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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20250327095641.131483-1-ubizjak@gmail.com

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index bbaf75ea6703bf767a34234d5ec48a9b8d16cada..eebbc8889e70f5003d08e090dc193657a821bc3f 100644 (file)
--- 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);
 }
 
 /**