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Comment by qingcharles | Hacker News Reader

As someone who wrote x86 optimization code professionally in the 90s, do we need to do this manually still in 2025?

Can we not just write tests and have some LLM try 10,000 different algorithms and profile the results?

Or is an LLM unlikely to find the optimal solution even with 10,000 random seeds?

Just asking. Optimizing x86 by hand isn't the easiest, because to think through it you start to have to try and fit all the registers in your mind and work through the combinations. Also you need to know how long each instruction combination will take; and some of these instructions have weird edge cases that take vastly longer or quicker to run that is hard for a human to take into account.

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I guess your question could be rephrased as "couldn't we come up with better compilers?" (LLM-based or not, brute force based or not).

I don't have an answer but I believe that a lot of effort has been put in making (very smart) compilers already, so if it's even possible I doubt it's easy.

I also believe there are some cases where it's simply not possible for a compiler to beat handwritten assembly : indeed there is only so much info you can convey in a C program, and a developer who's aware of the whole program's behavior might be able to make extra assumptions (not written in the C code) and therefore beat a compiler. I'm sure people here would be able to come up with great practical examples of this.

While using a LLM might not be the best approach, it would be interesting to know if there are some tools these days that can automate this.

Like, I should be able to give the compiler a hot loop and a week, and see what it can come up with.

One potential pitfall I can see is that there are a lot of non-local interactions in moderns systems. We have large out-of-order buffers, many caching layers, complex branch predictors, and an OS running other tasks at the same time, and a dozen other things.

What is optimal on paper might not be optimal in the real world.

> Like, I should be able to give the compiler a hot loop and a week, and see what it can come up with.

There are optimization libraries which can find the optimum combination of parameters for an objective, like Optuna.

It would be enough to expose all the optimization knobs that LLVM has, and Optuna will find the optimum for a particular piece of code on a particular test payload.

What you're looking for is https://en.wikipedia.org/wiki/Superoptimization.

Collaborators have actually superoptimized some of the more complicated Highway ops on RISC-V, with interesting gains, but I think the approach would struggle with largish tasks/algorithms?

I have tried with Grok3 and Claude. They both seem to have an understanding of the algorithms and data patterns which is more than I expected but then just guess a solution that's often nonsensical.

You would need to be very careful about verifying the output. Having an LLM generate patterns and then running them through a SAT solver might work, but usually it's only really feasible for short sequences of code.

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