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Peter Anvin" , Greg KH , Boqun Feng , Miguel Ojeda , Christoph Hellwig , rust-for-linux , David Airlie , linux-kernel@vger.kernel.org, ksummit@lists.linux.dev Content-Transfer-Encoding: quoted-printable Message-Id: References: <326CC09B-8565-4443-ACC5-045092260677@zytor.com> <2025021954-flaccid-pucker-f7d9@gregkh> <2nn05osp-9538-11n6-5650-p87s31pnnqn0@vanv.qr> <2025022052-ferment-vice-a30b@gregkh> <9B01858A-7EBD-4570-AC51-3F66B2B1E868@zytor.com> <20250221183437.1e2b5b94@pumpkin> To: Linus Torvalds X-Mailer: Apple Mail (2.3826.400.131.1.6) > On Feb 21, 2025, at 11:12=E2=80=AFAM, Linus Torvalds = wrote: >=20 > On Fri, 21 Feb 2025 at 10:34, David Laight = wrote: >>=20 >> 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. >>=20 >> 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. >=20 > 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=E2=80=99s with default compiler flags. On 32-bit x86, Linux = passes -freg-struct-return which avoids this problem. But I don=E2=80=99t= know whether or not there=E2=80=99s anything similar on other = architectures. This could be easily answered by checking actual kernel = binaries, but I didn=E2=80=99t :)=