Re: [RFC] While processing a struct die, store the method's address in its fn_field

[Doug Evans <dje at google dot com>]

On Mon, Nov 24, 2014 at 7:54 AM, Siva Chandra <sivachandra@google.com> wrote:
> [The tests in this patch depend on this patch:
> https://sourceware.org/ml/gdb-patches/2014-11/msg00479.html.  Also, it
> adds a new dwarf2 test which I am not very sure I got it right.]
>
> While processing a struct die, store the method's address in its fn_field.
>
> This enables calling the method when its physname is missing and its
> minsym cannot be discovered, but its address (DW_AT_low_pc) is available.
> For example, this happens for the operator() methods of c++11 lambdas.
> Consider the following C++ code:
>
> int
> main ()
> {
>   auto lambda = [] (int j) { return j + 113; };
>   return lambda (-113);
> }
>
> When compiled with g++, the DWARF corresponding to the lambda's operator()
> shows up under the DWARF for the function main as follows:
>
> DW_TAG_subprogram
>   DW_AT_name                  "operator()"
>   DW_AT_type                  <0x0000002d>
>   DW_AT_artificial            yes(1)
>   DW_AT_low_pc                0x00400566
>   DW_AT_high_pc               <offset-from-lowpc>19
>   DW_AT_frame_base            len 0x0001: 9c: DW_OP_call_frame_cfa
>   DW_AT_object_pointer        <0x0000010f>
>   DW_AT_GNU_all_call_sites    yes(1)
>   DW_TAG_pointer_type
>     DW_AT_byte_size             0x00000008
>     DW_AT_type                  <0x000000b9>
>   DW_TAG_formal_parameter
>     DW_AT_name                  "__closure"
>     DW_AT_type                  <0x0000011b>
>     DW_AT_artificial            yes(1)
>     DW_AT_location              len 0x0002: 9168: DW_OP_fbreg -24
>   DW_TAG_const_type
>     DW_AT_type                  <0x00000109>
>   DW_TAG_formal_parameter
>     DW_AT_name                  "j"
>     DW_AT_decl_file             0x00000001
>     DW_AT_decl_line             0x00000004
>     DW_AT_type                  <0x0000002d>
>     DW_AT_location              len 0x0002: 9164:DW_OP_fbreg -28
>
> There is no physname and the minsym corresponding to the the operator()
> method does not demangle to its qualified name as specified by the DWARF.
> However, since DW_AT_low_pc is available, it can be used to create a value
> corresponding to the method in value.c:value_fn_field and subsequently be
> passed to call_function_by_hand to invoke it.
>
> gdb/ChangeLog:
>
> 2014-11-24  Siva Chandra Reddy  <sivachandra@google.com>
>
>         * dwarf2read.c (dwarf2_add_member_fn): Note the methods address
>         if its DT_AT_low_pc is available.
>         * gdbtypes.h (struct fn_field): New field ADDR.
>         (TYPE_FN_FIELD_ADDR): New macro.
>         * value.c (value_fn_field): Use address of the method if
>         available.
>
> gdb/testsuite/ChangeLog:
>
> 2014-11-24  Siva Chandra Reddy  <sivachandra@google.com>
>
>         * gdb.dwarf2/dw2-member-function-addr.S: New file.
>         * gdb.dwarf2/dw2-member-function-addr.exp: New file.

Hi.

[I have to admit I hadn't realized gdb used low_pc for the entry point.
Or more likely forgotten. :-)
There's nothing to say that low_pc is the entry point,
but gdb has been using this since forever I'm guessing,
and if it wasn't the entry point presumably the compiler could emit
a DW_TAG_entry_point record.]

Adding new fields to types or symbols is a big deal.
I'd like to understand why things are failing better.
For normal functions gdb gets the start address
from BLOCK_START (SYMBOL_BLOCK_VALUE (sym)).