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Date: Fri, 24 Mar 2017 19:00:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
Message-ID: <20170324180025.tumlczzf4ji3uzpo@hirez.programming.kicks-ass.net>
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On Fri, Mar 24, 2017 at 06:23:42PM +0100, Peter Zijlstra wrote:
> I tried a few variants, but nothing really made it better.
> 
> Find the tiny.c file below; I'm using:
> 
>   gcc (Debian 6.3.0-5) 6.3.0 20170124
> 
> it has both an inline and an stmt-expr try_cmpxchg variant to play with;
> the 'expected' output is at the bottom (same as above left).
> 
> Note that clang doesn't compile this stuff due to missing features.
> 

static inline bool try_cmpxchg2(unsigned int *ptr, unsigned int *val, unsigned int new)
{
	unsigned int old = *val;
	bool success;

        asm volatile goto("lock cmpxchgl %[new], %[ptr]; "
                          "jnz %[label]"
                     : /* no output */
                     : [ptr] "+m" (*ptr),
                       [old] "+a" (old)
                       [new] "r" (new)
                     : "memory"
                     : label);
        return 1;

label:
	*val = old;
	return 0;
}


generates better code than the @cc output with extra if (!success), but
not as good as the original.