Re: [PATCH] DW_TAG_ptr_to_member_type for PMF and DW_AT_{,rvalue_}reference for those

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Nov 02, 2016 at 11:31:25AM -0400, Jason Merrill wrote:
> On Wed, Nov 2, 2016 at 10:31 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > It uses Alex' LANG_HOOKS_GET_PTRMEMFN_TYPE langhook.  I've tried
> > to think about https://gcc.gnu.org/ml/gcc-patches/2011-05/msg00227.html
> > and we even have such a langhook now, modified_type_die
> > uses lang_hooks.types.get_debug_type, but
> > 1) it is just called in modified_type_die and not in
> >    gen_type_die_with_usage, that looks weird
> 
> How much of a problem is that?  modified_type_die calls gen_type_die,
> does that not cover the cases needed here?

If e.g. on the ref-3.C testcase from the patch I put breakpoint on
both gen_ptr_to_mbr_type_die and modified_type_die (the latter only for
type->base.code == RECORD_TYPE), then I see first:
#0  gen_ptr_to_mbr_type_die (type=<record_type 0x7ffff1727150>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>, 
    class_type=<record_type 0x7ffff1709540 S>, member_type=<method_type 0x7ffff1709738>) at ../../gcc/dwarf2out.c:23128
#1  0x0000000000c3a7e5 in gen_type_die_with_usage (type=<record_type 0x7ffff1727150>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>, usage=DINFO_USAGE_DIR_USE)
    at ../../gcc/dwarf2out.c:24428
#2  0x0000000000c3ab92 in gen_type_die (type=<record_type 0x7ffff1727150>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:24491
#3  0x0000000000c3caed in gen_decl_die (decl=<var_decl 0x7ffff7ff9ea0 o1>, origin=<tree 0x0>, ctx=0x0, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:25117
#4  0x0000000000c3b13e in process_scope_var (stmt=<block 0x7ffff17129c0>, decl=<var_decl 0x7ffff7ff9ea0 o1>, origin=<tree 0x0>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:24620
#5  0x0000000000c3b1bb in decls_for_scope (stmt=<block 0x7ffff17129c0>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:24645
and only afterwards:
#0  modified_type_die (type=<record_type 0x7ffff1727150>, cv_quals=0, reverse=false, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:12328
#1  0x0000000000c2f03b in add_type_attribute (object_die=<dw_die_ref 0x7ffff1738aa0 DW_TAG_variable <parent=0x7ffff1738910 DW_TAG_subprogram>>, 
    type=<record_type 0x7ffff1727150>, cv_quals=0, reverse=false, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:20346
#2  0x0000000000c354f4 in gen_variable_die (decl=<var_decl 0x7ffff7ff9ea0 o1>, origin=<tree 0x0>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:22688
#3  0x0000000000c3cb8e in gen_decl_die (decl=<var_decl 0x7ffff7ff9ea0 o1>, origin=<tree 0x0>, ctx=0x0, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:25138
#4  0x0000000000c3b13e in process_scope_var (stmt=<block 0x7ffff17129c0>, decl=<var_decl 0x7ffff7ff9ea0 o1>, origin=<tree 0x0>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:24620
#5  0x0000000000c3b1bb in decls_for_scope (stmt=<block 0x7ffff17129c0>, 
    context_die=<dw_die_ref 0x7ffff1738910 DW_TAG_subprogram <parent=0x7ffff15c7000 DW_TAG_compile_unit>>) at ../../gcc/dwarf2out.c:24645

which means if gen_type_die or gen_type_die_with_usage doesn't
use the langhook, then we'd emit a completely useless { __pfn; __delta }
struct into debug info first, and then in modified_type_die used
the langhook, get OFFSET_TYPE and probably create the
DW_TAG_ptr_to_member_type.  So I think we really need that.

> > 2) it is used for something Ada-ish I really don't know how to test etc.
> >    to be able to find out if it is safe to call it in
> >    gen_type_die_with_usage too
> 
> You could find an Ada test that uses the code and verify that the
> output stays the same?

I can try to find the patch that introduced it and if it contained any
testcases.

> > 3) most importantly, if the C++ version of this langhook would create
> >    OFFSET_TYPE on the fly, I don't know how to ensure effective sharing
> >    of DW_TAG_ptr_to_member_type nodes with the same DW_AT_type
> >    and DW_AT_containing_type; unless the C++ langhook adds some extra
> >    hash table that caches already created OFFSET_TYPEs or something similar,
> >    it would create a new OFFSET_TYPE each time it is called
> 
> build_offset_type already uses a hash table.

Ah, ok.

> > Also, I really don't know how well does GDB (especially older releases)
> > handle DW_TAG_ptr_to_member_type for PMF, so the patch wraps that currently
> > with if (dwarf_version >= 5).  Quick grep revealed that GDB has code to
> > handle the __pfn/__delta fields.  So, can I ask somebody from the GDB
> > team to test this patch with that if (dwarf_version >= 5) replaced
> > with if (1) and see if it works properly with current GDB as well as say
> > 4-5 years old one (e.g. with -gdwarf-2 or -gdwarf-3)?  If yes, we
> > should emit it unconditionally.
> 
> This all makes sense to me.
> 
> > +      if (dwarf_version >= 5)
> > +       {
> > +         tree class_type = lang_hooks.types.get_ptrmemfn_type (type, 0);
> > +         if (class_type != NULL_TREE)
> 
> This can be
> 
> if (dwarf_version >= 5)
>   if (tree class_type = lang_hooks.types.get_ptrmemfn_type (type, 0))

Ok.

	Jakub