Re: [PATCH] [DWARF] Fix hierarchy of debug information for offload kernels.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jul 21, 2021 at 7:55 PM Hafiz Abid Qadeer
<abid_qadeer@mentor.com> wrote:
>
> On 19/07/2021 17:41, Richard Biener wrote:
> > On July 19, 2021 6:13:40 PM GMT+02:00, Hafiz Abid Qadeer <abid_qadeer@mentor.com> wrote:
> >> On 19/07/2021 11:45, Richard Biener wrote:
> >>> On Fri, Jul 16, 2021 at 10:23 PM Hafiz Abid Qadeer
> >>> <abid_qadeer@mentor.com> wrote:
> >>>>
> >>>> On 15/07/2021 13:09, Richard Biener wrote:
> >>>>> On Thu, Jul 15, 2021 at 12:35 PM Hafiz Abid Qadeer
> >>>>> <abid_qadeer@mentor.com> wrote:
> >>>>>>
> >>>>>> On 15/07/2021 11:33, Thomas Schwinge wrote:
> >>>>>>>
> >>>>>>>> Note that the "parent" should be abstract but I don't think
> >> dwarf has a
> >>>>>>>> way to express a fully abstract parent of a concrete instance
> >> child - or
> >>>>>>>> at least how GCC expresses this causes consumers to
> >> "misinterpret"
> >>>>>>>> that.  I wonder if adding a DW_AT_declaration to the late DWARF
> >>>>>>>> emitted "parent" would fix things as well here?
> >>>>>>>
> >>>>>>> (I suppose not, Abid?)
> >>>>>>>
> >>>>>>
> >>>>>> Yes, adding DW_AT_declaration does not fix the problem.
> >>>>>
> >>>>> Does emitting
> >>>>>
> >>>>> DW_TAG_compile_unit
> >>>>>   DW_AT_name    ("<artificial>")
> >>>>>
> >>>>>   DW_TAG_subprogram // notional parent function (foo) with no code
> >> range
> >>>>>     DW_AT_declaration 1
> >>>>> a:    DW_TAG_subprogram // offload function foo._omp_fn.0
> >>>>>       DW_AT_declaration 1
> >>>>>
> >>>>>   DW_TAG_subprogram // offload function
> >>>>>   DW_AT_abstract_origin a
> >>>>> ...
> >>>>>
> >>>>> do the trick?  The following would do this, flattening function
> >> definitions
> >>>>> for the concrete copies:
> >>>>>
> >>>>> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> >>>>> index 82783c4968b..a9c8bc43e88 100644
> >>>>> --- a/gcc/dwarf2out.c
> >>>>> +++ b/gcc/dwarf2out.c
> >>>>> @@ -6076,6 +6076,11 @@ maybe_create_die_with_external_ref (tree
> >> decl)
> >>>>>    /* Peel types in the context stack.  */
> >>>>>    while (ctx && TYPE_P (ctx))
> >>>>>      ctx = TYPE_CONTEXT (ctx);
> >>>>> +  /* For functions peel the context up to namespace/TU scope.  The
> >> abstract
> >>>>> +     copies reveal the true nesting.  */
> >>>>> +  if (TREE_CODE (decl) == FUNCTION_DECL)
> >>>>> +    while (ctx && TREE_CODE (ctx) == FUNCTION_DECL)
> >>>>> +      ctx = DECL_CONTEXT (ctx);
> >>>>>    /* Likewise namespaces in case we do not want to emit DIEs for
> >> them.  */
> >>>>>    if (debug_info_level <= DINFO_LEVEL_TERSE)
> >>>>>      while (ctx && TREE_CODE (ctx) == NAMESPACE_DECL)
> >>>>> @@ -6099,8 +6104,7 @@ maybe_create_die_with_external_ref (tree
> >> decl)
> >>>>>         /* Leave function local entities parent determination to
> >> when
> >>>>>            we process scope vars.  */
> >>>>>         ;
> >>>>> -      else
> >>>>> -       parent = lookup_decl_die (ctx);
> >>>>> +      parent = lookup_decl_die (ctx);
> >>>>>      }
> >>>>>    else
> >>>>>      /* In some cases the FEs fail to set DECL_CONTEXT properly.
> >>>>>
> >>>>
> >>>> Thanks. This solves the problem. Only the first hunk was required.
> >> Second hunk
> >>>> actually causes an ICE when TREE_CODE (ctx) == BLOCK.
> >>>> OK to commit the attached patch?
> >>>
> >>> I think we need to merge the TYPE_P peeling and FUNCTION_DECL peeling
> >> into
> >>> one loop since I suppose we can have a nested function in class
> >> scope.
> >>> So sth like
> >>>
> >>> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> >>> index 82783c4968b..61228410b51 100644
> >>> --- a/gcc/dwarf2out.c
> >>> +++ b/gcc/dwarf2out.c
> >>> @@ -6073,8 +6073,12 @@ maybe_create_die_with_external_ref (tree decl)
> >>>      }
> >>>    else
> >>>      ctx = DECL_CONTEXT (decl);
> >>> -  /* Peel types in the context stack.  */
> >>> -  while (ctx && TYPE_P (ctx))
> >>> +  /* Peel types in the context stack.  For functions peel the
> >> context up
> >>> +     to namespace/TU scope.  The abstract copies reveal the true
> >> nesting.  */
> >>> +  while (ctx
> >>> +        && (TYPE_P (ctx)
> >>> +            || (TREE_CODE (decl) == FUNCTION_DECL
> >>> +                && TREE_CODE (ctx) == FUNCTION_DECL)))
> >>>      ctx = TYPE_CONTEXT (ctx);
> >>>    /* Likewise namespaces in case we do not want to emit DIEs for
> >> them.  */
> >>>    if (debug_info_level <= DINFO_LEVEL_TERSE)
> >>>
> >> This causes an ICE,
> >> internal compiler error: tree check: expected class 'type', have
> >> 'declaration' (function_decl)
> >>
> >> Did you intend something like this:
> >>
> >> diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
> >> index 561f8b23517..c61f0041fba 100644
> >> --- a/gcc/dwarf2out.c
> >> +++ b/gcc/dwarf2out.c
> >> @@ -6121,3 +6121,8 @@ maybe_create_die_with_external_ref (tree decl)
> >> -  /* Peel types in the context stack.  */
> >> -  while (ctx && TYPE_P (ctx))
> >> -    ctx = TYPE_CONTEXT (ctx);
> >> +  /* Peel types in the context stack.  For functions peel the context
> >> up
> >> +     to namespace/TU scope.  The abstract copies reveal the true
> >> nesting.  */
> >> +  while (ctx
> >> +       && (TYPE_P (ctx)
> >> +           || (TREE_CODE (decl) == FUNCTION_DECL
> >> +               && TREE_CODE (ctx) == FUNCTION_DECL)))
> >> +    ctx = TYPE_P (ctx) ? TYPE_CONTEXT (ctx) : DECL_CONTEXT (ctx);
> >> +
> >
> > Yes, of course.
> >
> >>
> >>> if that works it's OK.  Can you run it on the gdb testsuite with
> >> -flto added
> >>> as well please (you need to do before/after comparison since IIRC
> >> adding
> >>> -flto will add a few fails).
>
> GDB testsuite shows some extra fails which mainly happen because testcase assumes that you can
> access the local variable of enclosing function from the nested function (or omp parallel region).
> After this change, the nested functions are no longer children of the enclosing function so those
> tests fail.

I think you should consult with gdb folks on this - the functions are
still children of the enclosing
function as seen in the abstract instance.  Just the concrete instance
is put in another place.
But yes, that was what I expected as bad side-effect of the change.
Now I wonder how to fix
that - even for offloading a "good" debugger could allow debugging
both the host and the target
and DTRT when printing a variable from the containing function on the
target (lookup the variable
on the host).

So I think we need to get to an agreement between the debug info
producer and consumer here.
Usually the DWARF spec is not of much help here.

Richard.

> The problem that prompted this patch happened for parent function that did not have a code range i.e
> a notional parent.  I was wondering if we should update the ctx only for such parents instead of all
> function as we did above.
>
> Thanks,
> --
> Hafiz Abid Qadeer
> Mentor, a Siemens Business