thedigitalengel

In any case, Java's stack-based bytecode is really a register machine in disguise. It isn't a general stack machine -- for instance, at a given "PC" the shape of the expression stack (i.e. the number and "basic" types of elements in it) needs to be constant.

The right way to look at Java's bytecode stream is opcodes for a register machine with _implicit_ input and output registers. Mapping it to a compiler IR would not be easy in the general case if that weren't true.

Thank you for taking time out to comment! :)

> But you said that the JMM guarantees that all reads in C_i - C_(i-1) > will see writes in C_(i-1); this means that C_3 which reads > `tuple.nonVolatileF` will see the write to that variable in C_2, no?

Talking about such things in English is ambiguous. The formal statement is

"For any read r ∈ C_i −C_(i−1), we have W_i(r) ∈ C_(i−1) and W(r) ∈ C_(i−1)".

A clearer way to state the clause in English is "writes seen by reads in C_i - C_(i-1) belong to C_(i-1);". I can justify an execution as long as reads (ultimately) seen any write in the commit previous to the one it is in, subject to happens before consistency. To prevent causality loops, the JMM allows a read to see a write that happened before it in the commit that introduces it. Then, to allow certain interesting optimizations, the JMM allows us the bait-and-switch the write a read saw -- "Each read r in C_i − C_(i−1) must see writes in C_(i−1) in both E_i and E, but may see a different write in E_i from the one it sees in E.".

The non-volatile read is _allowed_ to see the non-volatile write (i.e. such an execution exists, this is the question I ask in the exercise), but it doesn't have to. The transform in question is illegal because the transformed program allows behavior that the untransformed program didn't allow -- the transform breaks semantics.

> Since you're constructing an execution, why didn't you interleave > the execution to make this trivially true?

I don't understand this, make /what/ trivially true? In any case, the transformed program has a data race, and so observationally it doesn't need to have a sequentially consistent execution. Specifically, it is allowed to show behavior that cannot be described by any interleaving of the instructions streams of the individual threads.

> Also, can't you wrap the writer in an atomic block during > transformation?

What purpose would that serve?

> Should the first 'illegal' read legal?

Well it depends. :) I did mean "illegal", though there certainly are many illegal transforms that look legal and vice versa.

Unfortunately, the solution ultimately proposed doesn't work in this case:

  int _a = 50;
  int result = min(num, _a);

you end up expanding to "int _a = _a;" which creates a new int _a, and assigns it to itself.

Thank you for taking out time to comment. :)

> - It doesn't address how you decide which edges to speculatively > consider executable. > > - Optimizers intentionally do not play the "what if" game and try > optimizing things multiple different ways to see what wins or what > gains might be had. It simply gets too expensive (in compile time) > too quickly.

My use case was that you already have a set of edges that profiling tells you is "rarely taken", and you wish to know if eliding any of those edges improves optimization. Normally JIT compilers end up installing traps in most of these edges, hoping that the cost of the occasional side exit to the interpreter is worth the added performance in the fast case. I wanted a way to push the decision on whether to install a trap for a specific edge to later in the optimization process.

> - There are already other ways to achieve similar effect, > e.g. superblock formation or simple tail duplication (which can be > done for specific effect, e.g. see branch threading in LLVM).

I don't disagree here. I'll have to admit that this approach is somewhat "out there", in its current form it doesn't seem very practical.

(Disclaimer: I know nothing about Ruby, but I know some things about JIT compilers)

Another way to handle this is to assume that Fixnum#+ hasn't changed when compiling a method that is using it (maybe add a check at method entry); but when it does get redefined you "deoptimize" the methods that you compiled while holding that assumption.

A quick solution involves starting at a corner, and placing each queen one horizontal and two vertical (a constrained knight's move, basically) away from the previous one. This works for an 8x8 board, but does not generalize to an NxN board (it is easy to see that this won't work when N is divisible by 3). Determining which N's it exactly works for is a math, not a programming puzzle, however. :)

I'm from the same place as the artagnon. Anti-weed laws are almost never enforced, and weed is incredibly cheap (you'll get enough weed to get 10 people stoned out of their minds for the price of a bottle of beer). And the ease of storage means you can easily maintain a huge stash.