sillycross

> I just meant that even being aware this is a naive copy-and-patch JIT, my first impression was that the code was slightly worse than I expected.

> "there’s only so much one can do without touching the data model"

You probably want to look at the other link in that PR, which demonstrated how well copy-and-patch can do for another dynamic language (Lua): [1]

Of course, whether or not CPython could eventually make it to that point (or even further) is a different story: they are under a way tighter constraint than just developing something for academia. But copy-and-patch can do a lot even for dynamic languages :)

[1] https://sillycross.github.io/2023/05/12/2023-05-12/

That's correct. Lua function calls are not that easy to remove, as function is first-class value in Lua so can be redefined at any time. To remove a function call, you need speculative compilation and OSR-exit, which is outside the job of the baseline JIT.

Look at the CPS-version of the add.

That's an interesting approach :) Though it only works if the control flow in the language is exactly "paired" (no continue/break, no goto, etc), I guess?

I agree with your point, but I want to point out that Deegen also provided APIs to hide all the details of inline caching. The bytecode only specifies the body lambda and the effect lambdas, and Deegen lowers it to an efficient implementation automatically, including exotic optimizations that fuses the cached IC effect into the opcode so one indirect branch can be avoided.

If LuaJIT interpreter were to employ IC, it would have to undergo a major rewrite (due to its assembly nature) to have the equally efficient code as LJR (that we generate automatically). This is one advantage of our meta-compiler approach.

Finally, although no experiment is made, my subjective opinion is already in the article:

> LuaJIT’s hand-written assembly interpreter is highly optimized already. Our interpreter generated by Deegen is also highly optimized, and in many cases, slightly better-optimized than LuaJIT. However, the gain from those low-level optimizations are simply not enough to beat LuaJIT by a significant margin, especially on a modern CPU with very good instruction-level parallelism, where having a few more instructions, a few longer instructions, or even a few more L1-hitting loads have negligible impact on performance. The support of inline caching is one of the most important high-level optimizations we employed that contributes to our performance advantage over LuaJIT.

That is, if you compare the assembly between LJR and LuaJIT interpreter, I believe LJR's assembly is slightly better (though I would anticipate only marginal performance difference). But that's also because we employed a different boxing scheme (again...). If you force LJR to use LuaJIT's boxing scheme, I guess the assembly code would also be similar since LJR's hand-written assembly is already optimal or at least very close to optimal.

Clang/LLVM accepts (little-known but documented) flags to let you align code block using a custom alignment, though it only works at file-level. See [1].

However, I'm not sure if doing so is useful or necessary. Interpreter performance is sensitive to code layout (which affects hardware branch predictor accuracy), but I don't think there is a general way to optimize the code layout to make the branch predictor as happy as possible.

So if you changed your code alignment and saw a perf change, it's more likely caused by the random perf gain/loss due to code layout change, not because that 1-byte-alignment is better than 16-byte-alignment or vice versa.

[1] https://easyperf.net/blog/2018/01/25/Code_alignment_options_...

I just want to point out that LLVM is not a runtime dependency. It is only a build-time dependency if you want to build LJR from source. Once LJR is built, it is stand-alone and does not need LLVM at runtime.

Yeah, you should do it at LLVM IR level.

The copy-and-patch paper is also written by me. Deegen a follow-up work of copy-and-patch, and it will use copy-and-patch as a tool to build its JIT tiers in the future.

> More recently I have been experimenting with a new calling convention that uses no callee-saved registers to work around this, but the results so far are inconclusive. The new calling convention would use all registers for arguments, but allocate registers in the opposite order of normal functions, to reduce the chance of overlap. I have been calling this calling convention "reverse_cc".

As explained in the article, LLVM already has the calling convention you are exactly looking for: the GHC convention (cc 10). You can use it to "pin" registers by passing arguments at the right spot. If you pin your argument in a callee-saved register of the C calling conv, it won't get clobbered after you do a C call.

Yes, the object can become unreachable, but we won't know it until the end of the current GC cycle (and the whole purpose of GC is to figure that information out). So yes, even if it has become unreachable, it is live (and must be live) until the end of the cycle.

I think the point you missed is: the function 'cellContainsLiveObject' is not used by GC, it is used by allocator to tell if the cell is available for allocation. So it's fine if the function returns true but the object is actually unreachable, but not the other way around.

> He (Abbot) proposed instead an alternative framework called Merit, Fairness, and Equality (MFE) whereby university applicants are treated as individuals and evaluated through a rigorous and unbiased process based on their merit and qualifications alone

I am in full support with this, though it seems to me this is too idealized to be practical in practice. How can one reach a fair judgement of a student only based on a 1000-word essay in his/her application (which might not even be written by him/herself)?

However, I'm still saddened by that the MIT response to this incident is simply "it is Abbot's right of free expression to say whatever he wants", but nothing about what he actually said, or whether it at least makes some sense. It's as if MIT treated Abbot as an unknowing child whose nonsense words shall be tolerated, which is disturbing. Below is part of the mail list letter I received:

> Freedom of expression is a fundamental value of the Institute.

> I believe that, as an institution of higher learning, we must ensure that different points of view – even views that some or all of us may reject – are allowed to be heard and debated at MIT. Open dialogue is how we make each other wiser and smarter.

> This commitment to free expression can carry a human cost. The speech of those we strongly disagree with can anger us. It can disgust us. It can even make members of our own community feel unwelcome and illegitimate on our campus or in their field of study.

> I am convinced that, as an institution, we must be prepared to endure such painful outcomes as the price of protecting free expression – the principle is that important.

> Amazingly, we find that the GCC compilers is able to compile Travis’ is_empty C++ function to constant-time code.

It's actually an interesting example where undefined behavior allowed compiler optimization:

(1) dereferencing an invalid pointer is UD

(2) signed integer overflow is UD.

This allows the compiler to assume that the program never crashes and the counter never overflows. The loop is then optimized out knowing that it is read-only thus has no side-effects.

> The slide also shows that AT&T retains “cloud storage internet/web browsing” data for 1 year.

I never thought before that ISPs would really keep track of every user's browsing history, but apparently as cheap as the disks are today, this has become true. Can't think of any use of this data other than for mass surveillance.

Not sure what you mean by 'emulator', but I will assume you meant ISA for a different hardware architecture.

> The emulator would identify hot code blocks, generate C like constructs (or ideally the AST)

The idea behind copy-and-patch should be able to handle your use case of quickly translating code blocks in another ISA to native instructions.

However, I think Pochi's metaprogramming capabilities might not be too relevant here. After all, you are translating from a block of CPU instructions (in another ISA). It's probably not necessary or helpful to translate them back to C-like control flow only to compile them again.

> The ability to call back to host methods, handle exception semantics and all the while being totally oblivious to the platform

I'm not sure what you mean here. Yes Pochi supports intuitive inter-operation with the host program (call methods, handle exception etc). This is important for metaprogramming use case (e.g., generating a program that executes a SQL query), but I don't see what it has to do with emulating a program in another architecture.

Put the innocent and the defenseless at life risk for the enjoyment of the others.

Probably reasonable if R<1 since it's only going to affect a limited number of people, but this is absurd for countries where the R is still >1. This is effectively putting everyone at life risk.

I am the author.

> which do JIT "compilation" in the business-logic/abstract sense of the term?

If I understood what you said correctly, you mean translating something to SQL?

The problem this paper is trying to solve is how we can generate binary code fast. If your target (i.e. the stuff you want to generate in the end) is not binary code, but some high level representation like a SQL text, then I think it doesn't have much to do with the technique in our paper.

Well in some sense yes. Template compiler is a very old idea, and this is a template-compiler-styled compiler.

The interesting part is the various improvements to this old idea that allows us to both compile fast and generate good code.

Java is static-typed already and only has a low degree of dynamism, so the importance of doing so is much less (only to reduce the initialization time I believe).

Interesting post. Seems similar to the approach of HPHPC for PHP, where the code is first run offline in a simulated environment to collect type information, and then use these speculations to generate static-typed code ahead-of-time.

Would be interested to see a performance comparison with V8 JIT (instead of V8 interpreter).

Very interesting paper.

The paper is very long and mostly math so I only skimmed it. It seems to me (as a practitioner) the key insight to alleviate primary clustering effects is the following:

At high loads, when you rebuild the hash table, you put k/2 "fake tombstones" evenly across the table, where k is the # of free slots in the table. This way, when one performs an insertion, the iteration length is going to be short (thanks to the evenly spreaded tombstone), thus alleviating the primary clustering effects.

I'm very interested in seeing some empirical numbers for graveyard hashing in both RAM and external storage use case.

As already mentioned, the difference is that Windows is monopolized by a commercial company, is not open-sourced and does not accept outside contribution.

You may also be interested by MS Edge's decision to abandon their ChakraCore engine, which is similar to what I reasoned above [1]

[1] https://blogs.windows.com/windowsexperience/2018/12/06/micro...

> Seriously though, the glaring lack of information and supporting evidence is kinda the point

Yeah that's the point.

I would say, instead of blaming Trump for punching a hole in the cornerstone of democracy, it just demonstrated how fragile the system already is. So many people are already eagerly ready to do that to win. Trump only dog-whistled them by doing so as president.

The website you linked doesn't even list any evidence supporting its claim that "statistical patterns of voting fraud can be detected", not to mention the "easily reproducible" part. It only has a single page, claiming that there is fraud and asking users to fill forms.

According to the paper, the leak occurs when the battery is close to full (>95%). In that case, since the battery is almost full, the power drained from the charger reflects the concurrent power being consumed by the device.

I initially thought this is easily fixable (for example, by always request full power), but then I realized it isn't, since the extra power would then have to be dissipated as heat, which is a bad cellphone UX as well.

IMO browser is a tool, just like operating system. The ultimate praise of a tool is "it works". For browser this means rendering every web page correctly, and for OS this means working on every hardware configuration correctly.

So if we already have a working tool, why do we want to spend the redundant work to invent more tools that solves the same problem, or to maintain the other tools so they are all standard-compliant?

Of course, it's a different story if the existing tool is monopolized by a commercial company. But Chrome is open sourced, and there is a committee steering the future Web standards.

So imo we don't need another Chrome, just like we don't need another Linux.

I agree that it's very important to clearly communicate the designs to the users. However, I feel that it's practically hard for many reasons. For example, it's hard to argue in the document why something is not done (e.g. why are entities referenced by pointer instead of string name). And also keep in mind that the document is read by people of various degrees of expertise. It's hard to make all of them happy.

In fact, when I first started using LLVM, I created a basic block and put everything into it. Then LLVM complains, and I learnt that a basic block must be a list of straight-lined operations followed by a branch. At that moment I was feeling similar to the author in this post: "Why is there such a bizzare rule that makes me harder to write my program?" But after I was forced to rewrite my code to conform with this rule, surprisingly, I found my program logic much easier to understand. And when I gradually knew a bit more about LLVM, I understood that the basic block rule is there for other very good reasons as well.

So is this good design? Clearly it is. This design decision not only helps with the library itself, but also forces the users to write code in a less error-prone way.

However, is it possible to justify this in the document, so another user won't have to go through my initial frustration? I am doubtful. At least my personal feeling is that, I won't be able to understand why it is designed this way, unless I actually have played around and experienced it myself.

I hope this clarifies my point on why sometimes it is hard to communicate design philosophies.

I definitely agree that it would be better if LLVM has a more flattened learning curve and a more accessible manual.

I'm only pointing out that many "problems" listed in the post are intentional design choices for good reasons. They are not downsides that should be improved.

Not LLVM expert, but I don't agree with some of your arguments.

> My side of the code generator had to recognize when a variable had already been defined and keep track of its pointer

For human, it's natural to write code text that reference each variable by its name. However, for a compiler, it's really error prone (and inefficient) to reference a variable by its string name (for example, think about shadowing). The natural way to reference an entity is by its object pointer, which is what LLVM does. This is especially true considering LLVM is designed to perform various complex transformations.

> There is a pass called mem2reg that will convert to SSA, but it needs you to allocate and store variables in memory (instead of in registers).

The purpose of mem2reg is to make your job easier. It's weird to say that it "needs" you to allocate allocas for your variables: that's what it allows you to do (for your own convenience). If you prefer to generate PHI nodes directly, you can just do so.

> LLVM IR has opinions about variable scope

Not sure what you are referencing to. LLVM only has 'alloca', which knows nothing about "scope". It must be defined before being referenced -- but this is true for everything in SSA.

So (at least some of) the former Afghan gov officials are not even using their own email server, and just store whatever possibly-sensitive information on Google servers..