Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user

[Kirill A. Shutemov]

On Tue, Jan 17, 2023 at 02:05:22PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 11, 2023 at 03:37:27PM +0300, Kirill A. Shutemov wrote:
> 
> >  #define __untagged_addr(untag_mask, addr)
> >  	u64 __addr = (__force u64)(addr);				\
> > -	s64 sign = (s64)__addr >> 63;					\
> > -	__addr &= untag_mask | sign;					\
> > +	if (static_branch_likely(&tagged_addr_key)) {			\
> > +		s64 sign = (s64)__addr >> 63;				\
> > +		__addr &= untag_mask | sign;				\
> > +	}								\
> >  	(__force __typeof__(addr))__addr;				\
> >  })
> >  
> > #define untagged_addr(addr) __untagged_addr(current_untag_mask(), addr)
> 
> Is the compiler clever enough to put the memop inside the branch?

Hm. You mean current_untag_mask() inside static_branch_likely()?

But it is preprocessor who does this, not compiler. So, yes, the memop is
inside the branch.

Or I didn't understand your question.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov