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

[Tom de Vries]

On 2/23/23 14:44, Jan Beulich wrote:
> On 23.02.2023 14:27, Tom de Vries wrote:
>> 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 think both should be allowed. In a complex build system it may be
> different entities setting "how" and "whether". (To me "none" falls in
> the "whether" category together with "force", and it also can be seen
> as falling in the "how" category together with "zlib" etc. In Linux
> Kconfig, for example, I'd see this being expressed as first a "whether"
> choice [yes/maybe/forced] and then a "how" choice dependent upon
> "whether != none".)
> 

I gave this approach a try.

Thanks,
- Tom

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