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From: "Jan Mikkelsen" <janm@transactionsite.com>
To: "Kevin Mills" <kmills@aventail.com>,
	"FreeBSD Hackers" <freebsd-hackers@FreeBSD.org>
Subject: Re: atomic operations
Date: Tue, 26 Sep 2000 08:54:48 +1100
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Kevin Mills <kmills@aventail.com> wrote:
>I found the atomic_* functions in <machine/atomic.h>, but noticed that they
>have no return value.  What I need is a function that increments/decrements
>the given value *and* returns the new value in an atomic operation.  I
>suppose this is possible, yes?  How would one modify the assembly to make
>this work?


Atomic decrement, in the Intel style:

long atomic_decrement(volatile long* address)
{
  asm {
    mov ecx, [address]
    mov eax, -1
    lock xadd [ecx], eax
    dec eax
  }
 /* Return value in EAX */
}

An untested conversion into the GNU/AT&T style:

long atomic_decrement(volatile long* address)
{
 asm("movl 8(%ebp),%ecx");
 asm("movl $-1, %eax");
 asm("lock xaddl %eax,(%ecx)");
 asm("decl %eax");
 /* Return value in %eax */
}

Deriving increment is straightforward.

I haven't looked at the GNU inline assembler notation for indicating
register usage.  I'd be curious to see what is should look like.

Jan Mikkelsen




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