[llvm] Move X86-specific MCSymbolRefExpr::VariantKind to X86MCExpr::Specifier (PR #132149)

[Fangrui Song via llvm-commits]

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@@ -338,10 +340,29 @@ unsigned X86ELFObjectWriter::getRelocType(MCContext &Ctx, const MCValue &Target,
   MCFixupKind Kind = Fixup.getKind();
   if (Kind >= FirstLiteralRelocationKind)
     return Kind - FirstLiteralRelocationKind;
-  MCSymbolRefExpr::VariantKind Modifier = Target.getAccessVariant();
-  X86_64RelType Type = getType64(Kind, Modifier, IsPCRel);
+  auto Specifier = X86MCExpr::Specifier(Target.getAccessVariant());
+  switch (Specifier) {
+  case X86MCExpr::VK_GOTTPOFF:
+  case X86MCExpr::VK_INDNTPOFF:
+  case X86MCExpr::VK_NTPOFF:
+  case X86MCExpr::VK_GOTNTPOFF:
+  case X86MCExpr::VK_TLSCALL:
+  case X86MCExpr::VK_TLSDESC:
+  case X86MCExpr::VK_TLSGD:
+  case X86MCExpr::VK_TLSLD:
+  case X86MCExpr::VK_TLSLDM:
+  case X86MCExpr::VK_TPOFF:
+  case X86MCExpr::VK_DTPOFF:
+    if (auto *S = Target.getSymA())
+      cast<MCSymbolELF>(S->getSymbol()).setType(ELF::STT_TLS);
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MaskRay wrote:

https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers#tls-symbols

> When a symbol is defined in a section with the SHF_TLS flag (Thread-Local Storage), GNU assembler assigns it the type STT_TLS in the symbol table. For undefined TLS symbols, the process differs: GCC and Clang don’t emit explicit labels. Instead, assemblers identify these symbols through TLS-specific relocation specifiers in the code, deduce their thread-local nature, and set their type to STT_TLS accordingly.

The legacy generic code uses `ELFObjectWriter::fixSymbolsInTLSFixups` to set `STT_TLS`. The better way is to do this in `getRelocType`, which I have done for AArch64, PowerPC, and RISC-V.

https://github.com/llvm/llvm-project/pull/132149