[llvm] [RISCV] Add support for vendor relocations on Xqci extensions (PR #135400)

[Fangrui Song via llvm-commits]

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@@ -1435,6 +1435,21 @@ void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
                         : C;
   FixedValue = usesRela(TO, FixupSection) ? 0 : Addend;
 
+  // Some RISC-V relocations need a marker relocation that appears before the
+  // relocation in question.
+  unsigned PreRelocType;
+  MCSymbol *PreRelocSymbol;
+  uint64_t PreRelocAddend;
+  if (Backend.fixupNeedsMarkerELFRelocation(Asm, *Fragment, Fixup, PreRelocType,
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MaskRay wrote:

(MCAssembler.cpp and ELFObjectWriter.cpp have significant tech debt. I try to clean up them from time to time. Just noticed that my latest change 19730e37ec5d3513d188a75999c964f2ef4b9509 might need you to rebase.)

I think we should refrain from adding new hooks. (I added `Backend.handleAddSubRelocations` in the past, and on reflection we might need some cleanup so that it could be combined with getRelocType).

Can you add an extra fixup when `fixup_riscv_qc_e_branch` is appended? Or the hook is here because in some scenarios the fixup can be resolved and you don't want to add a redundant marker?

https://github.com/llvm/llvm-project/pull/135400