Re: C aggregate passing (Rust kernel policy)

comex
2025-02-21T20:07:30Z

> On Feb 21, 2025, at 11:12 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> On Fri, 21 Feb 2025 at 10:34, David Laight <david.laight.linux@gmail.com> wrote:
>>
>> As Linus said, most modern ABI pass short structures in one or two registers
>> (or stack slots).
>> But aggregate returns are always done by passing a hidden pointer argument.
>>
>> It is annoying that double-sized integers (u64 on 32bit and u128 on 64bit)
>> are returned in a register pair - but similar sized structures have to be
>> returned by value.
>
> No, they really don't. At least not on x86 and arm64 with our ABI.
> Two-register structures get returned in registers too.

This does happen on older ABIs though.

With default compiler flags, two-register structures get returned on the stack on 32-bit x86, 32-bit ARM, 32-bit MIPS, both 32- and 64-bit POWER (but not power64le), and 32-bit SPARC. On most of those, double-register-sized integers still get returned in registers.

I tested this with GCC and Clang on Compiler Explorer:
https://godbolt.org/z/xe43Wdo5h

Again, that’s with default compiler flags. On 32-bit x86, Linux passes -freg-struct-return which avoids this problem. But I don’t know whether or not there’s anything similar on other architectures. This could be easily answered by checking actual kernel binaries, but I didn’t :)