[cfe-dev] [PATCH] libc++: help diagnosing the following std::atomic compile error

David Chisnall David.Chisnall at cl.cam.ac.uk
Tue May 21 06:11:16 PDT 2013

On 21 May 2013, at 01:55, Howard Hinnant <hhinnant at apple.com> wrote:

> I've enclosed a proposed patch for fixing <atomic>.  This patch changes us from using _Atomic(T) for the stored type, to just T, but aligned to _Atomic(T).  

This may still end up with the std::atomic<> types lacking padding in structures that makes it possible to act on them correctly.  For example, what do you get when you do:

struct {
	std::atomic<char> x;
	char y;

y should be sufficiently after x that they can be operated on independently (at least from an ISA perspective - if they're in the same cache line then most RISCy CPUs will do a bit of redundant work anyway).  Even though the alignment of y is 1, we (typically) should be adding 3 bytes of padding after x.


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