Re: [PATCH v2 4/5] riscv: bitops: Use __riscv_has_extension_likely

From: Yury Norov <yury.norov@gmail.com>
To: Vivian Wang <wangruikang@iscas.ac.cn>
Cc: "Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Rasmus Villemoes" <linux@rasmusvillemoes.dk>,
	"Charlie Jenkins" <charlie@rivosinc.com>,
	"Xiao Wang" <xiao.w.wang@intel.com>,
	"Christoph Müllner" <christoph.muellner@vrull.eu>,
	"Vivian Wang" <uwu@dram.page>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/5] riscv: bitops: Use __riscv_has_extension_likely
Date: Thu, 21 Aug 2025 10:44:05 -0400	[thread overview]
Message-ID: <aKcwtXDJKRf4O_tF@yury> (raw)
In-Reply-To: <20250821-riscv-altn-helper-wip-v2-4-9586fa702f78@iscas.ac.cn>

On Thu, Aug 21, 2025 at 05:16:34PM +0800, Vivian Wang wrote:
> Use __riscv_has_extension_likely() to check for RISCV_ISA_EXT_ZBB,
> replacing the use of asm goto with ALTERNATIVE.
> 
> The "likely" variant is used to match the behavior of the original
> implementation using ALTERNATIVE("j %l[legacy]", "nop", ...).
> 
> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
> ---
>  arch/riscv/include/asm/bitops.h | 32 ++++++++------------------------
>  1 file changed, 8 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
> index d59310f74c2ba70caeb7b9b0e9221882117583f5..f70ccc0c2ffb86a6fda3bc373504143d0c6a1093 100644
> --- a/arch/riscv/include/asm/bitops.h
> +++ b/arch/riscv/include/asm/bitops.h
> @@ -47,9 +47,8 @@
>  
>  static __always_inline unsigned long variable__ffs(unsigned long word)
>  {
> -	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
> -				      RISCV_ISA_EXT_ZBB, 1)
> -			  : : : : legacy);
> +	if (!__riscv_has_extension_likely(0, RISCV_ISA_EXT_ZBB))
> +		return generic___ffs(word);

So, on the previous round you spent quite a lot of time explaining how
'unlikely()' version is handy over '!likely()', and now use just the
latter. I feel really lost about how the code generation should look.

Can you please share bloat-o-meter report against this patch? Can you
also show an example of code generation before and after? Have you
tried the 'unlikely()` one? How the output looks?

>  	asm volatile (".option push\n"
>  		      ".option arch,+zbb\n"

Yeah, now the diff is much cleaner. Thanks.

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