Re: [PATCH 3/6] gdb/testsuite/dwarf: emit type unit sections as COMDAT

[Andrew Burgess <aburgess@redhat.com>]

Simon Marchi <simon.marchi@efficios.com> writes:

> From: Simon Marchi <simon.marchi@polymtl.ca>
>
> In an effort to generate ELF files and DWARF info as close as possible
> as an actual compiler would generate, change how we emit type unit
> sections to emit each type unit in its own section, in COMDAT section
> groups.
>
> We currently emit all type units in a single, standard section (either
> .debug_info or .debug_types, depending on the DWARF version).  Compilers
> emit each type unit in its own .debug_types section.  Each section is
> placed in a COMDAT section group with a signature based on the type
> unit's signature.  This lets the linker deduplicate identical type units
> by discarding section groups with identical signatures (keeping only one
> group with a given signature).
>
> Looking at a .s file generated by gcc, we can see that this is how it
> specifies the section for a type unit:
>
>     .section .debug_info,"G",@progbits,wi.006fd2152a3054a6,comdat
>
> The "G" flag tells the assembler to place the section in a section
> group with signature "wi.006fd2152a3054a6".  That string was generated
> from the type unit, signature.  Finally, the comdat linkage field
> indicates that the section group should have the COMDAT flag.
>
> Update the tu proc to emit a section with these properties.  In this
> case, it's mandatory to specify the type of the section (progbits).
>
> Update the _section proc to accept the new options "group_signature" and
> "linkage".
>
> As a result, if you look at the .debug_types section in a .o file from
> gcc:
>
>     $ readelf -g main.o
>     COMDAT group section [    1] `.group' [wt.006fd2152a3054a6] contains 2 sections:
>        [Index]    Name
>        [   11]   .debug_types
>        [   12]   .rela.debug_types
>
>     COMDAT group section [    2] `.group' [wt.c621aa8e3c23e450] contains 2 sections:
>        [Index]    Name
>        [   13]   .debug_types
>        [   14]   .rela.debug_types
>
> And a .o file created by our DWARF assembler:
>
>     $ readelf -g testsuite/outputs/gdb.dwarf2/struct-with-sig/struct-with-sig1.o
>     COMDAT group section [    1] `.group' [sig.0x0000000000000001] contains 2 sections:
>        [Index]    Name
>        [   10]   .debug_types
>        [   11]   .rela.debug_types
>
>     COMDAT group section [    2] `.group' [sig.0x0000000000000002] contains 2 sections:
>        [Index]    Name
>        [   12]   .debug_types
>        [   13]   .rela.debug_types
>
> In both cases, in the fully linked files, there is a single .debug_types section
> containing the two type units, as expected.
>
> Before this patch, when I run gdb.dwarf2/fission-with-type-unit.exp, the
> resulting .dwo file has a single .debug_info.dwo.  After this patch it
> has two: one with the type unit and one with the compile unit (the test
> uses DWARF 5).  Based on what I see gcc generate when using "-gdwarf-5
> -gsplit-dwarf -fdebug-types-section", this is what we expect.

I think I followed all of this.

Approved-By: Andrew Burgess <aburgess@redhat.com>

Thanks,
Andrew


>
> Change-Id: Ie1954dc697fe100b5dbe664d148c71fa02888d96
> ---
>  gdb/testsuite/lib/dwarf.exp | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
> index 5536b7656b8c..c3c35d43607d 100644
> --- a/gdb/testsuite/lib/dwarf.exp
> +++ b/gdb/testsuite/lib/dwarf.exp
> @@ -1075,6 +1075,8 @@ namespace eval Dwarf {
>  	parse_options {
>  	    { flags "" }
>  	    { type "" }
> +	    { group_signature "" }
> +	    { linkage "" }
>  	}
>  
>  	set directive ".section $name"
> @@ -1097,6 +1099,14 @@ namespace eval Dwarf {
>  	    append directive ", %$type"
>  	}
>  
> +	if { $group_signature != "" } {
> +	    append directive ", $group_signature"
> +	}
> +
> +	if { $linkage != "" } {
> +	    append directive ", $linkage"
> +	}
> +
>  	_emit "        $directive"
>      }
>  
> @@ -1717,7 +1727,13 @@ namespace eval Dwarf {
>  	    set _abbrev_section "$_abbrev_section.dwo"
>  	}
>  
> -	_section $section
> +	# The specific format of the signature is arbitrary.
> +	_section $section {
> +	    flags G
> +	    type progbits
> +	    group_signature sig.$signature
> +	    linkage comdat
> +	}
>  
>  	set cu_num [incr _cu_count]
>  
> -- 
> 2.51.2