Does x86 have an atomic increment that keeps the value that was stored?

Question

I have found lock inc addr but that doesn't keep a copy of the stored value around and even a read immediately after it in the same thread could come after a competing write.

The best solution I have found is a load/inc/cas loop.

In C++11 std::atomic, [`fetch_add`](http://en.cppreference.com/w/cpp/atomic/atomic/fetch_add) `+ 1` does what you want. Or more simply, [`operator++`](http://en.cppreference.com/w/cpp/atomic/atomic/operator_arith), but then you can't use a weaker `memory_order_relaxed` to make it faster on non-x86 if you don't need a barrier. (`lock xadd` implements `fetch_add`, returning the old value). — Peter Cordes, Sep 02 '17 at 08:16
[Can num++ be atomic for 'int num'?](https://stackoverflow.com/q/39393850/995714) — phuclv, Jan 14 '18 at 08:37

score 11 · Accepted Answer · edited Mar 21 '09 at 06:59

11

lock xadd is your friend.

edited Mar 21 '09 at 06:59

Nathan Fellman

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answered Mar 21 '09 at 06:13

Anton Tykhyy

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score 3 · Answer 2 · edited Oct 19 '09 at 03:19

3

see atomic_impl.h for more x86/x86_64 atomic primitives and usage.

edited Oct 19 '09 at 03:19

BCS

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answered Oct 18 '09 at 12:36

Test

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Does x86 have an atomic increment that keeps the value that was stored?

2 Answers2

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