Re: [PATCH] Making it easier to set breakpoints in gdb on pass->execute methods

[David Malcolm <dmalcolm at redhat dot com>]

On Wed, 2014-07-30 at 10:07 +0200, Richard Biener wrote:
> On Tue, Jul 29, 2014 at 10:44 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> > A complaint I heard at Cauldron with the C++ification of GCC passes is
> > that it's become much more difficult to set breakpoints on the execute
> > hooks of a pass, now that the passes are classes within anonymous
> > namespaces.
> >
> > When this was first done, the execute methods were trivial
> > implementations that called into the existing named functions, which are
> > still easy to put a breakpoint on by name (assuming you know the name of
> > the function), but some of these have now been converted so that the
> > "execute" method is the body of the pass.
> >
> > I did some experimentation, on this box with
> > gdb-7.6.50.20130731-19.fc20.x86_64 and gcc trunk r212913 (the latter
> > from a week ago).
> >
> > You *can* set a breakpoint by name on such an execute method, but it's
> > tedious to type:
> > (gdb) break '(anonymous namespace)::pass_expand::execute'
> > Breakpoint 7 at 0x655220: file ../../src/gcc/cfgexpand.c, line
> >
> > ...since tab-completion doesn't work well:
> >
> > (gdb) break '(a<TAB>
> > does tab complete to:
> > (gdb) break '(anonymous namespace)::
> >
> > but typing anything else then hitting tab returns back to:
> > (gdb) break '(anonymous namespace)::
> >
> > Is anyone else seeing this?
> 
> Yes, I filed a gdb bug about this.
> 
> > I had a go at implementing a workaround, for the lack of tab completion
> > (and the general verbosity) using gdbhooks.py.
> >
> > Attached is a patch to gdbhooks.py which adds a new command
> > "break-on-pass" to gdb; in particular, it locates and parses passes.def,
> > so that it can tab-complete on pass classnames:
> >
> > Example of use from the script: putting a breakpoint on "final", i.e.
> > classname "pass_final":
> >
> >   (gdb) break-on-pass
> > Press <TAB>; it autocompletes to "pass_":
> >   (gdb) break-on-pass pass_
> > Press <TAB>:
> >   Display all 219 possibilities? (y or n)
> > Press "n"; then type "f":
> >   (gdb) break-on-pass pass_f
> > Press <TAB> to autocomplete to pass classnames beginning with "pass_f":
> >   pass_fast_rtl_dce              pass_fold_builtins
> >   pass_feedback_split_functions  pass_forwprop
> >   pass_final                     pass_fre
> >   pass_fixup_cfg                 pass_free_cfg
> > Type "in<TAB>" to complete to "pass_final":
> >   (gdb) break-on-pass pass_final
> > ...and hit <RETURN>:
> >   Breakpoint 6 at 0x8396ba: file ../../src/gcc/final.c, line 4526.
> > ...and we have a breakpoint set; continue execution:
> >   (gdb) cont
> >   Continuing.
> >   Breakpoint 6, (anonymous namespace)::pass_final::execute (this=0x17fb990) at ../../src/gcc/final.c:4526
> >   4526    virtual unsigned int execute (function *) { return rest_of_handle_final (); }
> >
> > This assumes you've suitably enabled gdbhooks.py, as documented at the
> > top of that file.
> >
> > Thoughts?
> 
> Works for me though I'm not able to review python ;)

I've gone ahead and committed it to trunk as r213646; hope this reduces
the pain somewhat.

> Richard.
> 
> > Dave
> >
> > gcc/
> >         * gdbhooks.py (find_gcc_source_dir): New helper function.
> >         (class PassNames): New class, locating and parsing passes.def.
> >         (class BreakOnPass): New command "break-on-pass".
> >