Re: [PATCH] gas: Add --force-compress-debug-sections

[Tom de Vries <tdevries@suse.de>]

On 2/23/23 14:08, Jan Beulich wrote:
> On 23.02.2023 13:45, Tom de Vries via Binutils wrote:
>> Gas has an option --compress-debug-sections that allows it to generate
>> compressed debug sections.
>>
>> That does not guarantee that the debug sections are in fact compressed:
>> ...
>> $ gcc ~/hello.c -Wa,-gdwarf-5 -c -Wa,--compress-debug-sections=zstd
>> $ readelf -S -W hello.o | grep " .debug"
>>    [ 9] .debug_line       PROGBITS         0000a8 000053 00      0   0  1
>>    [11] .debug_line_str   PROGBITS         0000fb 000025 01  MS  0   0  1
>>    [12] .debug_info       PROGBITS         000120 000039 00      0   0  1
>>    [14] .debug_abbrev     PROGBITS         000159 000028 00      0   0  1
>>    [15] .debug_aranges    PROGBITS         000190 000030 00      0   0 16
>>    [17] .debug_str        PROGBITS         0001c0 000039 01  MS  0   0  1
>> ...
>>
>> Sensibly so, they're only compressed if that provides a size benefit.
>>
>> However, for the purposes of testing components consuming dwarf
>> we may want the sections to be compressed regardless.
>>
>> Add a new option --force-compress-debug-sections that ignores the size
>> heuristic, such that we have instead:
>> ...
>> $ gcc ~/hello.c -Wa,-gdwarf-5 -c -Wa,--compress-debug-sections=zstd \
>>    -Wa,--force-compress-debug-sections
>> $ readelf -S -W hello.o | grep " .debug"
>>    [ 9] .debug_line       PROGBITS         0000a8 000064 00   C  0   0  8
>>    [11] .debug_line_str   PROGBITS         000110 000046 01 MSC  0   0  8
>>    [12] .debug_info       PROGBITS         000158 000046 00   C  0   0  8
>>    [14] .debug_abbrev     PROGBITS         0001a0 000049 00   C  0   0  8
>>    [15] .debug_aranges    PROGBITS         0001f0 000034 00   C  0   0  8
>>    [17] .debug_str        PROGBITS         000228 00005a 01 MSC  0   0  8
>> ...
>>
>> Advertised as:
>> ...
>> $ as --help 2>&1 | grep compress
>>    --compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi|zstd}]
>>                            compress DWARF debug sections
>>    --nocompress-debug-sections
>>                            don't compress DWARF debug sections
>>    --force-compress-debug-sections
>>                            force compression of DWARF debug sections
> 
> No objection in principle, but have you considered making this a new
> sub-option to --compress-debug-sections, i.e. compress-debug-sections=force?

I did consider adding a "force-" prefix variant for all the non-none 
sub-options, but decided to go with the simplest solution first.

Your suggestion, --compress-debug-sections=force is more orthogonal, 
though it breaks the pattern that all the sub-options are mutually 
exclusive.

We could have it be standalone, so you'd do: 
--compress-debug-sections=zstd --compress-debug-sections=force.

Or instead combined: --compress-debug-sections=force,zstd.  Harder to 
parse though, I suppose.

I guess this last one would be my preference, because it makes it clear 
force is in a different category than zlib/zstd.

Thanks,
- Tom

> (I've actually been puzzled by --nocompress-debug-sections, which looks to
> be no different than --compress-debug-sections=none.)