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

Doug Evans
2014-11-24T20:22:59Z

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)).