[PATCH 0/2] Make C front end share the C++ tree representation of loops and switches

[Sandra Loosemore <sandra at codesourcery dot com>]

This patch series lays some groundwork for the project to redo the
OpenACC "kernels" region support in GCC, described in Thomas
Schwinge's recent talk at the GNU Cauldron:

https://gcc.gnu.org/wiki/cauldron2019talks?action=AttachFile&do=view&target=OpenACC+kernels-cauldron2019.pdf

Briefly, the larger goal is to make the compiler recognize "for" loops
that are candidates for parallelization, and treat them as if they
were annotated with "#pragma acc loop auto".  This is fairly
straightforward to do on the output of the Fortran and C++ front ends,
which both produce a high-level parse tree representation of the loop.
OTOH, the C front end currently lowers loops into a goto form very
early, making it hard both to recognize "for" loops and to
differentiate between (valid) structured and (invalid) unstructured
control flow in the body of the loop.

So, the immediate goal of this patch sequence is to make the C front
end produce output using the same high-level tree representation as
the C++ front end already emits: specifically, to produce FOR_STMT,
WHILE_STMT, and DO_STMT for loops, BREAK_STMT and CONTINUE_STMT
instead of gotos, and also SWITCH_STMT instead of SWITCH_EXPR since
that's mixed up in the handling for "break".  Besides this being
helpful to OpenACC implementation, there's also some argument to be
made that sharing more code between the respective C family front ends
is good from an engineering perspective, etc.

Part 1 of the patch set moves the definitions and support
(genericization, pretty-printers, dumpers, etc) for those tree nodes
out of the cp/ front end and into the common c-family/ code.  Part 2
hacks up the C front end to emit the now-shared data structures.

I bootstrapped and regression-tested this on x86_64-linux-gnu.  There
are a few regressions involving these tests:

gcc.dg/tree-ssa/pr77445-2.c
gcc.dg/tree-ssa/ssa-dce-3.c
gcc.dg/tree-ssa/ssa-dom-thread-7.c

I looked at these briefly and it seems like the problem here is that
the output of the C front end after gimplification is different than
it used to be, since I swapped in the C++ genericization code (it
lowers to a LOOP_EXPR instead of directly to goto form, then the
LOOP_EXPR gets lowered during gimplification).  TBH, I don't really
understand what optimizations these test cases are trying to test, and
whether the pattern-matching is too fragile, whether the form of the code
isn't something that actually tests the optimization, or whether the
optimization is just not working on this input.  So I'm not sure what
to do about those failures!  :-(  I could change the genericizer to
emit the same goto form that the C front end formerly emitted instead
of the C++-style output, but that might just be papering over bugs
elsewhere.  Any advice on how to proceed here is welcome.  If the
patches are OK otherwise, maybe just file bugzilla issues for these
regressions?

-Sandra


Sandra Loosemore (2):
  Move loop and switch tree data structures from cp/ to c-family/.
  Change C front end to emit structured loop and switch tree nodes.

 gcc/c-family/c-common.c             |  24 +++
 gcc/c-family/c-common.def           |  24 +++
 gcc/c-family/c-common.h             |  53 ++++-
 gcc/c-family/c-dump.c               |  38 ++++
 gcc/c-family/c-gimplify.c           | 414 ++++++++++++++++++++++++++++++++++++
 gcc/c-family/c-pretty-print.c       |  92 +++++++-
 gcc/c/c-decl.c                      |  18 +-
 gcc/c/c-lang.h                      |   3 +-
 gcc/c/c-objc-common.h               |   2 +
 gcc/c/c-parser.c                    | 125 ++++++-----
 gcc/c/c-tree.h                      |  21 +-
 gcc/c/c-typeck.c                    | 227 ++++++--------------
 gcc/cp/cp-gimplify.c                | 347 ++----------------------------
 gcc/cp/cp-objcp-common.c            |  13 +-
 gcc/cp/cp-tree.def                  |  23 --
 gcc/cp/cp-tree.h                    |  40 ----
 gcc/cp/cxx-pretty-print.c           |  78 -------
 gcc/cp/dump.c                       |  31 ---
 gcc/doc/generic.texi                |  56 +++--
 gcc/objc/objc-act.c                 |   6 +-
 gcc/testsuite/gcc.dg/gomp/block-7.c |  12 +-
 21 files changed, 859 insertions(+), 788 deletions(-)

-- 
2.8.1