linux-kernel - Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

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Message-ID: <aA8q4Ot-1zTzv_Kt@gmail.com>
Date: Mon, 28 Apr 2025 09:14:40 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
	Arnd Bergmann <arnd@...db.de>, Arnd Bergmann <arnd@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Alexander Usyskin <alexander.usyskin@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mateusz Jończyk <mat.jonczyk@...pl>,
	Mike Rapoport <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case
 handling to C


* Ingo Molnar <mingo@...nel.org> wrote:

> And once we remove 486, I think we can do the optimization below to 
> just assume the output doesn't get clobbered by BS*L in the 
> zero-case, right?
> 
> In the text size space it's a substantial optimization on x86-32 
> defconfig:
> 
>         text	   data	       bss	     dec	    hex	filename
>   16,577,728    7598826    1744896      25921450        18b87aa vmlinux.vanilla      # CMOV+BS*L
>   16,577,908	7598838	   1744896	25921642	18b886a	vmlinux.linus_patch  # if()+BS*L
>   16,573,568	7602922	   1744896	25921386	18b876a	vmlinux.noclobber    # BS*L

And BTW, *that* is a price that all of non-486 x86-32 was paying for 
486 support...

And, just out of intellectual curiosity, I also tried to measure the 
code generation price of the +1 standards-quirk in the fls()/ffs() 
interface as well:

         text	   data	       bss	     dec	    hex	filename
   16,577,728   7598826    1744896      25921450        18b87aa vmlinux.vanilla      # CMOV+BS*L
   16,577,908	7598838	   1744896	25921642	18b886a	vmlinux.linus_patch  # if()+BS*L
   16,573,568	7602922	   1744896	25921386	18b876a	vmlinux.noclobber    # BS*L
   ..........
   16,573,552	7602922	   1744896	25921370	18b875a	vmlinux.broken       # BROKEN: 0 baseline instead of 1

... and unless I messed up the patch, it seems to have a surprisingly 
low impact - maybe because the compiler can amortize its cost by 
adjusting all dependent code mostly at build time, so the +1 doesn't 
end up being generated most of the time?

Thanks,

	Ingo

===============================>

This broken patch is broken: it intentionally breaks the ffs()/fls() 
interface in an attempt to measure the code generation effects of 
interface details.

NOT-Signed-off-by: <anyone@...where.anytime>
---
 arch/x86/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index e3e94a806656..21707696bafe 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -318,7 +318,7 @@ static __always_inline int variable_ffs(int x)
 	    : "=r" (r)
 	    : ASM_INPUT_RM (x), "0" (-1));
 
-	return r + 1;
+	return r;
 }
 
 /**
@@ -362,7 +362,7 @@ static __always_inline int fls(unsigned int x)
 	    : "=r" (r)
 	    : ASM_INPUT_RM (x), "0" (-1));
 
-	return r + 1;
+	return r;
 }
 
 /**