linux-kernel - [PATCH 2/2] x86/bootflag: Use __builtin

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Message-ID: <20250129154920.6773-2-ubizjak@gmail.com>
Date: Wed, 29 Jan 2025 16:47:51 +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@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 2/2] x86/bootflag: Use __builtin_parity() when available

Compilers (GCC and clang) provide optimized __builtin_parity() function
that returns the parity of X, i.e. the number of 1-bits in X modulo 2.

Use __builtin_parity() built-in function to optimize parity(). This
improves generated parity code to just:

  57:	84 db                	test   %bl,%bl
  59:	7a 5c                	jp     b7 <sbf_init+0xa7>

where TEST instruction sets parity flag (PF) in EFLAGS, exercised by the
follow-up jump instruction.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/kernel/bootflag.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/bootflag.c b/arch/x86/kernel/bootflag.c
index 4d89a2d80d0f..d63247d7abd4 100644
--- a/arch/x86/kernel/bootflag.c
+++ b/arch/x86/kernel/bootflag.c
@@ -22,6 +22,9 @@ int sbf_port __initdata = -1;	/* set via acpi_boot_init() */
 
 static bool __init parity(u8 v)
 {
+#if __has_builtin(__builtin_parity)
+	int x = __builtin_parity(v);
+#else
 	int x = 0;
 	int i;
 
@@ -29,7 +32,7 @@ static bool __init parity(u8 v)
 		x ^= (v & 1);
 		v >>= 1;
 	}
-
+#endif
 	return !!x;
 }
 
-- 
2.42.0