linux-kernel - [PATCH 1/2] x86/bitops: Use TZCNT mnemonic in <asm/bitops.h>

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Message-ID: <20250325175215.330659-1-ubizjak@gmail.com>
Date: Tue, 25 Mar 2025 18:52:01 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 1/2] x86/bitops: Use TZCNT mnemonic in <asm/bitops.h>

Current minimum required version of binutils is 2.25,
which supports TZCNT instruction mnemonic.

Replace "REP; BSF" in variable__{ffs,ffz}() function
with this proper mnemonic.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 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 100413aff640..bbaf75ea6703 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -248,7 +248,7 @@ arch_test_bit_acquire(unsigned long nr, const volatile unsigned long *addr)
 
 static __always_inline unsigned long variable__ffs(unsigned long word)
 {
-	asm("rep; bsf %1,%0"
+	asm("tzcnt %1,%0"
 		: "=r" (word)
 		: ASM_INPUT_RM (word));
 	return word;
@@ -267,7 +267,7 @@ static __always_inline unsigned long variable__ffs(unsigned long word)
 
 static __always_inline unsigned long variable_ffz(unsigned long word)
 {
-	asm("rep; bsf %1,%0"
+	asm("tzcnt %1,%0"
 		: "=r" (word)
 		: "r" (~word));
 	return word;
-- 
2.42.0