Fwd: [whopr] Design/implementation alternatives for the driver and WPA

["Ollie Wild" <aaw@google.com>]

Reposting to the gcc list since my first email got bounced.

Ollie



On Tue, Jun 3, 2008 at 7:26 PM, Chris Lattner <clattner@apple.com> wrote:

> This is a very interesting design, and a very nice evolution from the previous proposal.  I'm not completely clear on the difference between LTO and whopr here.  Is LTO the mode "normal people" will use, and whopr is the mode where "people with huge clusters" will use?  Will LTO/whopr support useful optimization on common multicore machines?

WHOPR is just an extension of the original LTO proposal.  It seeks to
augment the LTO design by providing a mechanism for parallelizing the
final (link-time) optimization phase.  The design has been based on a
distcc-like distributed compilation model, so it should be beneficial
even to those with small to moderate sized clusters.  This doesn't
preclude parallelization on multi-core machines (and that has been
discussed to some degree), but I at least have treated that as a
secondary consideration.  A good example of this is in the WPA
discussion below.  On a multicore machine, repackaging doesn't make a
lot of sense because the compiler can efficiently cherry-pick function
bodies from different files.  However, in a distcc compiler farm, the
entirety of a file must be transferred, so this would result in a lot
of excess network overhead.

> Are you focusing on inlining here as a specific example, or is this the only planned IPA optimization that can use summaries?  It seems unfortunate to design a system where inlining is the only real IPO transformation you can do.  Does adding new interprocedural optimizations require adding whole new phases?

The WPA document is a cleaned up transcription of an internal document
I wrote.  During the transcription, some context got lost.  It's not
meant to be a description of a final implementation but rather a
pro/con comparison between two possible draft implementations.  The
goal is to get some basic infrastructure in place so that we can start
experimenting with it and better parallelize additional work.
Inlining is chosen as an initial feature because it's relatively easy
to implement and can be (coarsely) handled without support for
serializing IPA summary information.  Other IPA passes (e.g.
inter-procedural constant propagation) require additional
serialization capabilities (which Kenneth Zadeck is working on now).

Ollie