linux-kernel - Re: [PATCH v2 4/5] riscv: bitops: Use __riscv_has_extension

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Message-ID: <aK6AKTnUtthEnEyy@yury>
Date: Tue, 26 Aug 2025 23:48:57 -0400
From: Yury Norov <yury.norov@...il.com>
To: Vivian Wang <wangruikang@...as.ac.cn>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Charlie Jenkins <charlie@...osinc.com>,
	Xiao Wang <xiao.w.wang@...el.com>,
	Christoph Müllner <christoph.muellner@...ll.eu>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Vivian Wang <uwu@...m.page>
Subject: Re: [PATCH v2 4/5] riscv: bitops: Use __riscv_has_extension_likely

On Fri, Aug 22, 2025 at 01:46:19AM +0800, Vivian Wang wrote:
> On 8/21/25 22:44, Yury Norov wrote:
> 
> > 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@...as.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.
> 
> It's not "handy". The operative part is "has_extension", and both
> functions return true if the extension is available and false if not.
> Functionally:
> 
>     if (likely()) <-- equivalent --> if (unlikely())
>     if (!likely()) <-- equivalent --> if (!unlikely())
> 
> Whereas:
> 
>     if (likely()) <-- **opposite of** --> if (!unlikely())
>     if (unlikely()) <-- **opposite of** --> if (!likely())
> 
> All the asm goto dispatch stuff work like this:
> static_branch_{likely,unlikely}, (arm64)
> alternative_has_cap_{likely,unlikely},
> __riscv_has_extension_{likely,unlikely}. Maybe it's confusing, but it
> seems to be the convention.
> 
> And, codegen-wise:
> 
> ALTERNATIVE("j %l[no_alt]", "nop", ...) -> likely() ALTERNATIVE("nop",
> "j %l[has_alt]", ...) -> unlikely()
> 
> Since the original code has the "likely" pattern, using "if (likely())"
> preserves it. Whatever the codegen was, it's still the same.
> 
> > 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?
> 
> Thanks for the tip on bloat-o-meter. I'll take a look tomorrow.
> 
> >>  	asm volatile (".option push\n"
> >>  		      ".option arch,+zbb\n"
> > Yeah, now the diff is much cleaner. Thanks.
> 
> This is why the condition at the top needed to be "!has_extension". So
> the structure can be:
> 
>     if (!has_extension)
>         return sw_version;
> 
>     multi_line
>     zbb_version;
> 
> If I used "if (has_extension)" the code would need be
> 
>     if (has_extension) {
>         multi_line
>         zbb_version;
>     } else {
>         sw_version;
>     }
> 
> And whether it was "likely" or "unlikely" had no bearing on the
> structure of the code.

OK, I see. Sorry for confusion.

Acked-by: Yury Norov (NVIDIA) <yury.norov@...il.com>