also diminishing returns at some
>>point: how much faster is it in practice to pass/return 13 values in
>>registers instead of 8 of them in registers and the remaining 5 on
>>the stack?  I expect a 13-arg function to perform an amount
>>of work that will dwarf the extra work of going through the stack.
> I certainly have a use for as many arguments as the ABI provides,

Ah, yes, machine-generated code can always defy intuitions about what
is "typical".  🙂


        Stefan