Re: DWARF for extern variable

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

On Mon, Aug 23, 2021 at 11:18 PM Indu Bhagat via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hello,
>
> What is the expected DWARF for extern variable in the following cases? I
> am seeing that the DWARF generated is different with gcc8.4.1 vs gcc-trunk.
>
> Testcase 1
> ----------
> extern const char a[];
>
> int foo()
> {
>    return a != 0;
> }
>
> Testcase 1 Behavior
> ---------------------
> - gcc-trunk has _no_ DWARF for variable a.

Use -fno-eliminate-unused-debug-symbols (the comparison is folded away
very early it seems).  Then we get similar DWARF as with GCC 8.

> - gcc8.4.1 generates following DW_TAG_variable for extern variable a.
> But does not designate it as a non-defining decl (IIUC,
> DW_AT_specification is used for such cases?).

It has DW_AT_declaration.

>
> <..>
>   <1><31>: Abbrev Number: 2 (DW_TAG_array_type)
>      <32>   DW_AT_type        : <0x48>
>      <36>   DW_AT_sibling     : <0x3c>
>   <2><3a>: Abbrev Number: 3 (DW_TAG_subrange_type)
>   <2><3b>: Abbrev Number: 0
>   <1><3c>: Abbrev Number: 4 (DW_TAG_const_type)
>      <3d>   DW_AT_type        : <0x31>
>   <1><41>: Abbrev Number: 5 (DW_TAG_base_type)
>      <42>   DW_AT_byte_size   : 1
>      <43>   DW_AT_encoding    : 6        (signed char)
>      <44>   DW_AT_name        : (indirect string, offset: 0x1df6): char
>   <1><48>: Abbrev Number: 4 (DW_TAG_const_type)
>      <49>   DW_AT_type        : <0x41>
>   <1><4d>: Abbrev Number: 6 (DW_TAG_variable)
>      <4e>   DW_AT_name        : a
>      <50>   DW_AT_decl_file   : 1
>      <51>   DW_AT_decl_line   : 1
>      <52>   DW_AT_decl_column : 19
>      <53>   DW_AT_type        : <0x3c>
>      <57>   DW_AT_external    : 1
>      <57>   DW_AT_declaration : 1
> <..>
>
> -----------------------------------------------------------
>
> Testcase 2
> ----------
> extern const char a[];
> const char a[] = "testme";
>
> Testcase 2 Behavior
> --------------------
> - Both gcc-trunk and gcc8.4.1 generate two DW_TAG_variable DIEs (the
> defining decl holds the reference to the non-defining decl via
> DW_AT_specification)
> - But gcc8.4.1 does not generate any DWARF for the type of the defining
> decl (const char[7]) but gcc-trunk does.
>
> ## DWARF for testcase 2 with gcc-trunk is as follows:
> <...>
>   <1><22>: Abbrev Number: 2 (DW_TAG_array_type)
>      <23>   DW_AT_type        : <0x39>
>      <27>   DW_AT_sibling     : <0x2d>
>   <2><2b>: Abbrev Number: 5 (DW_TAG_subrange_type)
>   <2><2c>: Abbrev Number: 0
>   <1><2d>: Abbrev Number: 1 (DW_TAG_const_type)
>      <2e>   DW_AT_type        : <0x22>
>   <1><32>: Abbrev Number: 3 (DW_TAG_base_type)
>      <33>   DW_AT_byte_size   : 1
>      <34>   DW_AT_encoding    : 6        (signed char)
>      <35>   DW_AT_name        : (indirect string, offset: 0x2035): char
>   <1><39>: Abbrev Number: 1 (DW_TAG_const_type)
>      <3a>   DW_AT_type        : <0x32>
>   <1><3e>: Abbrev Number: 6 (DW_TAG_variable)
>      <3f>   DW_AT_name        : a
>      <41>   DW_AT_decl_file   : 1
>      <42>   DW_AT_decl_line   : 1
>      <43>   DW_AT_decl_column : 19
>      <44>   DW_AT_type        : <0x2d>
>      <48>   DW_AT_external    : 1
>      <48>   DW_AT_declaration : 1
>   <1><48>: Abbrev Number: 2 (DW_TAG_array_type)
>      <49>   DW_AT_type        : <0x39>
>      <4d>   DW_AT_sibling     : <0x58>
>   <2><51>: Abbrev Number: 7 (DW_TAG_subrange_type)
>      <52>   DW_AT_type        : <0x5d>
>      <56>   DW_AT_upper_bound : 6
>   <2><57>: Abbrev Number: 0
>   <1><58>: Abbrev Number: 1 (DW_TAG_const_type)
>      <59>   DW_AT_type        : <0x48>
>   <1><5d>: Abbrev Number: 3 (DW_TAG_base_type)
>      <5e>   DW_AT_byte_size   : 8
>      <5f>   DW_AT_encoding    : 7        (unsigned)
>      <60>   DW_AT_name        : (indirect string, offset: 0x2023): long
> unsigned int
>   <1><64>: Abbrev Number: 8 (DW_TAG_variable)
>      <65>   DW_AT_specification: <0x3e>
>      <69>   DW_AT_decl_line   : 2
>      <6a>   DW_AT_decl_column : 12
>      <6b>   DW_AT_type        : <0x58>

I suppose having both a DW_AT_specification and a DW_AT_type
is somewhat at odds.  It's likely because the definition specifies
the size of the array while the specification does not.  Not sure
what should be best done here.

Richard.

>      <6f>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0
> (DW_OP_addr: 0)
>   <1><79>: Abbrev Number: 0
>
> ## DWARF for testcase 2 with gcc8.4.1 is as follows:
>   <1><21>: Abbrev Number: 2 (DW_TAG_array_type)
>      <22>   DW_AT_type        : <0x38>
>      <26>   DW_AT_sibling     : <0x2c>
>   <2><2a>: Abbrev Number: 3 (DW_TAG_subrange_type)
>   <2><2b>: Abbrev Number: 0
>   <1><2c>: Abbrev Number: 4 (DW_TAG_const_type)
>      <2d>   DW_AT_type        : <0x21>
>   <1><31>: Abbrev Number: 5 (DW_TAG_base_type)
>      <32>   DW_AT_byte_size   : 1
>      <33>   DW_AT_encoding    : 6        (signed char)
>      <34>   DW_AT_name        : (indirect string, offset: 0x1e04): char
>   <1><38>: Abbrev Number: 4 (DW_TAG_const_type)
>      <39>   DW_AT_type        : <0x31>
>   <1><3d>: Abbrev Number: 6 (DW_TAG_variable)
>      <3e>   DW_AT_name        : a
>      <40>   DW_AT_decl_file   : 1
>      <41>   DW_AT_decl_line   : 1
>      <42>   DW_AT_decl_column : 19
>      <43>   DW_AT_type        : <0x2c>
>      <47>   DW_AT_external    : 1
>      <47>   DW_AT_declaration : 1
>   <1><47>: Abbrev Number: 5 (DW_TAG_base_type)
>      <48>   DW_AT_byte_size   : 8
>      <49>   DW_AT_encoding    : 7        (unsigned)
>      <4a>   DW_AT_name        : (indirect string, offset: 0x1df2): long
> unsigned int
>   <1><4e>: Abbrev Number: 7 (DW_TAG_variable)
>      <4f>   DW_AT_specification: <0x3d>
>      <53>   DW_AT_decl_line   : 2
>      <54>   DW_AT_decl_column : 12
>      <55>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0
> (DW_OP_addr: 0)
>   <1><5f>: Abbrev Number: 0
>
> Thanks
> Indu