Re: [PATCH] riscv: jump_label: mark arguments as const to satisfy asm constraints

From: Heiko Stuebner <heiko@sntech.de>
To: Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Jason Baron <jbaron@akamai.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tom Rix <trix@redhat.com>, Samuel Holland <samuel@sholland.org>,
	linux-riscv@lists.infradead.org
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, Jisheng Zhang <jszhang@kernel.org>
Subject: Re: [PATCH] riscv: jump_label: mark arguments as const to satisfy asm constraints
Date: Tue, 11 Oct 2022 09:32:30 +0200	[thread overview]
Message-ID: <2432835.C4sosBPzcN@phil> (raw)
In-Reply-To: <20221006064028.548-1-jszhang@kernel.org>

Am Donnerstag, 6. Oktober 2022, 08:40:28 CEST schrieb Jisheng Zhang:
> Samuel reported that the static branch usage in cpu_relax() breaks
> building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]:
> 
> In file included from <command-line>:
> ./arch/riscv/include/asm/jump_label.h: In function 'cpu_relax':
> ././include/linux/compiler_types.h:285:33: warning: 'asm' operand 0
> probably does not match constraints
>   285 | #define asm_volatile_goto(x...) asm goto(x)
>       |                                 ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro
> 'asm_volatile_goto'
>    41 |         asm_volatile_goto(
>       |         ^~~~~~~~~~~~~~~~~
> ././include/linux/compiler_types.h:285:33: error: impossible constraint
> in 'asm'
>   285 | #define asm_volatile_goto(x...) asm goto(x)
>       |                                 ^~~
> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro
> 'asm_volatile_goto'
>    41 |         asm_volatile_goto(
>       |         ^~~~~~~~~~~~~~~~~
> make[1]: *** [scripts/Makefile.build:249:
> arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
> make: *** [arch/riscv/Makefile:128: vdso_prepare] Error 2
> 
> Maybe "-Os" prevents GCC from detecting that the key/branch arguments
> can be treated as constants and used as immediate operands. Inspired
> by x86's commit 864b435514b2("x86/jump_label: Mark arguments as const to
> satisfy asm constraints"), and as pointed out by Steven in [2] "The "i"
> constraint needs to be a constant.", let's do similar modifications to
> riscv.
> 
> Tested by CC_OPTIMIZE_FOR_SIZE + gcc and CC_OPTIMIZE_FOR_SIZE + clang.

I ran into the same build-issue when enabling CC_OPTIMIZE_FOR_SIZE
and this patch fixes it, so

Tested-by: Heiko Stuebner <heiko@sntech.de>


> [1]https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@sholland.org/
> [2]https://lore.kernel.org/all/20210212094059.5f8d05e8@gandalf.local.home/
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/include/asm/jump_label.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/jump_label.h b/arch/riscv/include/asm/jump_label.h
> index 38af2ec7b9bf..6d58bbb5da46 100644
> --- a/arch/riscv/include/asm/jump_label.h
> +++ b/arch/riscv/include/asm/jump_label.h
> @@ -14,8 +14,8 @@
>  
>  #define JUMP_LABEL_NOP_SIZE 4
>  
> -static __always_inline bool arch_static_branch(struct static_key *key,
> -					       bool branch)
> +static __always_inline bool arch_static_branch(struct static_key * const key,
> +					       const bool branch)
>  {
>  	asm_volatile_goto(
>  		"	.option push				\n\t"
> @@ -35,8 +35,8 @@ static __always_inline bool arch_static_branch(struct static_key *key,
>  	return true;
>  }
>  
> -static __always_inline bool arch_static_branch_jump(struct static_key *key,
> -						    bool branch)
> +static __always_inline bool arch_static_branch_jump(struct static_key * const key,
> +						    const bool branch)
>  {
>  	asm_volatile_goto(
>  		"	.option push				\n\t"
> 


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