Re: [boost] [atomic] (op)_and_test naming

Subject: Re: [boost] [atomic] (op)_and_test naming
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-01-26 17:00:10

On 01/26/18 19:54, Andrey Semashev wrote:
> On 01/26/18 19:32, Peter Dimov via Boost wrote:
>>
>> The interface strikes me as heavily influenced by what g++ can 
>> (__builtin_constant_p) and cannot (figure out that the result is not 
>> needed) do. The order of adding the functions probably also plays a 
>> part; were `op_and_test` added first, `opaque_op` probably wouldn't 
>> have been.
> 
> No, all extra ops were added at the same time. I was also planning to 
> add a generalized `read_modify_write` operation but didn't do it because 
> I don't have the hardware with TSX.
> 
> It's true though that my main testing compiler is gcc. Clang doesn't 
> seem to support __builtin_constant_p, which makes it fail to convert 
> "add" to "inc" in add_and_test and opaque_add. OTOH, Intel compiler does 
> support it and generates better code for add_and_test and opaque_add 
> than for fetch_add. Maybe I should switch clang to the generic emulation 
> backend.
> 
> If the code can be improved for other compilers, I welcome suggestions 
> and patches.
> 
>> It's interesting to play and see what gets generated when. For 
>> instance, clang++ 3.6 figures out by itself that in `x1.fetch_and_add( 
>> 1 );` the result is not used, and generates `lock inc`.
>>
>> https://godbolt.org/g/7TFQ2V
>>
>> How does it manage to do that, if you're using assembly `xadd`, I 
>> don't know.
> 
> `fetch_add` is implemented in terms of instrinsics (I assume, clang 
> supports __atomic* intrinsics, so those should be used). It is expected 
> that there is no difference to `std::atomic` in the standard operations 
> on recent compilers.

Sorry, forgot to paste the link:

https://github.com/boostorg/atomic/blob/develop/include/boost/atomic/detail/ops_gcc_atomic.hpp#L104

The backend is selected here:

https://github.com/boostorg/atomic/blob/develop/include/boost/atomic/detail/operations_lockfree.hpp
https://github.com/boostorg/atomic/blob/develop/include/boost/atomic/detail/platform.hpp