(pointersgonewild.com)
The old, enum based value type used a single big match statement to dispatch between all possible type combinations. Their assembler output looks like the match gets compiled to something like a big stack of nested if statements.
The new code uses an explicit fast path check with a dispatch into a tagged 'cold' path when the common case isn't hit. The generated code is a single upfront branch for the fast path that exits immediately, with a dispatch into the slow path in a separate function.
This would be contributing significantly to the performance improvements. The old path requires taking several branches even on the hot path. The new code has a single, highly predictable branch that skips all the messy dispatch for the other types.
This could have been implemented for the enum based value type, and I would expect to see a jump in performance there too even without the new compact value type. There will be a much higher branch predictor hit rate with the explicit fast path.
One cannot expect a compiler to come up with such encoding.
One could, however, imagine a sufficiently expressive language that allows the developer to specify the encoding schema without resorting to raw 64-bit words.
(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)
Unless, again, the title was edited at some point?
I'm not sure being this pedantic is particularly useful in titles though...
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)
That... might actually be an improvement?
Like just open Claude Code and ask it to find optimizations. That's the right place for this kind of optimization.
This new code also supplies similar abstractions. That actual specific code is much harder to reason about, but most users--and even the next person who works on the interpreter--simply won't care, or even know what is going on underneath the hood. The abstractions provided by the author do that work and apparently do it cleanly.
For most use-cases, that extra hand-written code isn't worth it. But in specific cases it can be, and the author has actually measured the value and determined that it is.
Also, Rust does try to do some of these optimizations itself. These aren't exposed in the stable language to let you do some more advanced things, but it wouldn't be impossible for you to get the best of both worlds by letting you communicate this stuff more directly to the compiler. Right now those things are more like "this value is where you should put the tag" than the more advanced stuff here, though. Would be cool to see someday!
Rust provides for example NonZeroU8 which is an 8-bit unsigned integer that's never zero, leaving it with 255 possible values and a convenient niche. You cannot make one of these yourself directly, because the mechanism used by Rust itself is a deliberately perma-unstable compiler-only proc macro which says "Hey compiler, I promise I only ever use bit patterns 0x01 through 0xFF inclusive".
Today you can either - hide a NonZero type inside your type and use that to get the niche, or, use an enum itself which automatically knows ever pattern it didn't use is a niche. In the future a hypothetical "Pattern Types" feature would let you make such types yourself as easily as Rust does
Personally I would like to make a Balanced set of types, like BalanacedI8 (the 8-bit integers except the most negative, so -127 to +127 inclusive) because I think lots of people have a use for types like i8 or i32 but don't need their unbalanaced most-negative value and could re-purpose it this way. And you can make such types... indeed I have... but it's only really practical in unstable Rust.
For a system language I wish Rust would support such things rather than coming with NonZero hacks.
NonZero isn't a hack: it's an example of a common pattern. If pattern types were available today, you'd still want NonZero, as an example of a pretty standard pattern.
The idea is, as always: prove out the specific version, then generalize.
(typep 3 '(or (integer 0 10) (integer 50 100)));; => TThe bit operations involved are pretty simple and won't take you long to figure out even if you've never done them before.
In the case of a tagged object in Rust, depending on how well the compiler can wrangle through it, you might even be able to add a `.unpack()` method that returns a pretty enum from a packed value, that you can pattern-match on or whatever, and let the compiler remove all the code of unpacking unused cases.
(using that directly for the addition example would end up less efficient of course, but still most likely beneficial. It's after this when there's a potential true readability vs performance tradeoff)
...unless it's supported by the language as a first class feature. See for example C++ and RVO.
this is brilliant, love it. stealing this idea immediately.
enum Value {
Float(f64),
Ptr(*const T),
}
That being said, the optimization is complex that may be insufficient:
> For my boxing scheme, I picked a bias value such that the lowest two bits end up being 10. That 1 in bit index 1 indicates that doubles can't be directly compared for equality. Amazingly, we only lose two bits of exponent, and we keep the full precision of the mantissa, meaning we lose no significant digits in the flonum representation.
This suggests the optimization needs more information about specifically how you want to box the float. There probably is some primitives worth considering standardizing to make this kind of optimization possible so that the tunable parameters are passed as const generic values.
enum Value {
SimpleFloat(64bit value),
ComplexNan(Heap pointer)
Ptr(*const T)
}
I'd be very unhappy if a compiler silently did this to me - it would make performance extremely hard to reason about. But it's not quite as bad as changing the semantics.