Re: Memory ordering

On 11/17/2024 6:03 AM, jseigh wrote:

On 11/16/24 16:21, Chris M. Thomasson wrote:
 

Fwiw, in C++ std::memory_order_consume is useful for traversing a node based stack of something in RCU. In most systems it only acts like a compiler barrier. On the Alpha, it must emit a membar instruction. Iirc, mb for alpha? Cannot remember that one right now.

 That got deprecated.  Too hard for compilers to deal with.  It's now
same as memory_order_acquire.

Strange! C++ has:
https://en.cppreference.com/w/cpp/atomic/atomic_signal_fence
That only deals with compilers, not the arch memory order... Humm...
Interesting Joe!

Which brings up an interesting point.  Even if the hardware memory
memory model is strongly ordered, compilers can reorder stuff,
so you still have to program as if a weak memory model was in
effect.   Or maybe disable reordering or optimization altogether
for those target architectures.

Indeed. The compiler needs to know about these things. Iirc, there was an old post over c.p.t that deals with a compiler (think it was GCC) that messed up a pthread try lock for a mutex. It's a very old post. But I remember it for sure.