linux-kernel - Re: undefined reference to `__aarch64_cas4

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Message-ID: <CAJNi4rNTzkuZeya8f=tBVOn_dqgLbLf1A+3X4vHqiSa_sgDbUA@mail.gmail.com>
Date: Tue, 9 Jan 2024 10:52:23 +0800
From: richard clark <richard.xnu.clark@...il.com>
To: Mark Rutland <mark.rutland@....com>
Cc: gcc-help@....gnu.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: undefined reference to `__aarch64_cas4_sync' error on arm64
 native build

On Mon, Jan 8, 2024 at 6:51 PM Mark Rutland <mark.rutland@....com> wrote:

>
> This is due to a difference in default options between the two compilers; the
> kernel isn't linked against libc in either case.
>
> Your native compiler evidently has -moutline-atomics enabled in its default
> options. With that enabled, the builtin atomics generate calls to out-of-line
> functions which the kernel itself does not provide, and hence those result in a
> link-time error.
>
> Your cross-compiler evidently does not have -moutline-atomics enabled in its
> default options. Without that enabled, the builtin atomics generate inline
> atomic instructions rather than function calls. Since these don't depend on
> external functions there's no link-time error.
>
> If you pass 'mno-outline-atomics' to your native compiler, the problem should
> disappear.
>
Right, this should be the root cause, just pass 'mno-outline-atomics'
to my native compiler, the kernel link time issue doesn't show. But, I
test a userland application with '__sync_val_compare_and_swap' used,
for the 'aarch64-linux-gnu-gcc test.c', the 'nm a.out' will output:
..
0000000000000960 t __aarch64_cas4_sync
0000000000011011 B __aarch64_have_lse_atomics
..
>From the above output, seems the cross compiler has
'-moutline-atomics' enabled by default, also with
'aarch64-linux-gnu-gcc -mno-outline-atomics test.c', the 'nm a.out'
doesn't show the '_aarch64_cas4_sync'. The same result for the native
build for the same test.c, this result can't explain why the
cross-compile linux kernel will not generate the link time error,...

>
> Mark.