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


On Tue, Jul 29, 2014 at 04:44:29PM -0400, David Malcolm 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.

Well, its a big annoyance of anonymous namespaces in general.

> 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.

The same holds for the gate method, but those are probably less
important in general.

> 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).

 I don't think anonymous namespaces ever worked well with gdb, and afaik
 it hasn't been fixed.

> Thoughts?

ugh, why can somebody just fix gdb is my first reaction, but if its
useful maybe its worth hacks.

Trev

> 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".
> 

> diff --git a/gcc/gdbhooks.py b/gcc/gdbhooks.py
> index 85608dc..aeb9199 100644
> --- a/gcc/gdbhooks.py
> +++ b/gcc/gdbhooks.py
> @@ -130,6 +130,7 @@ Instead (for now) you must access m_vecdata:
>    (gdb) p bb->preds->m_vecdata[1]
>    $21 = <edge 0x7ffff044d3b8 (4 -> 5)>
>  """
> +import os.path
>  import re
>  
>  import gdb
> @@ -476,4 +477,71 @@ gdb.printing.register_pretty_printer(
>      gdb.current_objfile(),
>      build_pretty_printer())
>  
> +def find_gcc_source_dir():
> +    # Use location of global "g" to locate the source tree
> +    sym_g = gdb.lookup_global_symbol('g')
> +    path = sym_g.symtab.filename # e.g. '../../src/gcc/context.h'
> +    srcdir = os.path.split(path)[0] # e.g. '../../src/gcc'
> +    return srcdir
> +
> +class PassNames:
> +    """Parse passes.def, gathering a list of pass class names"""
> +    def __init__(self):
> +        srcdir = find_gcc_source_dir()
> +        self.names = []
> +        with open(os.path.join(srcdir, 'passes.def')) as f:
> +            for line in f:
> +                m = re.match('\s*NEXT_PASS \((.+)\);', line)
> +                if m:
> +                    self.names.append(m.group(1))
> +PassNames()
> +
> +class BreakOnPass(gdb.Command):
> +    """
> +    A custom command for putting breakpoints on the execute hook of passes.
> +    This is largely a workaround for issues with tab-completion in gdb when
> +    setting breakpoints on methods on classes within anonymous namespaces.
> +
> +    Example of use: putting a breakpoint on "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 (); }
> +    """
> +    def __init__(self):
> +        gdb.Command.__init__(self, 'break-on-pass', gdb.COMMAND_BREAKPOINTS)
> +        self.pass_names = None
> +
> +    def complete(self, text, word):
> +        # Lazily load pass names:
> +        if not self.pass_names:
> +            self.pass_names = PassNames()
> +
> +        return [name
> +                for name in sorted(self.pass_names.names)
> +                if name.startswith(text)]
> +
> +    def invoke(self, arg, from_tty):
> +        sym = '(anonymous namespace)::%s::execute' % arg
> +        breakpoint = gdb.Breakpoint(sym)
> +
> +BreakOnPass()
> +
>  print('Successfully loaded GDB hooks for GCC')


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