Re: [PATCH 6/6] Add Propeller configuration for kernel build.

[Maksim Panchenko <max4bolt@gmail.com>]

On Sun, Jul 28, 2024 at 01:29:56PM -0700, Rong Xu wrote:
> Add the build support for using Clang's Propeller optimizer. Like
> AutoFDO, Propeller uses hardware sampling to gather information
> about the frequency of execution of different code paths within a
> binary. This information is then used to guide the compiler's
> optimization decisions, resulting in a more efficient binary.

Thank you for submitting the patches with the latest compiler features.

Regarding Propeller, I want to quickly mention that I plan to send a
patch to include BOLT as a profile-based post-link optimizer for the
kernel. I'd like it to be considered an alternative that is selectable
at build time.

BOLT also uses sampling, and the profile can be collected on virtually
any kernel (with some caveats).  There are no constraints on the
compiler (i.e., any version of GCC or Clang is acceptable), while Linux
perf is the only external dependency used for profile collection and
conversion. BOLT works on top of AutoFDO and LTO but can be used without
them if the user desires. The build overhead is a few seconds.

As you've heard from the LLVM discussion
(https://discourse.llvm.org/t/optimizing-the-linux-kernel-with-autofdo-including-thinlto-and-propeller)
and LPC talk (https://lpc.events/event/18/contributions/1921/), at Meta,
we've also successfully optimized the kernel and got similar results.

Again, this is a heads-up before the patch, and I would like to hear
what people think about having a binary optimizer as a user-selectable
alternative to Propeller.

Thanks,
Maksim