[PATCH] D75382: [lld] Initial commit for new Mach-O backend

[Peter Collingbourne via Phabricator via llvm-commits]

pcc added inline comments.


================
Comment at: lld/MachO/InputFiles.cpp:37
+// a Mach-O relocation has a bit indicating whether it refers a symbol or a
+// location within the same section. R_SCATTERED is that bit.
+//
----------------
int3 wrote:
> pcc wrote:
> > int3 wrote:
> > > pcc wrote:
> > > > I don't believe that this is true in general. In the case where R_SCATTERED is false, bit 27 of r_word1 (the `extern` bit) indicates whether the relocation refers to a symbol or a section address. You can see this by grepping for `rExtern` in the existing lld, and seeing how relocations with/without the bit are handled (e.g. http://llvm-cs.pcc.me.uk/tools/lld/lib/ReaderWriter/MachO/ArchHandler_arm64.cpp#434 ).
> > > This was originally written by @ruiu, but I *think* what this means is that R_SCATTERED determines whether the relocation is referring to something within the same section, and not whether it's referring to a symbol or a section. I'll rephrase things...
> > I think that both non-R_SCATTERED and R_SCATTERED can refer to locations within different sections. You can see this in the implementation of "atomByAddr" in the old linker:
> > http://llvm-cs.pcc.me.uk/tools/lld/lib/ReaderWriter/MachO/MachONormalizedFileToAtoms.cpp#527
> > which contains code to look up potentially different sections.
> > 
> > This function is called e.g. here:
> > http://llvm-cs.pcc.me.uk/tools/lld/lib/ReaderWriter/MachO/ArchHandler_arm64.cpp#444
> > 
> > and the "symbol" field (which becomes the first argument of that function) is filled in here:
> > http://llvm-cs.pcc.me.uk/tools/lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryUtils.h#144
> > (and code below to handle the non-R_SCATTERED case).
> > 
> > I would summarise the situation as being:
> > - non-scattered relocations may refer to either an external symbol or a location within a (same or different) section (index is specified in relocation, or inferred from address if 0)
> > - scattered relocations may only refer to a location within a (same or different) section (index is inferred from address).
> Thanks for the pointers! That makes sense. One question:
> 
> > section (index is specified in relocation, or inferred from address if 0)
> 
> Can a valid relocation actually have 0 as the section index? `Relocation::value` for a non-scattered relocation is always zero, so I'm not sure what address we could infer things from...
In this case, the address that we infer things from is stored inline in the section data (see `getImplicitAddend` here, or the second argument to `atomByAddr` in the old linker).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D75382/new/

https://reviews.llvm.org/D75382