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

[Rong Xu]

On Mon, Jul 29, 2024 at 2:53 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sun, Jul 28, 2024 at 01:29:59PM -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.
> >
> > The support requires a Clang compiler LLVM 19 or later, and the
> > create_llvm_prof tool
> > (https://github.com/google/autofdo/releases/tag/v0.30.1). This
>
> What's the relation between this and llvm-profgen? Is the above simply
> a google 'internal' proof of concept thing that will eventually make its
> way into llvm-profgen?
>
> It seems a bit weird LLVM landed propeller without the required profile
> generation tool.

AutoFDO historically required a third-party tool
(create_llvm_prof, mentioned above) in Clang.
AutoFDO in GCC also requires this tool (same source, different name).

llvm-profgen is a relatively new tool that was integrated directly into LLVM
(similar purpose, but different implementation).

AutoFDO and create_llvm_prof are not "proof of concept". They have been widely
used in Google for many years. Propeller currently uses create_llvm_prof as the
profile converting tool. But there is an effort to move to LLVM.