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

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 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 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 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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