[LLVMdev] FYI: Planning to remove ProfileInfo and related passes from LLVM

[Alastair Murray]

On 16/07/12 07:33, Chandler Carruth wrote:
> Can you start a thread with what your specific goals are?

This is to follow.


> I'd like to hear an explicit OK from you and your mentor before I remove
> anything, as I don't want to get in the way of any immediate progress or
> work you're doing as part of GSoC.

This is an explicit OK for you to remove ProfileInfo and related code. 
My mentor and I agree that it is better to work on code that will be 
sticking around.

The existing EdgeProfiling.cpp and the runtime code may be worth saving 
(these require ProfilingUtils.* and ProfileInfoTypes.h respectively).

Without a profile loader they aren't useful (even llvm-prof is based on 
ProfileInfoLoader), but as they count edge frequency it intuitively 
seems like the profiling data they generate could be used by a rewritten 
BranchProbability profile loader.

One note, if I plan on submitting patches to restore rewritten parts is 
destroying `svn blame` history a problem?


> The missing pieces are:
> 1) Ensuring that passes such as SimplifyCFG preserve as much branch
> weight metadata as possible. This is already an issue for metadata that
> comes from __builtin_expect() source code annotations, so it is
> something you can write tests for today and observe problems.
> 2) A pass to load profile data and attach metadata to branch
> instructions based on it.
> 3) Ensuring that #2 works with the profiles produced by an instrumented
> binary.
>
> It's important to note that #2 is easy. It's #1 and #3 that are really
> hard. When I have talked to others about #3, there has been the feeling
> that we would probably want to write a custom instrumentation pass that
> would add instrumentation to the right LLVM IR formations, in order to
> make sure that when it is read back in, the profile data matches up with
> the IR correctly, and is available early enough in the optimization
> passes to be used.

I understand that #1 is hard, but #3 isn't so clear to me.  The current 
ProfileInfo based -insert-edge-profiling and -profile-loader passes can 
already do this.  The instrumentation and loading passes have to occur 
at exactly the same point of their respective pass pipelines, but that 
is unavoidable.


Regards,
Alastair.