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Re: [PATCH 0/6] Conversion of gimple types to C++ inheritance (v3)


On 10/31/13 10:26, David Malcolm wrote:
The gimple statement types are currently implemented using a hand-coded
C inheritance scheme, with a "union gimple_statement_d" holding the
various possible structs for a statement.

The following series of patches convert it to a C++ hierarchy, using the
existing structs, eliminating the union. The "gimple" typedef changes
from being a
   (union gimple_statement_d *)
to being a:
   (struct gimple_statement_base *)

There are no virtual functions in the new code: the sizes of the various
structs are unchanged.

It makes use of "is-a.h", using the as_a <T> template function to
perform downcasts, which are checked (via gcc_checking_assert) in an
ENABLE_CHECKING build, and are simple casts in an unchecked build,
albeit it in an inlined function rather than a macro.

For example, one can write:

   gimple_statement_phi *phi =
     as_a <gimple_statement_phi> (gsi_stmt (gsi));

and then directly access the fields of the phi, as a phi.  The existing
accessor functions in gimple.h become somewhat redundant in this
scheme, but are preserved.

The earlier versions of the patches made all of the types GTY((user))
and provided hand-written implementations of the gc and pch marker
routines.  In this new version we rely on the support for simple
inheritance that I recently added to gengtype, by adding a "desc"
to the GTY marking for the base class, and a "tag" to the marking
for all of the concrete subclasses.  (I say "class", but all the types
remain structs since their fields are all publicly accessible).

As noted in the earlier patch, I believe this is a superior scheme to
the C implementation:

   * We can get closer to compile-time type-safety, checking the gimple
     code once and downcasting with an as_a, then directly accessing
     fields, rather than going through accessor functions that check
     each time.  In some places we may want to replace a "gimple" with
     a subclass e.g. phis are always of the phi subclass, to get full
     compile-time type-safety.

   * This scheme is likely to be easier for newbies to understand.

   * Currently in gdb, dereferencing a gimple leads to screenfuls of text,
     showing all the various union values.  With this, you get just the base
     class, and can cast it to the appropriate subclass.

   * With this, we're working directly with the language constructs,
     rather than rolling our own, and thus other tools can better
     understand the code. (e.g. doxygen).

Again, as noted in the earlier patch series, the names of the structs
are rather verbose.  I would prefer to also rename them all to eliminate
the "_statement" component:
   "gimple_statement_base" -> "gimple_base"
   "gimple_statement_phi"  -> "gimple_phi"
   "gimple_statement_omp"  -> "gimple_omp"
etc, but I didn't do this to mimimize the patch size.  But if the core
maintainers are up for that, I can redo the patch series with that
change also, or do that as a followup.

The patch is in 6 parts; all of them are needed together.
And that's part of the problem. There's understandable resistance to (for example) the as_a casting.

There's a bit of natural tension between the desire to keep patches small and self-contained and the size/scope of the changes necessary to do any serious reorganization work. This set swings too far in the latter direction :-)

Is there any way to go forward without the is_a/as_a stuff? ie, is there're a simpler step towards where we're trying to go that allows most of this to go forward now rather than waiting?


   * Patch 1 of 6: This patch adds inheritance to the various gimple
     types, eliminating the initial baseclass fields, and eliminating the
     union gimple_statement_d.   All the types remain structs.  They
     become marked with GTY(()), gaining GSS_ tag values.

   * Patch 2 of 6: This patch ports various accessor functions within
     gimple.h to the new scheme.

   * Patch 3 of 6: This patch is autogenerated by "refactor_gimple.py"
     from https://github.com/davidmalcolm/gcc-refactoring-scripts
     There is a test suite "test_refactor_gimple.py" which may give a
     clearer idea of the changes that the script makes (and add
     confidence that it's doing the right thing).
     The patch converts code of the form:
       {
         GIMPLE_CHECK (gs, SOME_CODE);
         gimple_subclass_get/set_some_field (gs, value);
       }
     to code of this form:
       {
         some_subclass *stmt = as_a <some_subclass> (gs);
         stmt->some_field = value;
       }
     It also autogenerates specializations of
         is_a_helper <T>::test
     equivalent to a GIMPLE_CHECK() for use by is_a and as_a.
Conceptually I'm fine with #1-#3.


   * Patch 4 of 6: This patch implement further specializations of
     is_a_helper <T>::test, for gimple_has_ops and gimple_has_mem_ops.
Here's where I start to get more concerned.


   * Patch 5 of 6: This patch does the rest of porting from union access
     to subclass access (all the fiddly places that the script in patch 3
     couldn't handle).

   * Patch 6 of 6: This patch updates the gdb python pretty-printing
     hook.
Conceptually #5 and #6 shouldn't be terribly controversial.

THe question is can we move forward without patch #4, even if that means we aren't getting the static typechecking we want?

Jeff


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