LLVM IR atomics: difference between unordered and monotonic?

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1

Hi All,

I am reading about LLVM IR atomics (http://llvm.org/docs/Atomics.html) and get confused about the difference between “Unordered” and “Monotonic”. In particular, I am not sure I understand the statement of “It essentially guarantees that if you take all the operations affecting a specific address, a consistent ordering exists.”. For me, it means that for the following example, if {r1,r2} = {1,2}, {r3,r4} = {2,1} is not allowed under “Monotonic” but allowed under “Unordered”. Is this all how “Monotonic” is stronger than “Unordered”? If not, can anyone provide an example where other behaviors are allowed under “Unordered” but not “Monotonic”?

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If you understand the difference between the Java and C++ memory
model, "Unordered" is how to represent ordinary Java variables, while
"Monotonic" is how to represent memory_order_relaxed operations on
C++11 atomics.

If you don't understand the Java and C++ memory models, read
N2480: A Less Formal Explanation of the Proposed C++ Concurrency Memory Model and
http://unladen-swallow.googlecode.com/files/journal.pdf (why is there
no better location for this file?).

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Another litmus test is that with "unordered" variables, it's permissible to run:

T1 T2
W(X)=1 r1=R(X)
                r2=R(X)
                r3=R(X)

With X initially 0, and see r1==r3==0, but r2==1. Consider optimizing
code where &X == x == y, and the code was actually "r1=*x; r2=*y;
r3=*x;". Monotonic rules this out.

Jeffrey