linux-kernel - [tip: x86/asm] x86/bitops: Simplify variable_ffz() as variable_

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Message-ID: <174320139120.14745.16708448269773254006.tip-bot2@tip-bot2>
Date: Fri, 28 Mar 2025 22:36:30 -0000
From: "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Brian Gerst <brgerst@...il.com>, Juergen Gross <jgross@...e.com>,
 "H. Peter Anvin" <hpa@...or.com>, Josh Poimboeuf <jpoimboe@...hat.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject:
 [tip: x86/asm] x86/bitops: Simplify variable_ffz() as variable__ffs(~word)

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     e29c5d0e5dc35ed8b8920b573925d8aa2f8bafb0
Gitweb:        https://git.kernel.org/tip/e29c5d0e5dc35ed8b8920b573925d8aa2f8bafb0
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Thu, 27 Mar 2025 10:56:28 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 28 Mar 2025 23:24:16 +01:00

x86/bitops: Simplify variable_ffz() as variable__ffs(~word)

Find first zero (FFZ) can be implemented by negating the
input and using find first set (FFS).

Before/after code generation comparison on ffz()-using
kernel code shows that code generation has not changed:

  # kernel/signal.o:

   text	   data	    bss	    dec	    hex	filename
  42121	   3472	      8	  45601	   b221	signal.o.before
  42121	   3472	      8	  45601	   b221	signal.o.after

md5:
   ce4c31e1bce96af19b62a5f9659842f1  signal.o.before.asm
   ce4c31e1bce96af19b62a5f9659842f1  signal.o.after.asm

[ mingo: Added code generation check. ]

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Link: https://lore.kernel.org/r/20250327095641.131483-1-ubizjak@gmail.com
---
 arch/x86/include/asm/bitops.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index bbaf75e..eebbc88 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -267,10 +267,7 @@ static __always_inline unsigned long variable__ffs(unsigned long word)

 static __always_inline unsigned long variable_ffz(unsigned long word)
 {
-	asm("tzcnt %1,%0"
-		: "=r" (word)
-		: "r" (~word));
-	return word;
+	return variable__ffs(~word);
 }

 /**