[PATCH] D91455: [XCOFF][AIX] Generate LSDA data and compact unwind section on AIX

[Jason Liu via Phabricator via cfe-commits]

jasonliu added inline comments.


================
Comment at: llvm/lib/MC/MCObjectFileInfo.cpp:889
+  CompactUnwindSection =
+      Ctx->getXCOFFSection(".eh_info_table", XCOFF::StorageMappingClass::XMC_RW,
+                           XCOFF::XTY_SD, SectionKind::getData());
----------------
daltenty wrote:
> jasonliu wrote:
> > daltenty wrote:
> > > I think this may have been discussed elsewhere, but why do we want to emit this as a RW section?
> > Yes, this has been discussed before. And it's a good question.
> > If we emit this as an RO section, then we could not put 
> > ```
> > .vbyte	4, GCC_except_table1
> > .vbyte	4, __xlcxx_personality_v1[DS]
> > ```
> > into this csect directly. 
> > If we do that, we would hit linker error during link time. It seems that for read only csects (RO, PR mapping classes), we could not have relocation types like `R_POS` or `R_NEG` because the result coming out of it is not link-time constant. And the relocation types in read only csects needs to be link time constant.
> > 
> > Extra work (both compiler and runtime) will be needed if we want to make this RO.
> Thanks for clarifying. I guess we'd presumably need to add the TOC indirections to get to the LSDA and personality routine's function descriptor,  similar  to elsewhere. 
> 
> Since this structure is version'd, seems like we have the flexibility to defer that work, so I guess that's OK.
> add the TOC indirections to get to the LSDA and personality routine's function descriptor
Yep, that's one potential way to solve it. The downside of it would be 
1. We might have more TC entries than we desired.
2. A small performance penalty to pay for the extra indirection. (But since we already on the EH path, and it's slow anyway, might not be that big of a deal).
But as you already mentioned, we don't have to solve this issue right now. 


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  https://reviews.llvm.org/D91455/new/

https://reviews.llvm.org/D91455