[PATCH] arm64: Remove AArch64 X30 redundant register alias

[PATCH] arm64: Remove AArch64 X30 redundant register alias

[Naohiro Tamura]

This trivial patch is my first one to the kernel community.
Please advice me if something is not right.

-- >8 --
Subject: arm64: Remove AArch64 X30 redundant register alias

This patch removes a redundant AArch64 register alias of X30.
According to the GNU as manual [1], "ip0, ip1, lr and fp are
automatically defined to alias to X16, X17, X30 and X29
respectively".

[1] https://sourceware.org/binutils/docs/as/AArch64-Directives.html

Signed-off-by: Naohiro Tamura <naohirot@fujitsu.com>
---
 arch/arm64/include/asm/assembler.h | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 89faca0e740d..c66efc1b5406 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -155,11 +155,6 @@ alternative_endif
 9999:	x;					\
 	_asm_extable	9999b, l
 
-/*
- * Register aliases.
- */
-lr	.req	x30		// link register
-
 /*
  * Vector entry
  */
-- 
2.17.1


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Re: [PATCH] arm64: Remove AArch64 X30 redundant register alias

[Marc Zyngier]

On Tue, 13 Jul 2021 14:29:45 +0100,
Naohiro Tamura <naohirot@fujitsu.com> wrote:
> 
> This trivial patch is my first one to the kernel community.
> Please advice me if something is not right.
> 
> -- >8 --
> Subject: arm64: Remove AArch64 X30 redundant register alias
> 
> This patch removes a redundant AArch64 register alias of X30.
> According to the GNU as manual [1], "ip0, ip1, lr and fp are
> automatically defined to alias to X16, X17, X30 and X29
> respectively".
> 
> [1] https://sourceware.org/binutils/docs/as/AArch64-Directives.html
> 
> Signed-off-by: Naohiro Tamura <naohirot@fujitsu.com>
> ---
>  arch/arm64/include/asm/assembler.h | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 89faca0e740d..c66efc1b5406 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -155,11 +155,6 @@ alternative_endif
>  9999:	x;					\
>  	_asm_extable	9999b, l
>  
> -/*
> - * Register aliases.
> - */
> -lr	.req	x30		// link register
> -
>  /*
>   * Vector entry
>   */

I seem to vaguely remember that this isn't always defined, see [1] for
a recent example. Dropping it altogether may end-up being pretty
disruptive.

Thanks,

	M.

[1] https://lore.kernel.org/linux-arm-kernel/20210526213128.GB20442@willie-the-truck

-- 
Without deviation from the norm, progress is not possible.

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Re: [PATCH] arm64: Remove AArch64 X30 redundant register alias

[Robin Murphy]

On 2021-07-14 10:54, Marc Zyngier wrote:
> On Tue, 13 Jul 2021 14:29:45 +0100,
> Naohiro Tamura <naohirot@fujitsu.com> wrote:
>>
>> This trivial patch is my first one to the kernel community.
>> Please advice me if something is not right.
>>
>> -- >8 --
>> Subject: arm64: Remove AArch64 X30 redundant register alias
>>
>> This patch removes a redundant AArch64 register alias of X30.
>> According to the GNU as manual [1], "ip0, ip1, lr and fp are
>> automatically defined to alias to X16, X17, X30 and X29
>> respectively".
>>
>> [1] https://sourceware.org/binutils/docs/as/AArch64-Directives.html
>>
>> Signed-off-by: Naohiro Tamura <naohirot@fujitsu.com>
>> ---
>>   arch/arm64/include/asm/assembler.h | 5 -----
>>   1 file changed, 5 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
>> index 89faca0e740d..c66efc1b5406 100644
>> --- a/arch/arm64/include/asm/assembler.h
>> +++ b/arch/arm64/include/asm/assembler.h
>> @@ -155,11 +155,6 @@ alternative_endif
>>   9999:	x;					\
>>   	_asm_extable	9999b, l
>>   
>> -/*
>> - * Register aliases.
>> - */
>> -lr	.req	x30		// link register
>> -
>>   /*
>>    * Vector entry
>>    */
> 
> I seem to vaguely remember that this isn't always defined, see [1] for
> a recent example. Dropping it altogether may end-up being pretty
> disruptive.

Right, the built-in alias was only introduced a few years ago in 
binutils 2.29 (where it initially caused problems itself by being a bit 
overzealous[2]), but our current minimum version is 2.23, so we still 
need this for older but still-supported toolchains (e.g. I expect the 
Linaro GCC 7-based releases with binutils 2.28 are still being used).

Robin.

[2] https://www.spinics.net/lists/arm-kernel/msg599929.html

> 
> Thanks,
> 
> 	M.
> 
> [1] https://lore.kernel.org/linux-arm-kernel/20210526213128.GB20442@willie-the-truck
> 

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RE: [PATCH] arm64: Remove AArch64 X30 redundant register alias

[naohirot]

Hi Robin, Marc,

Thank you for your quick feedbacks!

> From: Robin Murphy <robin.murphy@arm.com>
> Sent: Wednesday, July 14, 2021 8:39 PM

> On 2021-07-14 10:54, Marc Zyngier wrote:
> > On Tue, 13 Jul 2021 14:29:45 +0100,
> > Naohiro Tamura <naohirot@fujitsu.com> wrote:

> >> -/*
> >> - * Register aliases.
> >> - */
> >> -lr	.req	x30		// link register
> >> -
> >>   /*
> >>    * Vector entry
> >>    */
> >
> > I seem to vaguely remember that this isn't always defined, see [1] for
> > a recent example. Dropping it altogether may end-up being pretty
> > disruptive.
> 
> Right, the built-in alias was only introduced a few years ago in
> binutils 2.29 (where it initially caused problems itself by being a bit
> overzealous[2]), but our current minimum version is 2.23, so we still
> need this for older but still-supported toolchains (e.g. I expect the
> Linaro GCC 7-based releases with binutils 2.28 are still being used).

I confirmed that binutils 2.23 doesn't have the alias [3].
So I withdraw this patch.

[3] https://sourceware.org/binutils/docs-2.23/as/AArch64-Directives.html#AArch64-Directives

p.s. I have learned how to submit a patch 😊
Thanks.
Naohiro

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