Re: C aggregate passing (Rust kernel policy)

Kent Overstreet
2025-02-22T21:22:15Z

On Sat, Feb 22, 2025 at 12:54:31PM -0800, H. Peter Anvin wrote:
> VLIW and OoO might seem orthogonal, but they aren't – because they are
> trying to solve the same problem, combining them either means the OoO
> engine can't do a very good job because of false dependencies (if you
> are scheduling molecules) or you have to break them instructions down
> into atoms, at which point it is just a (often quite inefficient) RISC
> encoding. In short, VLIW *might* make sense when you are statically
> scheduling a known pipeline, but it is basically a dead end for
> evolution – so unless you can JIT your code for each new chip
> generation...

JITing for each chip generation would be a part of any serious new VLIW
effort. It's plenty doable in the open source world and the gains are
too big to ignore.

> But OoO still is more powerful, because it can do *dynamic*
> scheduling. A cache miss doesn't necessarily mean that you have to
> stop the entire machine, for example.

Power hungry and prone to information leaks, though.