Re: Questions about LTO infrastructure and pragma omp target

[Jakub Jelinek <jakub at redhat dot com>]

On Thu, Aug 15, 2013 at 05:36:39PM +0400, Ilya Verbin wrote:
> 2.  The second question, regarding #pragma omp target implementation.
> I'm going to reuse LTO approach in a prototype, that will produce 2
> binaries - for host and target architectures.  Target binary will contain
> functions outlined from omp target region and some infrastructure to run
> them.
> To produce 2 binaries we need to run gcc and ld twice.  At the first run
> gcc will generate object file, that contains optimized code for host and
> GIMPLE for target.  At the second run gcc will read the GIMPLE and
> generate optimized code for target.
> 
> So, the question is - what is the right place for the second run of gcc
> and ld?  Should I insert them into liblto_plugin.so?  Or should I create
> entirely new plugin, that will only call gcc and ld for target, without
> performing any LTO optimizations for host?
> Suggestions?

The rough plan (partly discussed at the accelerator BoF) was that we would
stream LTO bytecode into special section somewhere during ompexp pass or so
(note, right now LTO streaming streams everything in a TU, we'd want to
stream only the routines with "omp declare target" attribute, and outlined
#pragma omp target regions, and vars referenced from it and types etc.),
then have some other linker plugin (-fopenmp/-fopenacc) that would recognize
these special sections and run lto1 on those (if not -flto in some mode that
would just compile each TU separately or something), then we need to link
it together and let the linker put it into some section of the host binary
or shared library.

	Jakub