Remote query for structure layout

[Simon Marchi]

On 2021-03-30 4:35 p.m., Tim Newsome wrote:
> On Sun, Mar 28, 2021 at 5:00 AM <gdb-request@sourceware.org> wrote:
> 
>> I would like to propose a new command for the remote protocol that can be
>> used to
>> query structure layouts.
>> Essentially a `qSymbol` equivalent for the output of `ptype`.
>>
> 
> I'm really interested in this. I've been working on OpenOCD FreeRTOS
> support and as Thomas points out, it's a fragile mess.
> 
> There's another use case that this would be great for. FreeRTOS stores
> registers on the stack when it switches to another thread. OpenOCD needs to
> know what this register layout is, and it also depends on compile time
> options. With this mechanism I think it would be possible for FreeRTOS to
> define a struct that represents how the registers are laid out on the
> stack. E.g.:
> struct register_stacking_t {
>     uint32_t x1;
>     uint32_t x2;
>     uint32_t x3;
>     uint32_t x4;
> ...
>     uint32_t pc;
> };
> 
> Am I right that this will end up in the dwarf info even if the struct
> itself isn't used anywhere in the FreeRTOS code? This would simplify so
> much.

It doesn't seem like it:

    $ cat test.cpp
    struct foo
    {
      int a;
    };

    int main() {

    }
    $ g++ test.cpp -g3 -O0
    $ llvm-dwarfdump -F -color a.out
    a.out:  file format elf64-x86-64

    .debug_info contents:
    0x00000000: Compile Unit: length = 0x00000053, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x00000057)

    0x0000000b: DW_TAG_compile_unit
                  DW_AT_producer [DW_FORM_strp]     ("GNU C++14 10.2.0 -mtune=generic -march=x86-64 -g3 -O0")
                  DW_AT_language [DW_FORM_data1]    (DW_LANG_C_plus_plus)
                  DW_AT_name [DW_FORM_strp] ("test.cpp")
                  DW_AT_comp_dir [DW_FORM_strp]     ("/home/simark/build/lttng-ust/doc/examples/easy-ust")
                  DW_AT_low_pc [DW_FORM_addr]       (0x0000000000001119)
                  DW_AT_high_pc [DW_FORM_data8]     (0x000000000000000b)
                  DW_AT_stmt_list [DW_FORM_sec_offset]      (0x00000000)
                  DW_AT_GNU_macros [DW_FORM_sec_offset]     (0x00000000)

    0x00000031:   DW_TAG_base_type
                    DW_AT_byte_size [DW_FORM_data1] (0x04)
                    DW_AT_encoding [DW_FORM_data1]  (DW_ATE_signed)
                    DW_AT_name [DW_FORM_string]     ("int")

    0x00000038:   DW_TAG_subprogram
                    DW_AT_external [DW_FORM_flag_present]   (true)
                    DW_AT_name [DW_FORM_strp]       ("main")
                    DW_AT_decl_file [DW_FORM_data1] ("/home/simark/build/lttng-ust/doc/examples/easy-ust/test.cpp")
                    DW_AT_decl_line [DW_FORM_data1] (6)
                    DW_AT_decl_column [DW_FORM_data1]       (0x05)
                    DW_AT_type [DW_FORM_ref4]       (0x00000031 "int")
                    DW_AT_low_pc [DW_FORM_addr]     (0x0000000000001119)
                    DW_AT_high_pc [DW_FORM_data8]   (0x000000000000000b)
                    DW_AT_frame_base [DW_FORM_exprloc]      (DW_OP_call_frame_cfa)
                    DW_AT_GNU_all_call_sites [DW_FORM_flag_present] (true)

    0x00000056:   NULL

There might be some clever trick to force the compiler in emitting it
though, but I suppose it will always be a bit fragile, dependent on the
particular compiler.

Simon