https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113918
Bug ID: 113918
Summary: Incomplete DWARF5 debug information for anonymous
unions
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: tsqurt at outlook dot com
Target Milestone: ---
Created attachment 57421
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57421&action=edit
example.c
This is an academic finding and may not be a serious bug that urgently needs to
be fixed. Fixing it may help improve the user's debugging experience.
According to Programming languages — C, ISO/IEC9899:2017, Chapter 6, Section
7.2.1.13, the members of an anonymous structure or union are considered to be
members of the containing structure or union. However, this feature does not
seem to be reflected directly in the debug information.
To check this problem, I use gcc-13.2.0 on Ubuntu 20.04.6 LTS, with the source
code example.c attached.
$gcc -o a.elf -std=c17 -gdwarf-5 --pedantic example.c
$objdump -g a.elf
...
<1><2e>: Abbrev Number: 2 (DW_TAG_union_type)
<2f> DW_AT_byte_size : 4
<30> DW_AT_decl_file : 1
<31> DW_AT_decl_line : 4
<32> DW_AT_decl_column : 5
<33> DW_AT_sibling : <0x42>
...
According to DWARF Debugging Information Format Version 5, Chapter 5, Section
7.1, this union type declared in ‘A’ should have a DW_AT_export_symbols
attribute which indicates that member ‘x’ may be referenced as if ‘x’ is a
member of ‘A’.
To confirm that this issue is indeed a bug, I use GNU gdb (Ubuntu
9.2-0ubuntu1~20.04.1) 9.2, and start with shell command `gdb a.elf`.
$(gdb) b main
Breakpoint 1 at 0x40110a: file example.c, line 12.
$(gdb) run
Starting program: a.elf
Breakpoint 1, main () at example.c:12
$(gdb) expr x.z=20
$(gdb) expr x.y=20
gdb command line:1:2: error: ‘<U6720>’ has no member named ‘y’
Compilation failed.
In comparison, member ‘z’ does not share this problem. This behavior is
inconsistent with the C17 standard. I'm not sure if this problem should be down
to DWARF5, gdb or gcc, but I think gcc should be capable to address this issue.
Discussion:
GCC has specified that the language is C17, so the debugger can deduce that ‘y’
is a member of ‘A’. But in DWARF5, debugging information is
language-independent, so the debugger may not be responsible to make the above
deduction based on the language C17. GCC should either generate a
DW_AT_export_symbols attribute for the anonymous union or a descendant node for
‘A’.
I do not know whether this is a bug or a feature. Please confirm this problem.
