Re: Calling conventions (particularly 32-bit ARM)

On Mon, 6 Jan 2025 13:57:51 +0000, David Brown wrote:

I'm trying to understand the reasoning behind some of the calling
conventions used with 32-bit ARM.  I work primarily with small embedded
systems, so the efficiency of code on 32-bit Cortex-M devices is very
important to me - good calling conventions make a big difference.
>
No doubt most people here know this already, but in summary these
devices are a 32-bit load/store RISC architecture with 16 registers.
R0-R3 and R12 are scratch/volatile registers, R4-R11 are preserved
registers, R13 is the stack pointer, R14 is the link register and R15 is
the program counter.  For most Cortex-M cores, there is no
super-scaling,

SuperScalar

              out-of-order execution, speculative execution, etc., but
instructions are pipelined.
>
The big problem I see is the registers used for returning values from
functions.  R0-R3 can all be used for passing arguments to functions, as
32-bit (or smaller) values, pointers, in pairs as 64-bit values, and as
parts of structs.

Someone above mentioned a trick to pass back a 128-bit value.

But the ABI only allows returning a single 32-bit value in R0, or a
scalar 64-bit value in R0:R1.  If a function returns a non-scalar that
is larger than 32-bit, the caller has to allocate space on the stack for
the return type and pass a pointer to that space in R0.
>
To my mind, this is massively inefficient, especially when using structs
that are made up of two 32-bit parts.

I have seen subroutines that returned structures where the point
in the subroutine that puts values in the returned structure is
such that putting the structure in registers is less efficient
than returning the struct in registers--it all depends on how
the struct is laid out in memory. Doing the struct field assign-
ments in the middle of the subroutine (long path to return) is
often enough to sway which is more efficient.

Is there any good reason why the ABI is designed with such limited
register usage for returns?

Vogue in 1980 was to have 1 result passed back from subroutines.

                             Newer ABIs like RISC-V 32-bit and x86_64
can at least use two registers for return values.

My 66000 can pass up to 8 registers back as a aggregate result.

                                                   Modern compilers are
quite happy breaking structs into parts in individual registers - it's a
/long/ time since they insisted that structs occupied a contiguous block
of memory.  Can anyone give me an explanation why return types can't
simply use all the same registers that are available for argument
passing?

In My 66000 ABI they can and do.

I also think code would be a bit more efficient if there more registers
available for parameter passing and as scratch registers - perhaps 6
would make more sense.

Basically, here, there is competing pressure between the compiler
needing a handful of preserved registers, and the compiler being
more efficient if there were more argument/result passing registers.
My 66000 ABI has 8 argument registers, 7 temporary registers, 14
preserved registers, a FP, and a SP. IP is not part of the register
file. My ABI has a note indicating that the aggregations can be
altered, just that I need a good reason to change.
I looked high and low for codes using more than 8 arguments and
returning aggregates larger than 8 double words, and about the
only things I found were a handful of []print[]() calls.

In more modern C++ programming, it's very practical to use types like
std::optional<>, std::variant<>, std::expected<> and std::tuple<> as a
way of dealing safely with status and multiple return values rather than
using C-style error codes or passing manual pointers to return value
slots.  But the limited return registers adds significant overhead to
small functions.

C++ also has the:
try-throw-catch exception model which require new-and-fun stuff to
be thrown onto the stack.
constructors and destructors
new
Atomic stuff

Are there good technical reasons for the conventions on 32-bit ARM?  Or
is this all just historical from the days when everything was an "int"
and that's all anyone ever returned from functions?

At the time, there were good technical rational--which my have
faded in import as the years go by.

>
Thanks for any pointers or explanations here.