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Re: Is there a recursive debug_c_tree which decodes "variant tag"?


On 05/28/09 11:42, Ian Lance Taylor wrote:
> Larry Evans <cppljevans@suddenlink.net> writes:
>
>> While attempting to debug the compiler on:
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40092
>>
>> I'm having great difficulty understanding the output
>> of debug_c_tree or debug_tree because the child
>> tree's are not shown.
>
> Can you give an example of what you mean?  debug_tree does show child
> trees as I understand it, although at each level down it shows less
> information.  If you want more information about a particular child,
> then you can see the address in the shortened information, and you can
> call debug_tree on that address.

Thanks Ian. That helped.

>
>> Is there any such debug_tree function usable in
>> gdb or ddd which will show the actual tree
>> variant?
>
> Here again I don't understand what you mean.
>

From http://gcc.gnu.org/onlinedocs/gccint-html.tar.gz,
in Tree-overview.html#Tree-overview,
there's this:

  The central data structure used by the internal representation is
  the tree. These nodes, while all of the C type tree, are of many
  varieties. A tree is a pointer type, but the object to which it
  points may be of a variety of types.

A tree variant means one of these tree "varieties" mentioned in the
above quote.  I'm guessing the value returned by macro, TREE_CODE, is
some tag indicating which variety of tree.  What I meant by "show the
actual tree variant" is for the print function to decode the tag and
just print the particular variant for that tag.  However, I guess
decode_tree does that, but just prints the address of the child trees.
The user must manually call debug_tree on those children to see what
type they are.

I've tried that but it's not helping much.  The compiler is trying to
expand the pack expansion:

      integral_c
      < Integral
      , Values
      >...

in this template:

  template<typename Integral, Integral... Values>
  struct package_c
  : package
    < integral_c
      < Integral
      , Values
      >...
    >
  {
  };


at this instance:


     typedef
    package_c< int, 1,2,3>
  ints_type
  ;

where:

  template<typename Integral, Integral Value>
  struct integral_c
  {};

That works except when this is present:

    template
    < typename Package_C
    >
  struct impl_front
  ;
    template
    < typename Integral
    , Integral Head
    , Integral... Tail
    >
  struct impl_front
    < package_c
      < Integral
      , Head
      , Tail...
      >
    >
  {
          typedef
        integral_c<Integral,Head>
      type
      ;
  };


So, I'm trying to see what the tree looks like. AFAICT, the tree should, somehow, reflect a pattern expansion:

integral_c<Integral,Values>

and I was hoping, somehow, to see that or some error by using
debug_tree.  I expected to see a tree with children
representing Integral and Values, where Values was
a parameter pack. However, all I get at:

Breakpoint 3, make_pack_expansion (arg=0x7f62797cfe40) at <gccsrc>/gcc/cp/pt.c:2692

is the following:

(gdb) p arg
$1 = (tree) 0x7f62797cfe40
(gdb) pt arg
<record_type 0x7f62797cfe40 integral_c type_5 VOID
align 8 symtab 0 alias set -1 canonical type 0x7f62797c4cc0 context <namespace_decl 0x7f62797c4a80 expbug>
no-binfo use_template=1 interface-unknown
chain <type_decl 0x7f62797cff00 integral_c>>


Then, using the call debug_tree on some addression shown above:

(gdb) call debug_tree(0x7f62797cff00)
<type_decl 0x7f62797cff00 integral_c
type <record_type 0x7f62797cfe40 integral_c type_5 VOID
align 8 symtab 0 alias set -1 canonical type 0x7f62797c4cc0 context <namespace_decl 0x7f62797c4a80 expbug>
no-binfo use_template=1 interface-unknown
chain <type_decl 0x7f62797cff00 integral_c>>
public ignored decl_2 VOID file nested_integral_c_expansion.bug.cpp line 20 col 3
align 1 context <namespace_decl 0x7f62797c4a80 expbug>>


I can't tell from this whether this is an expansion pattern or not.
Further use of call debug_tree on other addresses shown is equally
uninformative :(

I may have to wait till the variadic template experts get around to
this.

Thanks anyway, Ian.

-regards,
Larry




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