[dodobirdlord]

This is grey, but shouldn’t be. I assume people are downing it because it doesn’t give examples of the optimization opportunities it claims. So here are two that I think are likely most significant.

1. Rust’s aliasing semantics are more powerful than C’s unless you use the ‘restrict’ keyword everywhere, which most people don’t. It’s well recognized that FORTRAN continues to generally outperform C in many numeric applications because FORTRAN compilers are free to perform optimizations that C compilers cannot or don’t, due to the fact that multi-aliasing memory is undefined behavior in FORTRAN. The rust compiler currently doesn’t pass the relevant hints to the generated LLVM IR due to a long history of LLVM having bugs in ‘noalias’ handling, in large part due to the fact that those code paths are so rarely executed while compiling C code, itself due to the relatively low usage of the ‘restrict’ keyword.

2. Implicit arena allocations. The Rust compiler has access to information about possible aliasing and overlapping lifetimes that it can use to replace multiple allocations with similar lifetimes with offsets into a single allocation that is then all freed together. This is a complicated topic, but work is ongoing to make this a reality.

[jashmatthews]

2 might sound overly optimistic like "Rust will eventually beat C" but V8 already has an allocation folding optimization which combines multiple objects into a single site. There's also run time profiling to try and figure out the allocation lifetime.

https://static.googleusercontent.com/media/research.google.c... and https://static.googleusercontent.com/media/research.google.c...

[mrec]

> work is ongoing to make [implicit arena allocations] a reality.

Ooh, interesting; I hadn't heard about that but it makes a lot of sense. Do you happen to have an RFC or issue link? My google-fu is failing me.