LTO Dead Field Elimination

[Erick Ochoa]

This patchset brings back struct reorg to GCC.

We’ve been working on improving cache utilization recently and would 
like to share our current implementation to receive some feedback on it.

Essentially, we’ve implemented the following components:

     Type-based escape analysis to determine if we can reorganize a type 
at link-time

     Dead-field elimination to remove unused fields of a struct at 
link-time

The type-based escape analysis provides a list of types, that are not 
visible outside of the current linking unit (e.g. parameter types of 
external functions).

The dead-field elimination pass analyses non-escaping structs for fields 
that are not used in the linking unit and thus can be removed. The 
resulting struct has a smaller memory footprint, which allows for a 
higher cache utilization.

As a side-effect a couple of new infrastructure code has been written 
(e.g. a type walker, which we were really missing in GCC), which can be 
of course reused for other passes as well.

We’ve prepared a patchset in the following branch:

   refs/vendors/ARM/heads/arm-struct-reorg-wip

We’ve also added a subsection in the GCC internals document to allow 
other compiler devs to better understand our design and implementation. 
A generated PDF can be found here:

    https://cloud.theobroma-systems.com/s/aWwxPiDJ3nCgc7F
    https://cloud.theobroma-systems.com/s/aWwxPiDJ3nCgc7F/download

    page: 719

We’ve been testing the pass against a range of in-tree tests and 
real-life applications (e.g. all SPEC CPU2017 C benchmarks). For 
testing, please see testing subsection in the gcc internals we prepared.

Currently we see the following limitations:

* It is not a "true" ipa pass yes. That is, we can only succeed with 
-flto-partition=none.
* Currently it is not safe to use -fipa-sra.
* Brace constructors not supported now. We handle this gracefully.
* Only C as of now.
* Results of sizeof() and offsetof() are generated in the compiler 
frontend and thus can’t be changed later at link time. There are a 
couple of ideas to resolve this, but that’s currently unimplemented.
* At this point we’d like to thank the GCC community for their patient 
help so far on the mailing list and in other channels. And we ask for 
your support in terms of feedback, comments and testing.

Thanks!