How do I get the future index of a symbol in the AsmPrinter stage?

[StevenYangCC]

How do I get the future index of a symbol in the AsmPrinter stage? Because I want to generate a section, and the content of this section is a structured data, in which the symbol’s index in symtab is needed, but I see that when llc generates ELF file, the symbol’s index is determined at the end, so I want to ask if there is any way to get the symbol’s future index in symtab in a more forward stage.

[luxufan]

Note:
The following answer was generated with the assistance of ChatGPT.

Can I get the “future” symbol‑table index during AsmPrinter?

Short answer: No. The index is assigned only when the ELF writer serializes the symbol table.
Instead: Emit a relocation (an MCFixup) that gets resolved at the object‑writing stage, or patch the section later in a custom writer. Below is a markdown‑formatted explainer.

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1 · Why the index isn’t known early

• The ELF .symtab is assembled after all sections are finalized.
• Late relocations can still create symbols, so numbering must wait.
• AsmPrinter runs before layout is frozen; indices don’t exist yet.
• LLVM assigns them in MCObjectWriter::writeObject()
  (ELFObjectWriter::writeSymbolTable for ELF).

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2 · Canonical workaround: record the symbol, not its index

// In your AsmPrinter or target streamer
auto *Sym = OutContext.getOrCreateSymbol("foo");
MCStreamer &OS = *OutStreamer;

// Emit 4‑byte placeholder; fixup refers to Sym
OS.emitValue(MCSymbolRefExpr::create(Sym, OutContext), 4);

• This creates an MCFixup.
• At write‑time, LLVM replaces that placeholder with the correct symbol‑table index (if you pick or define an appropriate relocation kind).

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3 · Alternative: patch during object writing

1. Subclass or wrap ELFObjectWriter.
2. After writeSymbolTable() (indices now fixed), iterate over your custom section’s buffer and overwrite the placeholders.
3. Plug your writer via Target->createELFObjectWriter().

This keeps everything inside the object‑file layer, but it still happens after AsmPrinter.

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4 · Why helpers like emitLabelPlusOffset() don’t help

Those helpers resolve at layout time and give you addresses, not symbol‑table indices, so they can’t satisfy this requirement.