Re: [boost] [atomic] Generalized read-modify-write operation

Subject: Re: [boost] [atomic] Generalized read-modify-write operation
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-09-01 21:24:28

On 2/09/2015 12:11, Andrey Semashev wrote:
>> (And the above is also supposed to use LL/SC on architectures where this
>> is cheaper than CAS, although I'm not sure if this is the case.)
>
> I'm not sure there are architectures that implement both CAS and LL/SC
> instructions, at least I'm not aware of such. On the architectures that
> support LL/SC, the instructions will be used to implement
> compare_exchange_weak. The modify function in this CAS loop will not be
> executed within the LL/SC region, which is why the additional load
> before the loop is required. There is also a probability of CAS failure.

Right.  I meant that on LL/SC architectures, compare_exchange_weak is 
"more native", while compare_exchange_strong is "more native" on CAS 
architectures.

> There is another point to consider. compare_exchange_weak/strong
> opereation on an LL/SC architecure is more complex than what is required
> to implement a simple RMW operation. For example, let's see it in
> Boost.Atomic code for ARM. Here is fetch_add, which can be used as a
> prototype of what could be done with a generic RMW operation:
>
>    "1:\n"
>    "ldrex   %[original], %[storage]\n"
>    "add     %[result], %[original], %[value]\n" // modify
>    "strex   %[tmp], %[result], %[storage]\n"
>    "teq     %[tmp], #0\n"
>    "bne     1b\n"
>
> Frankly, I'd like to be able to generate code like this for operations
> other than those defined by the standard atomic<> interface.

That's true, and so would I.

The danger is that what you can do while holding the X bit is quite 
limited, sometimes extremely so.  If the function call is *not* inlined 
then even that might be enough to tip it to always fail.  So you'd need 
a fallback to basic compare_exchange_weak (which should eventually 
succeed unless the machine is really hammered) in such cases.

Of course, that's basically what you said in the OP. :)