Subject: Re: [boost] [atomic] (op)_and_test naming
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2018-01-26 09:34:45
On 01/26/18 03:08, Peter Dimov via Boost wrote:
>> Â Â Â if (a.sub_and_test(1))
>>
>> Also, the generated code for these operations can potentially be more
>> efficient.
>
> While we're on the subject, on what architectures would opaque_sub be
> more efficient than sub_and_test?
On x86 and gcc < 7 opaque_sub allows to use "lock sub" or "lock dec"
without setting the bool according to the zero flag, i.e. it saves a
register and an instruction. Gcc 7 introduced the ability to return
flags from the asm statement, so the code can be written the same way.
Although I noticed that the compiler tends to save the flag into a
register early unless it is tested immediately, so in some cases
opaque_sub might still be preferable where it suits. AFAIK, other
compilers, including clang, don't support this feature.