Theseus: A modern experimental OS written from scratch in Rust

The really nice thing about Dynamo was that it was a relatively simple trace-based JIT compiler from native code to native code (plus a native code interpreter for non-hotspots). This meant that it would automatically inline hotspots across DLLs and through C++ virtual method dispatches (with appropriate guards to jump back to interpreter mode if the virtual method implementation didn't match or the PLT entry got modified). They didn't have to do any special-casing of the interpreter to handle virtual method calls or cross-DLL calls, it's just a natural consequence of a trace-based JIT from native code to native code.

The only downsides of something like Dynamo are (1) a bit of complexity and space usage (2) some startup overhead due to starting in interpretive mode and (3) if your program is abnormal in not having a roughly Zipf distribution of CPU usage, the overhead is going to be higher.

Ever since I read about Michael Franz et al.'s SafeTSA SSA-based JVM bytecode that more quickly generated higher-performing native code, I've had a long-term back-burner idea to write a C compiler that generates native code in a particular way (functions are all compiled to arrays of pointers to strait-line extended basic blocks) that makes tracing easier, and also storing a SafeTSA-like SSA bytecode along with the native code. That way, a Dynamo-like runtime wouldn't use an interpreter, and when it came to generate an optimized trace, it could skip the first step of decompiling native code to an SSA form. (Also, the SSA would be a bit cleaner as input for an optimizer, as the compilation-decompilation round-trip tends to make the SSA a bit harder to optimize, as shown by Franz's modification of Pizza/JikesRVM to run both SafeTSA and JVM bytecode.) Once you have your trace, you don't need on-stack replacement to get code in a tight loop to go into the optimized trace, you just swap one pointer to native code in the function's array of basic blocks. (All basic blocks are strait-line code, so the only way to loop is to jump back to the start of the same basic block via the array of basic block pointers.)

The background for HP Dynamo is that during the Unix wars, there were a bunch of RISC system vendors vying for both the high-end workstation and server markets. Sun had SPARC, SGI had MIPS, DEC had Alpha AXP (and earlier, some MIPS DECStations) and HP had PA-RISC. The HP Dynamo research project wanted to show that emulation via dynamic recompilation could be fast, so to get an apples-to-apples comparison for emulation overhead, they wrote a PA-RISC emulator for PA-RISC.

> easy to copy and to rember

That's a very important, useful and valueable property of an abstraction, yes. Though certainly not the only important property.

Or do you want one that takes you hours to understand and that you have to remind yourself of it and re-learn every week?

Many such distinctions that cannot be made rigorous that stick in various cultures but never existed in others simply exist because a man copies his fellow man, not because they are useful in achieving any goals.

Though, as I said, they exist to simplify reality. I assume he human brain works so it does, simply to conserve energy and admit an inaccurate though usually useful enough solution. The problem is when such fuzzy logic is mixed with actual rigorous logic and attempts are made to analyse the former with the latter — that I find a futile waste of time.

There is a very good reason that no clear answer has been found to the quæstion raised in The Ship of Theseus even after millennia of contemplation — there is none, and the quæstion is bereft of sense.

Given your example, thinking about who is beautiful and who isn't (not "Scientific Study", just thinking) would be pointless is well, and in reality, in practical objective reality of a lot of people, isn't.

> quickly leading to reductions to the absurd such as here.

That is the whole point of the original thought experiment, it shows exactly that.

But it leads there by showing not that the question itself is wrong. But by showing that if you try to apply scientific thinking to everything, you end up with absurdity. That there are many areas of life where intuition is a much more suited, practical, and result-rich method of thinking.

Here, let me join the party. I disagree with both of you, because mathematically it should be the case that applying "identity" to x should yield back x for all x.

What non-futile metaphysical questions are left, then? Is metaphysics completely futile? If so, is philosophy of science itself pseudoscience?

As far as I understand, most logical positivists would indeed reject philosophy of science. On the other hand, philosophy of science mostly put an end to logical positivism — and showed that we do need metaphysics after all. This paradigm shift also brought useful concepts such as falsifiability.

Of course, we are all free to choose whatever philosophy we want. I just personally find logical positivism and its rejection of metaphysics unconvincing.

We actually do have these investigations, and there is a scientific quantification of what is and isn't beautiful. Symmetry is a significant factor, but there are others - many others. Nancy Etcoff's research delves into this - she even wrote a book about it - Survival of the Prettiest: The Science of Beauty.

I reject that? No I don't. I need to stop putting words in I's mouth.

Identity is one of the most fundamental concepts, without it you would have a hard time splitting the universe into more than one distinct piece in order to reference and talk about them. You can only refer to this TV series because it is an entity with identity, equal to itself and not equal to any other thing in the universe.

And yes, it is a very hard problem, as Theseus illustrates one can not simply define identity using the atoms or some other kind of parts something is made of in the general case to define identity. But that just shows the difficulty of the problem, not that identity is a useless idea.

I am not a philosopher but it seems to me that without a notion of identity or equality you can not have more than one distinct thing. Well, probably someone has spend his entire life thinking about that, so maybe there are alternatives to identity and equality?

Precisely none of them in 3-5 decades has managed to deliver on the promise of microkernels prompting the question:

"Were huge microkernel advantages actually greatly oversold, is posix a massively debilitating factor that prevents microkernel based systems from delivering on their raison d'etre or was every single one of those implementations a poor one? [1]"

[1] including QNX