linux-kernel - [PATCH v2] x86/ACPI/boot: Improve __acpi_acquire_global

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Date:   Mon, 20 Mar 2023 22:20:12 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        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 v2] x86/ACPI/boot: Improve __acpi_acquire_global_lock

Improve __acpi_acquire_global_lock by using a temporary variable.
This enables compiler to perform if-conversion and improves generated
code from:

 ...
 72a:	d1 ea                	shr    %edx
 72c:	83 e1 fc             	and    $0xfffffffc,%ecx
 72f:	83 e2 01             	and    $0x1,%edx
 732:	09 ca                	or     %ecx,%edx
 734:	83 c2 02             	add    $0x2,%edx
 737:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)
 73b:	75 e9                	jne    726 <__acpi_acquire_global_lock+0x6>
 73d:	83 e2 03             	and    $0x3,%edx
 740:	31 c0                	xor    %eax,%eax
 742:	83 fa 03             	cmp    $0x3,%edx
 745:	0f 95 c0             	setne  %al
 748:	f7 d8                	neg    %eax

to:

 ...
 72a:	d1 e9                	shr    %ecx
 72c:	83 e2 fc             	and    $0xfffffffc,%edx
 72f:	83 e1 01             	and    $0x1,%ecx
 732:	09 ca                	or     %ecx,%edx
 734:	83 c2 02             	add    $0x2,%edx
 737:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)
 73b:	75 e9                	jne    726 <__acpi_acquire_global_lock+0x6>
 73d:	8d 41 ff             	lea    -0x1(%rcx),%eax

BTW: the compiler could generate:

	lea 0x2(%rcx,%rdx,1),%edx

instead of:

	or     %ecx,%edx
	add    $0x2,%edx

but unwated conversion from add to or when bits are known to be zero
prevents this improvement. This is GCC PR108477.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Len Brown <lenb@...nel.org>
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>
---
v2: Expand return statement.
---
 arch/x86/kernel/acpi/boot.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 1c38174b5f01..a08a4a7a03f8 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -1853,13 +1853,18 @@ early_param("acpi_sci", setup_acpi_sci);
 
 int __acpi_acquire_global_lock(unsigned int *lock)
 {
-	unsigned int old, new;
+	unsigned int old, new, val;
 
 	old = READ_ONCE(*lock);
 	do {
-		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
+		val = (old >> 1) & 0x1;
+		new = (old & ~0x3) + 2 + val;
 	} while (!try_cmpxchg(lock, &old, new));
-	return ((new & 0x3) < 3) ? -1 : 0;
+
+	if (val)
+		return 0;
+
+	return -1;
 }
 
 int __acpi_release_global_lock(unsigned int *lock)
-- 
2.39.2