On Wed, Jul 7, 2021 at 12:19 PM 'Marco Elver' via Clang Built Linux
[off-list ref] wrote:

What is the story with Rust and LTO? Intuitively, I would expect Rust
code to only perform optimally if the kernel is built with LTO
(currently only supported via Clang).

Cross-language LTO is possible and is used in other major projects.
For instance, Firefox seems to use it:

    https://bugzilla.mozilla.org/show_bug.cgi?id=1486042

However, we have not taken a look at it yet for the kernel.

Because if calls to every one of these helpers are real calls, I would
expect performance to be pretty poor. There's probably a reason these
are macros or inlinable functions.

Maybe -- we need to measure.

I would almost go so far and suggest that CONFIG_RUST be modified as
follows:

Even if the performance was unacceptable, there are other ways than
LTO to solve this, such as implementing the equivalent macro/function
on Rust's side.

Furthermore, we need to consider GCC too.

Thus I would not be premature in making it depend on LTO.

Cheers,
Miguel