linux-kernel - Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

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Date: Mon, 19 Feb 2024 11:56:24 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Fangrui Song <maskray@...gle.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org, 
	Jisheng Zhang <jszhang@...nel.org>, Dave Martin <Dave.Martin@....com>, 
	Peter Smith <peter.smith@....com>, llvm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

On Mon, 19 Feb 2024 at 11:03, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Fangrui,
>
> On Tue, Feb 6, 2024 at 8:46 AM Fangrui Song <maskray@...gle.com> wrote:
> > The generic constraint "i" seems to be copied from x86 or arm (and with
> > a redundant generic operand modifier "c"). It works with -fno-PIE but
> > not with -fPIE/-fPIC in GCC's aarch64 port.
>
> Thanks for your patch, which is now commit f9daab0ad01cf9d1 ("arm64:
> jump_label: use constraints "Si" instead of "i"") in v6.8-rc5.
>
> > The machine constraint "S", which denotes a symbol or label reference
> > with a constant offset, supports PIC and has been available in GCC since
> > 2012 and in Clang since 7.0. However, Clang before 19 does not support
> > "S" on a symbol with a constant offset [1] (e.g.
> > `static_key_false(&nf_hooks_needed[pf][hook])` in
> > include/linux/netfilter.h), so we use "i" as a fallback.
>
> https://gcc.gnu.org/releases.html says gcc-5 was released in 2015,
> i.e. after 2012 ...
>
> > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > Signed-off-by: Fangrui Song <maskray@...gle.com>
> > Link: https://github.com/llvm/llvm-project/pull/80255 [1]
> >
> > ---
> > Changes from
> > arm64: jump_label: use constraint "S" instead of "i" (https://lore.kernel.org/all/20240131065322.1126831-1-maskray@google.com/)
> >
> > * Use "Si" as Ard suggested to support Clang<19
> > * Make branch a separate operand
> >
> > Changes from v1:
> >
> > * Use asmSymbolicName for readability
>
> But it still fails on gcc-5:
>
>     arch/arm64/include/asm/jump_label.h:25:2: error: invalid 'asm':
> invalid operand
>       asm goto(
>       ^
>
> http://kisskb.ellerman.id.au/kisskb/buildresult/15129281/
>

How odd. godbolt.org has 5.4 and it seems perfectly happy with it.

https://godbolt.org/z/szzG3s59K