Re: [PATCH 1/6] riscv: Introduce use_alternative_{likely,unlikely}

[Yury Norov <yury.norov@gmail.com>]

On Wed, Aug 20, 2025 at 09:44:45PM +0800, Vivian Wang wrote:
> Introduce convenience helpers use_alternative_likely() and
> use_alternative_unlikely() to implement the pattern of using asm goto to
> check if an alternative is selected. Existing code will be converted in
> subsequent patches.
> 
> Similar to arm64 alternative_has_cap_{likely,unlikely}, but for riscv,
> alternatives are not all CPU capabilities.
> 
> Suggested-by: Aydın Mercan <aydin@mercan.dev>
> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
> ---
>  arch/riscv/include/asm/alternative-macros.h | 73 +++++++++++++++++++++++++++++
>  1 file changed, 73 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/alternative-macros.h b/arch/riscv/include/asm/alternative-macros.h
> index 231d777d936c2d29c858decaa9a3fa5f172efbb8..be9835b5e4eba03d76db3a73da19ac9e2981c4db 100644
> --- a/arch/riscv/include/asm/alternative-macros.h
> +++ b/arch/riscv/include/asm/alternative-macros.h
> @@ -158,4 +158,77 @@
>  	_ALTERNATIVE_CFG_2(old_content, new_content_1, vendor_id_1, patch_id_1, CONFIG_k_1,	\
>  					new_content_2, vendor_id_2, patch_id_2, CONFIG_k_2)
>  
> +/*
> + * use_alternative_{likely,unlikely}() returns true if the alternative is
> + * applied and false otherwise, but in a way where the compiler can optimize
> + * this check down to a nop instruction that's patched into a jump, or vice
> + * versa.
> + *
> + * Always returns false if the alternatives mechanism is not available.
> + *
> + * Usage example:
> + *   if (use_alternative_likely(0, RISCV_ISA_ZBB))
> + *
> + * Similar to static keys, "likely" means use a nop if the alternative is
> + * selected, and jump if unselected; "unlikely" is the other way around.
> + */
> +
> +#ifndef __ASSEMBLER__
> +
> +#include <linux/types.h>
> +
> +#ifdef CONFIG_RISCV_ALTERNATIVE
> +
> +static __always_inline bool use_alternative_likely(u16 vendor_id, u32 patch_id)
> +{
> +	BUILD_BUG_ON(!__builtin_constant_p(vendor_id));
> +	BUILD_BUG_ON(!__builtin_constant_p(patch_id));
> +
> +	asm goto(ALTERNATIVE("j %l[no_alt]", "nop", %[vendor_id], %[patch_id], 1)
> +		 :
> +		 : [vendor_id] "i"(vendor_id),
> +		   [patch_id] "i"(patch_id)
> +		 :
> +		 : no_alt);
> +
> +	return true;
> +
> +no_alt:
> +	return false;
> +}

Apart from those BUILD_BUG_ON()s, it looks similar to
__riscv_has_extension_likely(). Can you make sure you don't duplicate
it?

If so, can you describe what's the difference between those two in the
commit message?

> +static __always_inline bool use_alternative_unlikely(u16 vendor_id, u32 patch_id)
> +{
> +	BUILD_BUG_ON(!__builtin_constant_p(vendor_id));
> +	BUILD_BUG_ON(!__builtin_constant_p(patch_id));
> +
> +	asm goto(ALTERNATIVE("nop", "j %l[alt]", %[vendor_id], %[patch_id], 1)
> +		 :
> +		 : [vendor_id] "i"(vendor_id),
> +		   [patch_id] "i"(patch_id)
> +		 :
> +		 : alt);
> +
> +	return false;
> +
> +alt:
> +	return true;
> +}

This 'unlikely' version is just an negation of 'likely' one, and it
looks like an attempt to save on one negation. On the other hand, the
function is __always_inline, which means that compiler should normally
take care of it. Can you prove with objdump that it really works as
intended? I mean that 

        if (use_alternative_unlikely())
                do_something();

generates a better code than 
        
        if (!use_alternative_likely())
                do_something();

Thanks,
Yury

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