Re: [PATCH -next] arm64: Optimize the comparison of unsigned expressions to avoid compiling error

[Mark Rutland]

On Tue, Feb 21, 2023 at 09:27:40AM +0800, Lin Yujun wrote:
> while compile arch/arm64/include/asm/cpufeature.h with
> -Werror=type-limits enabled, errors shown as below:
> 
> ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_4kb_granule':
> ./arch/arm64/include/asm/cpufeature.h:653:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
>   return (val >= ID_AA64MMFR0_TGRAN4_SUPPORTED_MIN) &&
>               ^~
> ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_64kb_granule':
> ./arch/arm64/include/asm/cpufeature.h:666:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
>   return (val >= ID_AA64MMFR0_TGRAN64_SUPPORTED_MIN) &&
>               ^~

When is the `-Werror=type-limits` flag enabled by the build system?

We have patterns like this all over the kernel, and I don't think this is
indicative of a real problem, and I don't think that we need to change code to
make this warning disappear.

> Modify the return judgment statement, use
> "((val - min) < (val - max - 1))" to confirm that returns
> true in “min <= val <= max” cases, false in other cases.

That expression is far less clear than the existing code, so I do not think
that is a good idea.

> Fixes: 79d82cbcbb3d ("arm64/kexec: Test page size support with new TGRAN range values")

What functional error does this fix?

What configuration is broken?

Thanks,
Mark.

> Signed-off-by: Lin Yujun <linyujun809@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/cpufeature.h | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 03d1c9d7af82..0a6bda025141 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -54,6 +54,9 @@ enum ftr_type {
>  #define FTR_VISIBLE_IF_IS_ENABLED(config)		\
>  	(IS_ENABLED(config) ? FTR_VISIBLE : FTR_HIDDEN)
>  
> +#define IN_RANGE_INCLUSIVE(val, min, max)		\
> +	(((val) - (min)) < ((val) - (max) - 1))
> +
>  struct arm64_ftr_bits {
>  	bool		sign;	/* Value is signed ? */
>  	bool		visible;
> @@ -693,8 +696,9 @@ static inline bool system_supports_4kb_granule(void)
>  	val = cpuid_feature_extract_unsigned_field(mmfr0,
>  						ID_AA64MMFR0_EL1_TGRAN4_SHIFT);
>  
> -	return (val >= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN) &&
> -	       (val <= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX);
> +	return IN_RANGE_INCLUSIVE(val,
> +		ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN,
> +		ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX);
>  }
>  
>  static inline bool system_supports_64kb_granule(void)
> @@ -706,8 +710,9 @@ static inline bool system_supports_64kb_granule(void)
>  	val = cpuid_feature_extract_unsigned_field(mmfr0,
>  						ID_AA64MMFR0_EL1_TGRAN64_SHIFT);
>  
> -	return (val >= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN) &&
> -	       (val <= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX);
> +	return IN_RANGE_INCLUSIVE(val,
> +		ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN,
> +		ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX);
>  }
>  
>  static inline bool system_supports_16kb_granule(void)
> @@ -719,8 +724,9 @@ static inline bool system_supports_16kb_granule(void)
>  	val = cpuid_feature_extract_unsigned_field(mmfr0,
>  						ID_AA64MMFR0_EL1_TGRAN16_SHIFT);
>  
> -	return (val >= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN) &&
> -	       (val <= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX);
> +	return IN_RANGE_INCLUSIVE(val,
> +		ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN,
> +		ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX);
>  }
>  
>  static inline bool system_supports_mixed_endian_el0(void)
> -- 
> 2.34.1
>