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

Subject: Re: [boost] [atomic] (op)_and_test naming
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-26 17:27:09

Andrey Semashev wrote:

> 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. Maybe I should 
> switch clang to the generic emulation backend.

This would make it use `xadd` instead of `add`, see 
https://godbolt.org/g/V1iTbj. Adding `inc_and_test` seems more 
straightforward and consistent with the spirit of the current interface that 
lets the user express the exact operation instead of relying on the compiler 
to figure it out.

>  OTOH, Intel compiler does support it and generates better code for 
> add_and_test and opaque_add than for fetch_add.

Indeed it does, https://godbolt.org/g/QAbTZo.

I checked the g++ and clang++ versions and was going to say that nowhere 
does opaque_add generate better code than the dumb fetch_add, but it does on 
Intel.

> `fetch_add` is implemented in terms of instrinsics (I assume, clang 
> supports __atomic* intrinsics, so those should be used).

Ah right.

To get back on topic, I'm fine with making op_and_test return true on 
nonzero.