[Dwarf-Discuss] Alternate entry points

[Michael Eager]

On 10/24/2018 03:15 AM, Jakub Jelinek via Dwarf-Discuss wrote:
> On Wed, Oct 24, 2018 at 01:00:26AM -0400, Rafik Zurob via Dwarf-Discuss wrote:
>>> I dug up gfortran 5.4, which does not emit DW_TAG_entry_point with my
>>> simple example program.  Does anybody actually use it?
>>
>> IBM XL Fortran generates it.
>>
>> $ cat entry.f
>> subroutine foo(a)
>>    integer a
>>    real b
>>
>>    a = 5
>>    return
>> entry bar(b)
>>    b = 3.0
>>    return
>> end subroutine
>> $
>>
>> Gets the following DWARF:
> 
> That is kind of weird, because foo only has dummy argument a
> and entry bar only has b, there is nothing that tells the debug info
> consumer that the subprogram doesn't have (a, b) arguments.
> 
>>   <1><42>: Abbrev Number: 2 (DW_TAG_base_type)
>>      <43>   DW_AT_name        : INTEGER
>>      <4b>   DW_AT_byte_size   : 4
>>      <4c>   DW_AT_encoding    : 5        (signed)
>>   <1><51>: Abbrev Number: 2 (DW_TAG_base_type)
>>      <52>   DW_AT_name        : REAL
>>      <57>   DW_AT_byte_size   : 4
>>      <58>   DW_AT_encoding    : 4        (float)
>>   <1><59>: Abbrev Number: 4 (DW_TAG_subprogram)
>>      <5a>   DW_AT_name        : foo
>>      <5e>   DW_AT_low_pc      : 0x0
>>      <66>   DW_AT_high_pc     : 128
>>      <67>   DW_AT_decl_file   : 1
>>      <68>   DW_AT_decl_line   : 1
>>      <69>   DW_AT_external    : 1
>>      <6a>   DW_AT_frame_base  : 0x0 (location list)
>>   <2><72>: Abbrev Number: 5 (DW_TAG_formal_parameter)
>>      <73>   DW_AT_location    : 3 byte block: 91 20 6    (DW_OP_fbreg: 32;
>> DW_OP_deref)
>>      <77>   DW_AT_name        : a
>>      <79>   DW_AT_type        : 0x42
>>   <2><81>: Abbrev Number: 5 (DW_TAG_formal_parameter)
>>      <82>   DW_AT_location    : 3 byte block: 91 28 6    (DW_OP_fbreg: 40;
>> DW_OP_deref)
>>      <86>   DW_AT_name        : b
>>      <88>   DW_AT_type        : 0x51
>>   <2><90>: Abbrev Number: 6 (DW_TAG_entry_point)
>>      <91>   DW_AT_name        : bar
>>      <95>   DW_AT_low_pc      : 0x28
>>      <9d>   DW_AT_decl_file   : 1
>>      <9e>   DW_AT_decl_line   : 7
>>      <9f>   DW_AT_frame_base  : 1 byte block: 6e         (DW_OP_reg30
>> (r30))

This looks odd to me as well.

Looking at the DWARF, I would interpret it to say that foo takes two
arguments, a and b, and bar takes zero arguments.  Shouldn't the
DW_TAG_formal_parameter for b be a child of the DW_TAG_entry_point,
not the DW_TAG_subprogram?

-- 
Michael Eager    eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306