Re: [boost] [interprocess] atomic_write32 on GCC/x86/x68_64 lacks memory barrier

Subject: Re: [boost] [interprocess] atomic_write32 on GCC/x86/x68_64 lacks memory barrier
From: Lars Hagström (lars_at_[hidden])
Date: 2014-08-11 10:10:19

Hmm, should I add a fence to the Windows atomic_read32 variant too?
Reading the Boost.Atomics code it looks like it puts a _ReadWriteBarrier in
there...

/Lars

On Mon, Aug 11, 2014 at 3:38 PM, Lars Hagström <lars_at_[hidden]> wrote:

> Ok, I've managed, through trial and error and a bit of googling to make it
> less verbose. Is this correct?
>
> inline boost::uint32_t atomic_read32(volatile boost::uint32_t *mem)
> {
>   const boost::uint32_t val = *mem;
>   asm volatile ( "" ::: "memory" );
>   return val;
>
> }
>
>
> inline void atomic_write32(volatile boost::uint32_t *mem, boost::uint32_t
> val)
> {
>   asm volatile
>
>     (
>      "xchgl %0, %1"
>      : "+r" (val), "+m" (*mem)
>      :: "memory"
>     );
> }
>
> Does that "memory" statement at the end add a fence both before and after
> the assembly statement? Or have I misunderstood how this works?
>
> /Lars
>
> On Mon, Aug 11, 2014 at 3:21 PM, Lars Hagström <lars_at_[hidden]> wrote:
>
>> Thanks!
>>
>> As I said, I'm not an assembly programmer. I can copy/paste though, which
>> lead to the verbosity. :-)
>>
>>
>> On Mon, Aug 11, 2014 at 2:34 PM, Peter Dimov <lists_at_[hidden]> wrote:
>>
>>> Lars Hagström wrote:
>>>
>>>  inline void atomic_write32(volatile boost::uint32_t *mem,
>>>> boost::uint32_t val)
>>>> {
>>>>   __asm__ __volatile__ ( "" ::: "memory" );
>>>>   __asm__ (
>>>>            "xchgl %0, %1"
>>>>            : "+r" (val), "+m" (*mem)
>>>>            );
>>>>   __asm__ __volatile__ ( "" ::: "memory" );
>>>> }
>>>>
>>>
>>> This will work, but it's more verbose than it needs to be. You can put
>>> the __volatile__ and "memory" on the xchg asm statement and take out the
>>> two others.
>>>
>>> You'll also need to fix atomic_read32:
>>>
>>> {
>>>    uint32 r = *mem;
>>>
>>>    __asm__ __volatile__ ( "" ::: "memory" );
>>>    return r;
>>>
>>> }
>>>
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>>
>>
>