The article title is misleading. It is not that Rust compiler was not able to optimize some low-level operations. Rather the author came up with encoding schema that fit most things the interpreter dealt with into 64 bit. This replaced the previous schema that used 128 bit for everything but that can be directly mapped into Rust enums. The catch was that it was necessary to allocate some things on the heap and use pointer indirection but that was used for rare values so on average the new schema provided nice win.

One cannot expect a compiler to come up with such encoding.

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win311fwg 4 hours ago | parent | next [-]

What is misleading about the title? A custom encoding scheme is exactly what it suggests. Maybe it has been edited since your comment was posted?

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gpm 2 hours ago | parent | next [-]

I'd actually point at the other half of the title than the existing comment when being pedantic "Replacing [...] with a 64-Bit Word" isn't quite right, it was replaced with a manually packed 64-Bit Word and the occasional heap allocation.

I'm not sure being this pedantic is particularly useful in titles though...

	▲	Dylan16807 2 hours ago \| parent [-]
		It's worth calling out either way. It's not just an encoding scheme, it's a reasonably significant change in architecture.

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dymk 4 hours ago | parent | prev [-]

It wasn’t replacing one rust enum, it was replacing what are effectively multiple enums

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dzaima 4 hours ago | parent [-]

How so? It's replacing multiple enum variants, but just one enum, "enum Value".

(also; if anything, the title is implying the exact opposite of "Rust compiler was able to optimize ...", "Replacing a Rust [...] with [...]" is clearly moving away from Rust-magic to something else)

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Brian_K_White 2 hours ago | parent [-]

Probably in the sense that you can remove the word rust and nothing changes. It's not about some failure of rust to be efficient at enums, but the title says it is.

	▲	win311fwg 2 hours ago \| parent [-]
		'Enum' is ill-defined so the addition of Rust is significant as it indicates what one can expect with how data is structured. There is nothing in that speaks to the Rust compiler or Rust being inefficient or anything of the sort. It remains unclear where this idea is coming from. There is nothing in title that would send you there. Unless, again, the title was edited at some point?

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pbiggar 4 hours ago | parent | prev [-]

When I think of how a "compiler" could make these optimizations, I think the right place is an optimizing LLM (so, just a regular coding agent that you prompted to find optimizations like this one), making the changes in source at the request of the developer. That provides the dev with adequate input on whether they would like to opt-in to an unsafe optimization like this one. The compiler can continue to do deterministically-safe optimizations.

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gpm 2 hours ago | parent | next [-]

What I'd like to enable this use of LLMs more recklessly is a compiler with formal methods that lets me guarantee equivalence between the opaque optimized code and something actually understandable.

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trickypr 3 hours ago | parent | prev [-]

That seems like a horrible idea:

1. Do you really want the rust compiler to run at the speed of an llm?

2. Compiler optimisations are already extremely unpredictable with deterministic compilers[1], I hate to think how unpredictable your compiler would be.

3. What if someone else wants to build the software, do they have to decide on optimisations now? What if the optimisation depends on your features not available on old generations of CPU? (There is a reason we don’t compile with -march=native)

4. Compilers already have “unsafe” optimisations, but people rarely enable them (-ffast-math)

[1]: https://faultlore.com/blah/oops-that-was-important/

	▲	jmalicki 25 minutes ago \| parent \| next [-]
		> Do you really want the rust compiler to run at the speed of an llm? That... might actually be an improvement?
	▲	pbiggar 3 hours ago \| parent \| prev [-]
		You misunderstand me. I'm saying that the developers can make these optimizations with LLMs, at the source level, and thus they don't need to be added to compilers. Like just open Claude Code and ask it to find optimizations. That's the right place for this kind of optimization.