[LLVMdev] Reviewer for our Path Profiling Implementation

[Gergö Barany]

[Cc'd to a bunch of people who have in the past expressed interest in
profiling with LLVM.]

On Mon, Dec 06, 2010 at 10:39:37 -0800, Bob Wilson wrote:
> As far as I know, none of LLVM's standard passes make use of any profiling
> information.  If we are going to get any value from having profiling
> support in LLVM, so that it is worth the effort of maintaining that code,
> we need to figure out how we're going to use it.

Right. That's a lot of grunt work, and no one person or group will probably
want to do it, especially if the results will not be very visible to others.

For this reason, our group in Vienna would like to propose a bundling of
various people's efforts in profiling. In particular, we would be willing to
host an LLVM branch that is open to profiling-related contributions such as:
- various kinds of profilers
- patches to preserve/update profiling information across existing passes
- code that improves existing passes based on profiling info
- code that communicates profiling data to the backend (to get MBB profile
  data, profile-based spill weights etc.)

The idea is to be more open to such submissions than mainline LLVM is, so
that we can experiment with and compare different approaches. Hopefully,
some parts of this would converge to a state that would then deemed
acceptable for mainline LLVM. Even if not, we would have a common codebase
in which the more boring parts (such as preserving info across simple
passes) are solved once and for all.

> In my opinion, it is essential that LLVM passes be able to query the profile information with a standard API, and that they also be able to update the profile information to account for changes to the code (e.g., when a block is duplicated, when a loop is unrolled, etc.).  We need to have one API for those things regardless of whether the profile information came from __builtin_expect, simple heuristics, static analysis, or some kind of profile feedback.

Absolutely. Bringing together people with different approaches to profiling
should hopefully help us get a good picture about what this standard API
could look like; and then also provide a number of profilers and clients
conforming to that API.

To get some idea about the number of people who could be involved in a
project like this, I would like to ask for a quick show of hands: Who would
be interested in contributing code to LLVM-with-profiling? (Either actual
profiling code, or passes that use profiling information.)
Who would want to use the branch, even without contributing?

-- 
Gergö Barany, research assistant                   gergo at complang.tuwien.ac.at
Institute of Computer Languages        http://www.complang.tuwien.ac.at/gergo/
Vienna University of Technology                         Tel: +43-1-58801-58522
Argentinierstrasse 8/E185, 1040 Wien, Austria           Fax: +43-1-58801-18598