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x86/bitops: Simplify variable_ffz() as variable__ffs(~word)

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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
This commit is contained in:
Uros Bizjak 2025-03-27 10:56:28 +01:00 committed by Ingo Molnar
parent 0717b1392d
commit e29c5d0e5d
1 changed files with 1 additions and 4 deletions
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5
arch/x86/include/asm/bitops.h
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@ -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);
}
/**