[PATCH] D40523: [llvm-objcopy] Add --only-keep-debug

[Jake Ehrlich via Phabricator via llvm-commits]

jakehehrlich added a comment.

> Honestly, I'm a little confused by the complexity of this patch. It seems to me like changing a section to SHT_NOBITS should be pretty straightforward, since it's just changing the value of one field in a section header. Perhaps you could explain this a little more, and where necessary, add some code comments to the same effect.

The type is how we know how to cast things. It's effectively our RTTI (perhaps that wasn't a good choice). To be honest I think changing to SHT_NOBITS would probably be harmless but that wouldn't be the case for other section types and I'm not 100% sure it's safe. Moreover I don't know that I want to add the mental burdon of worrying about weather or not a class's section type changed or not at some point. Also when we call writeSection every writeSection would have to check if it was a NOBITS section or not. We could also check that case in the Object::writeSections. Layout would happen correctly I think so no changes would need to occur there. removeSectionReference could throw an error as when it should as well. In general it isn't clear to me what all the repercussions would be to just changing the type. If we create a new section then it's clear that there won't be any surprising issues.

> Also, looking at the man page (I haven't tested this), but it seems to me like it's not just allocatable sections that get replaced, but any non-debug section. In other words, I'd expect the inverse of the predicate for --strip-debug to be used as the predicate for replacing. Is that not what you've observed?

Yeah that doesn't wind up being what it does in practice and I don't think we really want it to do that since there are other things like .note, .comment, .gnu.warning, etc might be nice to have. As near as I can tell it only makes the allocated sections into NOBITS sections. In general since the debug information is quite big I think keeping around extra possibly helpful information isn't too big of a deal. It also saves us from explicitly keeping a bunch of particular magic sections.


Repository:
  rL LLVM

https://reviews.llvm.org/D40523