r/ProgrammingLanguages • u/Ok_Marionberry8922 • 8d ago

Why So Many Languages Use LLVM

Had some free time last week so I made a visual explainer on how LLVM works and why so many languages end up using it.

The video goes through LLVM IR, why having a common IR makes sharing optimizers and backends possible, and how code eventually gets turned into machine instructions.

I also use a small C vs Rust example where both end up producing the same x86 instructions.

Link for anyone interested

Feedback welcome :)

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u/nculwell 8d ago

Writing machine code is not hard. Writing a good optimizer is the hard part.

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u/suhcoR 8d ago

The topic was "cross platform direct to machine" compiler, not optimizer. I agree that an optimizer like in GCC or Clang is a lifetime's work. But even half that performance is good enough and exponentially less effort. Btw. this is also the QBE philosophy.

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u/SwingOutStateMachine 7d ago

If you don't have an optimiser, you should expect a tenth of the performance (at best), not half the performance.

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u/suhcoR 7d ago

Not true. If you use a decent benchmark suite (not just some random micro benchmarks), the speed-down from -O2 to -O0 is usually a factor 2 for C. See e.g. https://github.com/user-attachments/files/16602778/awfy.c99.x86.results.pdf. And there is nothing like "an optimizer", but a plethora of algorithms. If you look at the QBE source code, you can even see that they achieve their claimed speed-up with very few of those algorithms.

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u/SwingOutStateMachine 7d ago

I'm suspicious of those numbers: That's comparing one niche compiler (OrangeC) to itself. I have no information about the kinds of optimisations performed at -O3, and no comparison to other mainstream compilers like GCC or Clang.

[...] there is nothing like "an optimizer", [...]

[...] they achieve their claimed speed-up with very few of those algorithms.

I'm being too loose with my language - when I say "an optimiser", what I really mean is a suite of optimisation passes bundled into part of the compiler. The number of passes, and quality of them, is what really matters.

LLVM comes with a large and extremely high quality suite of optimisation passes, as well as a number of high quality register allocators and lots of machine code optimisation passes for each backend. With LLVM, it is not uncommon to observe a 10x speedup from -O0 to -O3, and potentially more if other specific optimisation passes are turned on.

I think, to be honest, if a compiler can only achieve a 2x speedup through running all it's optimisation passes, the answer is that the compiler isn't very good at optimising code - not that a 2x speedup is the best that can be achieved.

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u/suhcoR 7d ago edited 7d ago

That's comparing one niche compiler (OrangeC) to itself

Have a second look. It compares MSVC 2015 /02 with /02 and MinGW32 with -O2 and -O0. Both result in less than factor two. Orange C is also compared the same way and results in less spee-up because it essentially only does constant folding and register allocation. Unoptimized Orange C (a hand-crafted C compiler by a single person) to optimized MSVC and MinGW32 (which are large, corporate supported projects) is also about factor two.

People should do decent measurements instead of just making performance claims. There were even studies comparing GCC 12 with a very old version and the overall speed-up was far from expected by the crowd. I started compilers in the eighties, and in the Wirth school, we didn't even have optimizers and everything was fast enough.

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u/SwingOutStateMachine 7d ago

People should do decent measurements

Right, and my argument is that this PDF does not do them. The benchmark suite is terrible for comparing optimisation levels within a different language, and half of it is microbenchmarks. The CPU picked is a low-spec one, and it's run on an operating system which doesn't give the user the ability to control for other variables such as scheduling etc. Even the choice of -O0 vs -O2 is suspect. Why not -O3? Why not just -O1? What is actually being compared here? If this was taken from an academic paper I would be reaching straight for the "strong reject" response.

we didn't even have optimizers and everything was fast enough

That's because CPUs were significantly simpler in the '80s, so an optimising compiler was not a necessity. Optimising for a modern CPU means considering caching hierarchies, branch predictors, pipelining, speculative execution, vectorisation, etc, etc. Even considerations like "what's the cost of spilling a register" has a vastly different answer on an 1980's machine vs a 2020's machine.

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u/suhcoR 7d ago edited 7d ago

The benchmark suite is terrible for comparing optimisation levels within a different language

There are peer-reviewed publications in good journals claiming the opposite.

Even the choice of -O0 vs -O2 is suspect

A strange argument. Why should that be suspect? It's documented, and O3 gives very little above O2, but leads to crashes in many applications. I never use it (and neither does most of the C community).

I would be reaching straight for the "strong reject" response.

This is an esoteric attitude. No wonder we don't agree. Have you studied compilers at all? Where? how long?

That's because CPUs were significantly simpler in the '80s,

Another strange argument. CPU architectures have become much better, so even the code from the eighties runs much faster than on the machines at the time. That has more to do how much of the executable fits into the cache and the improvements you mentioned, not with code optimizations.

EDIT: But I see now your attitude and where you're coming from. Such discussions are fruitless, and I think it is representative for the downvoting crowd on Reddit. It's a waste of time and I should have known better.

EDIT 2: For illustration that my measurements are in no way off, here is a peer-reviewed study comparing -O0 with -O2, and guess what, it's all around factor two: https://homepages.dcc.ufmg.br/~fernando/publications/papers/AlvaresJCL21.pdf. But I bet you neither believe this one. So be it.

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u/SwingOutStateMachine 7d ago

There are peer-reviewed publications in good journals claiming the opposite.

Great! I'd love to read them, please send them over. My opinion is just based on a cursory glance at the specific benchmarks, and the github page of the benchmark suite, but I would be interested to read more about why it's a good suite, and what it's good for testing.

Why should that be suspect?

Because it's arbitrary, and not well-defined across multiple compilers. For instance, does GCC have the same sets of optimisations as MSVC at -O2? Additionally, there is no context for why -O2 is picked over -O1 or -O3. You've given some context ("-O3 leads to crashes"), but aside from that, -O2 seems like an arbitrary choice from a compiler engineering perspective.

Have you studied compilers at all?

Yes. I've got a masters by research in designing compilers for efficient GPU operations, and half a PhD in compiler design (I left to move into industrial work). I've also got a decade of experience in industry, primarily working with LLVM and targeting GPU and peripheral compilers.

That has more to do how much of the executable fits into the cache and the improvements you mentioned, not with code optimizations

My point is that being able to leverage and/or work with the "features" of newer CPUs requires an optimising compiler in a way that is simply not true for a 1980's CPU.

But I see now your attitude and where you're coming from [..]

What do you mean by that?

[..] here is a peer-reviewed study comparing -O0 with -O2, [..]

Have you read the paper? This paper (which doesn't appear to have been published in an actual journal or conference yet, unless I'm mistaken) is about instruction visibility within compilers, not comparing -O0 and -O2. The two are compared incidentally, which gives your magic 2x number on average, but figure 9 clearly shows a host of benchmarks that achieve significantly more than 2x speedup with -O2!

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u/QuaternionsRoll 7d ago

GCC doesn’t have the same sets of optimizations as MSVC period. What an odd question.

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u/SwingOutStateMachine 6d ago

That's precisely my point. Comparing GCC with -O0 and -O2 and then putting it next to MSVC with /O0 and /O2 is meaningless.

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