LTO Dead Field Elimination

Richard Biener richard.guenther@gmail.com
Mon Jul 27 12:36:06 GMT 2020


On Fri, Jul 24, 2020 at 5:43 PM Erick Ochoa
<erick.ochoa@theobroma-systems.com> wrote:
>
> 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

Just had some time to peek into this.  Ugh.  The code doesn't look like
GCC code looks :/  It doesn't help to have one set of files per C++ class (25!).
The code itself is undocumented - it's hard to understand what the purpose
of all the Walker stuff is.

You also didn't seem to know walk_tree () nor walk_gimple* ().

Take as example - I figured to look for IPA pass entries, then I see

+
+static void
+collect_types ()
+{
+  GimpleTypeCollector collector;
+  collector.walk ();
+  collector.print_collected ();
+  ptrset_t types = collector.get_pointer_set ();
+  GimpleCaster caster (types);
+  caster.walk ();
+  if (flag_print_cast_analysis)
+    caster.print_reasons ();
+  ptrset_t casting = caster.get_sets ();
+  fix_escaping_types_in_set (casting);
+  GimpleAccesser accesser;
+  accesser.walk ();
+  if (flag_print_access_analysis)
+    accesser.print_accesses ();
+  record_field_map_t record_field_map = accesser.get_map ();
+  TypeIncompleteEquality equality;
+  bool has_fields_that_can_be_deleted = false;
+  typedef std::set<unsigned> field_offsets_t;

there's no comments (not even file-level) that explains how type escape
is computed.

Sorry, but this isn't even close to be coarsely reviewable.

> 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!


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