[PATCH] D85056: [ELF] Add --keep-section to expose linkerscript KEEP directive as a linker flag

[Peter Smith via Phabricator via llvm-commits]

psmith added a comment.

In D85056#2217103 <https://reviews.llvm.org/D85056#2217103>, @MaskRay wrote:

> In D85056#2188969 <https://reviews.llvm.org/D85056#2188969>, @psmith wrote:
>
>> I do have some sympathy with wanting to use a command line option to keep an individual section from the command line as it has been useful in Arm's proprietary linker, although this is mainly due to the convenience of not having to create or modify another file.
>>
>> One observation I'd make about the proposed implementation is that it looks like it only implements a subset of the Linker Script KEEP command that supports precise matches of a section name. Quoting from the GNU linker manual:
>>
>>   When link-time garbage collection is in use (`--gc-sections'), it is often useful to mark sections that should not be eliminated. This is accomplished by surrounding an input section's wildcard entry with KEEP(), as in KEEP(*(.init)) or KEEP(SORT_BY_NAME(*)(.ctors)).
>>
>> That permits the full power of the input section description to discriminate via object, as there can be many sections with the same name. Wildcards can sometimes be useful too.
>>
>> I think it would be worth considering a richer interface for keep-sections. In Arm's proprietary linker we permitted a similar syntax as for the equivalent linker script. This did mean quoting or escaping parentheses but did provide equivalence in what could be achieved.
>
> Having a non-INSERT-AFTER/BEFORE SECTIONS command is considered an external linker script and can change the default layout decisions. I can also feel sympathy with the users but I am also wary of adding these non-orthogonal features. The recent D76482 <https://reviews.llvm.org/D76482> (`__build_id_start = .`) and this patch make me think of output section descriptions (a fragment of a SECTIONS command) which do not affect section layout.
>
> @psmith @grimar I think we probably should start a conversation with binutils about such a feature. If they find needs as well, we will have a common ground, it'd be great. They need to be given the decision making opportunity to reduce the risk they create a similar but incompatible feature in the future.
>
> One idea is:
>
>   OVERRIDE SECTIONS {
>     .foo : { KEEP(*(.foo)) }
>     .bar : { KEEP(*(.bar)) }
>     sym = .;   // symbol assignments are disallowed
>   }
>
> The output section descriptions will override .foo & .bar in the external linker script. If the external linker script does not describe .foo or .bar, the command will change the orphan sections.
>
> Another syntax:
>
>   SECTIONS {
>     .foo : { KEEP(*(.foo)) }
>     .bar : { KEEP(*(.bar)) }
>   } REPLACE .foo;

It is a possibility although I think we'd have to think pretty hard about the possible edge cases and how to explain what the limitations are. For example:

- What are the semantics? For a linker script I'd expect that we'd replace the OutputSection description if it existed. I'm not sure about the orphan case, I guess we'd want to insert it at the same place as if it were an Orphan.
- LLD doesn't have a default linker script, and is not transparent about the default non-script case. It might be harder than it looks to override its behaviour. I guess we could implement something like --verbose that spits out a linker script that gets close, but keeping it up to date would be challenging if we couldn't derive it.
- Could the replace break the default non-script case in any way? Would we use the non-script code-path or the script code-path? I guess this is possible with INSERT as well.

Sounds like it could do with a prototype to flush out the problems. Although no harm in asking binutils about the concept.


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