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@gmail.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/all/20230320212012.12704-1-ubizjak%40gmail.com

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 1c38174b5f0197355a96135b03858d6bebe3e84a..a08a4a7a03f8a8662de043b752611545d347b819 100644 (file)
--- 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)