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

[Diego Novillo <dnovillo@google.com>]

On Tue, Jun 3, 2008 at 22:26, Chris Lattner <clattner@apple.com> wrote:

> 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?

As Ollie said, WHOPR is just an extension on the LTO framework to
cater for scalability when building large applications.  As such, when
building large applications we expect not to be able to apply IPA
passes that rely on having the whole program callgraph and bodies
loaded in memory.

However, WHOPR does not limit IPA passes to summary-only.  That's why
you see the distinction between IPA_PASS and SIMPLE_IPA_PASS in the
pass manager.

> 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

No.  Just the first pass that we are going to concentrate for the
initial implementation.

>> == Design Philosophy ==
>> * The implementation provides complete transparency. Developers
>> should be able to take advantage of LTO without having to modify
>> existing build systems and/or Makefiles, all that's needed is to add
>> an LTO option (-flto).
>
> Ok.  How do you handle merging of optimization info?   If I build one .o
> file with -Os and one with -O3 who wins or what does this mean?  If I build
> one with -ffast-math and one without, does the right thing happen?

Right now, mixed optimization flags will likely cause trouble.  We
have not really talked about this issue in detail.  I expect many/most
of these issues will be orthogonal to the driver, though.  We've
talked a bit about different ways of encoding the options into the IR,
but there is nothing concrete yet.  It's in my list of things to
discuss at the next BoF.

> Also, where does debug info (i.e. DWARF for -g) get stored?  I'm not talking
> about people debugging the compiler, I'm talking about people who want to
> build an executable with debug info.

In the .o file.  We are generating regular .o files (for now).

> Is there a specific reason you don't use the LLVM LTO interface?  It seems
> to be roughly the same as your proposed interface:

Not really.  This is mostly the first iteration.  Rafael and Robert
will be able to tell you much more about this.  I'm not directly
working on this aspect.


Diego.