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Subject: Re: atomic_ref::required_alignment usefulness
From: Thiago Macieira (thiago_at_[hidden])
Date: 2020-02-10 10:24:24


On Monday, 10 February 2020 00:44:42 PST Andrey Semashev via Std-Discussion
wrote:
> Yes, x86 is one of such architectures, as is any CAS-based architectures
> I'm aware of. The example is mentioned in the standard:
> atomic_ref<complex<double>>. Assuming double is 64-bit and
> complex<double> is a 128-bit structure, it must be aligned to 16 bytes
> for atomic CAS(*), but alignof(complex<double>) is 8.
[snip]
> But given the definition of required_alignment in the standard, this
> code does not guarantee that ref.is_lock_free() will be true. An
> implementation where required_alignment is 8 and ref is implemented with
> a lock pool is allowed by the standard. Even if such atomic_ref
> implementation supports lock-free operations starting at alignment of 16.

No, it doesn't guarantee lock-free, it just guarantees atomicity. The need to
use a lock is usually related to size, not alignment. x86-64 will do a 16-byte
CAS lock-free, i386 will not, regardless of alignment.

> What I want is a constant that would be have the value of 16 in this
> case. I.e. the alignment that would make atomic_ref lock-free, if it can
> be at all for an object of such size. If it can't, let it be equal to
> alignof(T).
>
> (*) If it is not aligned, atomicity is not guaranteed on x86. I think, a
> hardware exception is generated, which can be handled by the kernel. The
> exception handler may emulate the atomic CAS (at terrible performance
> cost) or it may kill the process with SIGBUS. For our purposes of
> atomic_ref, let's assume this is not the desired behavior and unaligned
> atomics are banned.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel System Software Products

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