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Strictly speaking, the Java Memory Model derives from the data-race-free model of the early '90s. Java was the first programming language to explicitly incorpor

Strictly speaking, the Java Memory Model derives from the data-race-free model of the early '90s. Java was the first programming language to explicitly incorporate it as part of the specification, but the main derivation actually comes from the C++ memory model, which built into it the basic atomic memory model that most derivatives rely on--Java doesn't have the weaker atomics support that C++ added, just sequentially-consistent. Java also introduced some frankly confusing (and incorrect, per my understanding) semantics for what happens in the face of data races that everybody else just shrugged and said "let's make it be UB and call it a day."

hinkley 3y ago
Uh, this is not the history I recall at all. Why is Hanz Boehm referring to memory model fixes and the JMM in the C++11 spec (Java started tackling this problem in 2004, see Goetz) if Java copied C++?
- jcranmer 3y ago
  C++ extended the Java memory model to include atomics, and everyone else copied the C++ atomics memory model. Basically, it goes: * Original data-race-free model (early 90s) * Java 5 memory model (~2004-5) [first incorporated into the programming language, as I'm aware] * C++0x memory model (finalized 2007-2008, IIRC) [added atomics] * Compilers build their internal intrinsics on top of C++0x, sans consume * Everyone else adopts either the C++ memory model as is, or without consume (Sibling comment notes that Java 9 backports the atomics from C++).
hashmash 3y ago
> Java doesn't have the weaker atomics support that C++ added Yes it does, but it doesn't have first-class keywords to represent these modes. You have to use the VarHandle class, which is a bit kludgy. https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/lang/invoke/VarHandle.html https://docs.oracle.com/en/java/javase/20/docs/api/java.base...
- jcranmer 3y ago
  Okay, it looks like those were added in Java 9, which was after I stopped following Java (and well after the C++ memory model was largely settled, in 2007).
  - CyberDildonics 3y ago
    Okay, it seems strange to make a sweeping generalization while assuming java had stagnated.
  - re-thc 3y ago
    Before Java 9 it could have been done using Unsafe. VarHandle just provides a slightly higher level construct.
    - gpderetta 3y ago
      Was the behavior of more relaxed operations fully formalized in the way it fits with the rest of the memory model?
      - hashmash 3y ago
        Not with the Unsafe class. It was never intended to be used by anything except the built-in concurrency support classes, and it was just barely good enough. External libraries which made use of the Unsafe class did so by studying how it was used, and then assumed/concluded how it worked. Java 9 formalized everything with the VarHandle class, and it offers much more features than the Unsafe class.
        gpderetta 3y ago
        Thanks for the clarification. Java was obviously a trailblazer on formally introducing advanced memory models on practical languages, but it did take a few tries to get it correct.

hinkley 3y ago

Uh, this is not the history I recall at all. Why is Hanz Boehm referring to memory model fixes and the JMM in the C++11 spec (Java started tackling this problem in 2004, see Goetz) if Java copied C++?

jcranmer 3y ago
C++ extended the Java memory model to include atomics, and everyone else copied the C++ atomics memory model. Basically, it goes: * Original data-race-free model (early 90s) * Java 5 memory model (~2004-5) [first incorporated into the programming language, as I'm aware] * C++0x memory model (finalized 2007-2008, IIRC) [added atomics] * Compilers build their internal intrinsics on top of C++0x, sans consume * Everyone else adopts either the C++ memory model as is, or without consume (Sibling comment notes that Java 9 backports the atomics from C++).

hashmash 3y ago

> Java doesn't have the weaker atomics support that C++ added Yes it does, but it doesn't have first-class keywords to represent these modes. You have to use the VarHandle class, which is a bit kludgy. https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/lang/invoke/VarHandle.html https://docs.oracle.com/en/java/javase/20/docs/api/java.base...

jcranmer 3y ago
Okay, it looks like those were added in Java 9, which was after I stopped following Java (and well after the C++ memory model was largely settled, in 2007).
- CyberDildonics 3y ago
  Okay, it seems strange to make a sweeping generalization while assuming java had stagnated.
- re-thc 3y ago
  Before Java 9 it could have been done using Unsafe. VarHandle just provides a slightly higher level construct.
  - gpderetta 3y ago
    Was the behavior of more relaxed operations fully formalized in the way it fits with the rest of the memory model?
    - hashmash 3y ago
      Not with the Unsafe class. It was never intended to be used by anything except the built-in concurrency support classes, and it was just barely good enough. External libraries which made use of the Unsafe class did so by studying how it was used, and then assumed/concluded how it worked. Java 9 formalized everything with the VarHandle class, and it offers much more features than the Unsafe class.
      - gpderetta 3y ago
        Thanks for the clarification. Java was obviously a trailblazer on formally introducing advanced memory models on practical languages, but it did take a few tries to get it correct.