Re: [PATCH] riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y

[Jisheng Zhang]

On Thu, Sep 22, 2022 at 01:09:58AM -0500, Samuel Holland wrote:
> commit 8eb060e10185 ("arch/riscv: add Zihintpause support") broke
> building with CONFIG_CC_OPTIMIZE_FOR_SIZE enabled (gcc 11.1.0):
> 
>   CC      arch/riscv/kernel/vdso/vgettimeofday.o
> 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
> 
> Having a static branch in cpu_relax() is problematic because that
> function is widely inlined, including in some quite complex functions
> like in the VDSO. A quick measurement shows this static branch is
> responsible by itself for around 40% of the jump table.
> 
> Drop the static branch, which ends up being the same number of
> instructions anyway. If Zihintpause is supported, we trade the nop from
> the static branch for a div. If Zihintpause is unsupported, we trade the
> jump from the static branch for (what gets interpreted as) a nop.

Hi Samuel,

I'm not sure whether it's correct to remove static branch usage from
cpu_relax, but your report inspired my patch of constifying arguments
of arch_static_branch() and arch_static_branch_jump() [1]. Could you
please also test it?

Thanks very much

[1]https://lore.kernel.org/linux-riscv/20221006064028.548-1-jszhang@xxxxxxxxxx/T/#u

> 
> Fixes: 8eb060e10185 ("arch/riscv: add Zihintpause support")
> Signed-off-by: Samuel Holland <samuel@xxxxxxxxxxxx>
> ---
> 
>  arch/riscv/include/asm/hwcap.h          |  3 ---
>  arch/riscv/include/asm/vdso/processor.h | 25 ++++++++++---------------
>  2 files changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 6f59ec64175e..b21d46e68386 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -68,7 +68,6 @@ enum riscv_isa_ext_id {
>   */
>  enum riscv_isa_ext_key {
>  	RISCV_ISA_EXT_KEY_FPU,		/* For 'F' and 'D' */
> -	RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
>  	RISCV_ISA_EXT_KEY_MAX,
>  };
>  
> @@ -88,8 +87,6 @@ static __always_inline int riscv_isa_ext2key(int num)
>  		return RISCV_ISA_EXT_KEY_FPU;
>  	case RISCV_ISA_EXT_d:
>  		return RISCV_ISA_EXT_KEY_FPU;
> -	case RISCV_ISA_EXT_ZIHINTPAUSE:
> -		return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 1e4f8b4aef79..789bdb8211a2 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -4,30 +4,25 @@
>  
>  #ifndef __ASSEMBLY__
>  
> -#include <linux/jump_label.h>
>  #include <asm/barrier.h>
> -#include <asm/hwcap.h>
>  
>  static inline void cpu_relax(void)
>  {
> -	if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
>  #ifdef __riscv_muldiv
> -		int dummy;
> -		/* In lieu of a halt instruction, induce a long-latency stall. */
> -		__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> +	int dummy;
> +	/* In lieu of a halt instruction, induce a long-latency stall. */
> +	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
>  #endif
> -	} else {
> -		/*
> -		 * Reduce instruction retirement.
> -		 * This assumes the PC changes.
> -		 */
> +	/*
> +	 * Reduce instruction retirement.
> +	 * This assumes the PC changes.
> +	 */
>  #ifdef __riscv_zihintpause
> -		__asm__ __volatile__ ("pause");
> +	__asm__ __volatile__ ("pause");
>  #else
> -		/* Encoding of the pause instruction */
> -		__asm__ __volatile__ (".4byte 0x100000F");
> +	/* Encoding of the pause instruction */
> +	__asm__ __volatile__ (".4byte 0x100000F");
>  #endif
> -	}
>  	barrier();
>  }
>  
> -- 
> 2.35.1
>