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[lld-macho] Add options -(no_)keep_dwarf_unwind
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Authored by gkm on Thu, Jun 10, 6:18 PM.

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int3
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Restricted Project
Summary

DWARF unwind info was superseded by compact unwind as-of macOS 10.9 and iOS 7.0
(circa 2013). Most often, DWARF unwind info is entirely redundant and can be
discarded. In rare instances, complex unwind info cannot be represented by a
compact-unwind entry, and we fall-back to the DWARF frame info.

The intelligent approach is to filter-out all redundant entries in the
__TEXT,__eh_frame section, leaving only those that are necessary fallbacks
when compact unwind is insufficient. However, that's not easy, so unless and
until (maybe later today!) we have a use case that requires the intelligent
approach, we offer the choice of all or nothing.

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gkm created this revision.Thu, Jun 10, 6:18 PM
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gkm requested review of this revision.Thu, Jun 10, 6:18 PM
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The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

gkm edited the summary of this revision. (Show Details)Fri, Jun 11, 10:14 AM
gkm added a comment.Fri, Jun 11, 10:18 AM

The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

Yes, I will do it, but wanted this out quickly. (I changed the description to read "... unless and until (maybe later today!) we have a use case ..." :)

When I looked at the CU entries for Chromium Framework, I saw none that had type bits [24..27] == 4, which means UNWIND_X86_64_MODE_DWARF. What am I not seeing?

int3 added a comment.Fri, Jun 11, 10:21 AM

Yes, I will do it, but wanted this out quickly.

No great rush to commit this though :)

The intelligent approach isn't _that_ hard, is it? It's < 200 lines in ld64 I think. ld64-linked Chromium Framework leaves some __eh_frame around, so I think removing the whole section generally probably doesn't work.

Yes, I will do it, but wanted this out quickly. (I changed the description to read "... unless and until (maybe later today!) we have a use case ..." :)

When I looked at the CU entries for Chromium Framework, I saw none that had type bits [24..27] == 4, which means UNWIND_X86_64_MODE_DWARF. What am I not seeing?

I didn't look in detail, I just noticed that ld64 keeps some of the section. So I thought the bits that remain are probably needed? Not sure though.