[PATCH,RFC 0/3] Support for CTF in GCC

[Indu Bhagat <indu dot bhagat at oracle dot com>]

Background :
CTF is the Compact Ansi-C Type Format. It is a format designed to express some
characteristics (specifically Type information) of the data types in a C
program. CTF format is compact and fast; It was originally designed for
use-cases like dynamic tracing, online in-application debugging among others.

A patch to the binutils mailing list to add libctf is currently under
review (https://sourceware.org/ml/binutils/2019-05/msg00154.html and
	https://sourceware.org/ml/binutils/2019-05/msg00212.html).  libctf
provides means to create, update, read and manipulate CTF information.

This GCC patch set is preliminary work and the purpose is to gather comments and
feedback about CTF support in GCC.

(For technical introduction into the CTF format, the CTF header or
https://sourceware.org/ml/binutils/2019-04/msg00277.html will be useful.)

Project Details :
The project aims to add the support for CTF in the GNU toolchain. Adding CTF
support in the GNU toolchain will help the community in developing and 
converging the tools and use-cases where a light-weight debug format is needed.

De-duplication is a key aspect of the CTF format which ensures its compactness.
A parallel effort is ongoing to support de-duplication of CTF types at the
link-time.

In phase 1, we are making the compiler, linker and the debugger (GDB) capable
of handling the CTF format.

CTF format, in its present form, does not have callsite information.  We are
working on this as well. Once the CTF format extensions are agreed upon, the 
-gt1 option (see below) will begin to take form, in phase 2 of the project.

GCC RFC patch set :
Patch 1 is a simple addition of a new function lang_GNU_GIMPLE to check for
GIMPLE frontend.
Patch 2 and Patch 3 set up the framework for CTF support in GCC :
-- Patch 2 adds the new command line option for generating CTF. CTF generation
   is enabled in the compiler by specifying an explicit -gt or
   -gtLEVEL[LEVEL=1,2] :
    
    -gtLEVEL

    This is used to request CTF debug information and to specify how much CTF
    debug information, LEVEL[=0,1,2] can be specified. If -gt is specified
    (with no LEVEL), the default value of LEVEL is 2.

    -gt0 (Level 0) produces no CTF debug information at all. Thus, -gt0
    negates -gt.

    -gt1 (Level 1) produces CTF information for tracebacks only. This includes
    CTF callsite information, but does not include type information for other
    entities.

    -gt2 (Level 2) produces type information for entities (functions, variables
    etc.) at file-scope or global-scope only. This level of information can be
    used by dynamic tracers like DTrace.

--  Patch 3 adds the CTF debug hooks and initializes them if the required 
    user-level options are specified. 
    CTF debug hooks are wrappers around the DWARF debug hooks.

One of the main high-level design requirements that is relevant in the context
of the current GCC patch set is that - CTF and DWARF must be able to co-exist.
A user may want CTF debug information in isolation or with other debug formats.
A .ctf section is small and unlike other debug sections, ideally should not
need to be stripped out of the binary/executable.

High-level proposed plan (phase 1) :
In the next few patches, the functionality to generate contents of the CTF
section (.ctf) for a single compilation unit will be added.
Once CTF generation for a single compilation unit stabilizes, LTO and CTF
generation will be looked at.

Feedback and suggestions welcome.

Thanks

Indu Bhagat (3):
  Add new function lang_GNU_GIMPLE
  Add CTF command line options : -gtLEVEL
  Create CTF debug hooks

 gcc/ChangeLog                                   |  24 ++
 gcc/Makefile.in                                 |   3 +
 gcc/common.opt                                  |   9 +
 gcc/ctfout.c                                    | 171 +++++++++
 gcc/ctfout.h                                    |  41 +++
 gcc/debug.h                                     |   4 +
 gcc/doc/invoke.texi                             |  16 +
 gcc/flag-types.h                                |  13 +
 gcc/gengtype.c                                  |   4 +-
 gcc/langhooks.c                                 |   9 +
 gcc/langhooks.h                                 |   1 +
 gcc/opts.c                                      |  26 ++
 gcc/testsuite/ChangeLog                         |   7 +
 gcc/testsuite/gcc.dg/debug/ctf/ctf-1.c          |   6 +
 gcc/testsuite/gcc.dg/debug/ctf/ctf-preamble-1.c |  11 +
 gcc/testsuite/gcc.dg/debug/ctf/ctf.exp          |  41 +++
 gcc/testsuite/gcc.dg/debug/dwarf2-ctf-1.c       |   7 +
 gcc/toplev.c                                    |  24 ++
 include/ChangeLog                               |   4 +
 include/ctf.h                                   | 471 ++++++++++++++++++++++++
 20 files changed, 890 insertions(+), 2 deletions(-)
 create mode 100644 gcc/ctfout.c
 create mode 100644 gcc/ctfout.h
 create mode 100644 gcc/testsuite/gcc.dg/debug/ctf/ctf-1.c
 create mode 100644 gcc/testsuite/gcc.dg/debug/ctf/ctf-preamble-1.c
 create mode 100644 gcc/testsuite/gcc.dg/debug/ctf/ctf.exp
 create mode 100644 gcc/testsuite/gcc.dg/debug/dwarf2-ctf-1.c
 create mode 100644 include/ctf.h

-- 
1.8.3.1