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

On Mon, 19 Feb 2024 at 11:03, Geert Uytterhoeven [off-list ref] wrote:

Hi Fangrui,

On Tue, Feb 6, 2024 at 8:46 AM Fangrui Song [off-list ref] wrote:

quoted

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.

quoted

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 ...

quoted

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Fangrui Song <redacted>
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/ (local))

* 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

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