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Date:   Tue, 17 Jan 2023 16:57:03 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Kostya Serebryany <kcc@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Taras Madan <tarasmadan@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Bharata B Rao <bharata@amd.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until
 the first LAM user
Message-ID: <20230117135703.voaumisreld7crfb@box>
References: <20230111123736.20025-1-kirill.shutemov@linux.intel.com>
 <20230111123736.20025-9-kirill.shutemov@linux.intel.com>
 <Y8adEg2CYUSVpwtk@hirez.programming.kicks-ass.net>
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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