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

[Jan Beulich <jbeulich@suse.com>]

On 24.02.2023 11:52, Tom de Vries wrote:
> 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.

Any specific reason you chose + as the separator instead of the more
conventional , ? I also wouldn't see anything wrong with something
like "...=force,zstd,none" - the last one(s) win. That's no different
from specifying a second instance of the option. And without that it
looks as if the parsing would end up simpler.

Jan