linux-kernel - Re: C aggregate passing (Rust kernel policy)

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Message-ID: <B8A09E5A-D3A6-4A1F-94F6-9E301EE9BBE4@zytor.com>
Date: Sat, 22 Feb 2025 15:50:59 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
CC: Linus Torvalds <torvalds@...ux-foundation.org>,
        Ventura Jack <venturajack85@...il.com>, Gary Guo <gary@...yguo.net>,
        airlied@...il.com, boqun.feng@...il.com, david.laight.linux@...il.com,
        ej@...i.de, gregkh@...uxfoundation.org, hch@...radead.org,
        ksummit@...ts.linux.dev, linux-kernel@...r.kernel.org,
        miguel.ojeda.sandonis@...il.com, rust-for-linux@...r.kernel.org
Subject: Re: C aggregate passing (Rust kernel policy)

On February 22, 2025 1:22:08 PM PST, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
>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.
>

I think I know a thing or two about JITting for VLIW..  and so does someone else in this thread ;)