[X86] Fix RTTI proxy emission for 32-bit (llvm#78622)

32-bit x86 doesn't have an appropriate relocation type we can use to
elide the RTTI proxies, so we need to emit them. This would previously
cause crashes when using the relative vtable ABI for 32-bit x86.

llvm/lib/Target/X86/X86TargetMachine.cpp

@@ -113,6 +113,9 @@ static std::unique_ptr<TargetLoweringObjectFile> createTLOF(const Triple &TT) {
 
   if (TT.isOSBinFormatCOFF())
     return std::make_unique<TargetLoweringObjectFileCOFF>();
+
+  if (TT.getArch() == Triple::x86_64)
+    return std::make_unique<X86_64ELFTargetObjectFile>();
   return std::make_unique<X86ELFTargetObjectFile>();
 }
 

llvm/lib/Target/X86/X86TargetObjectFile.cpp

@@ -57,7 +57,7 @@ const MCExpr *X86ELFTargetObjectFile::getDebugThreadLocalSymbol(
   return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_DTPOFF, getContext());
 }
 
-const MCExpr *X86ELFTargetObjectFile::getIndirectSymViaGOTPCRel(
+const MCExpr *X86_64ELFTargetObjectFile::getIndirectSymViaGOTPCRel(
     const GlobalValue *GV, const MCSymbol *Sym, const MCValue &MV,
     int64_t Offset, MachineModuleInfo *MMI, MCStreamer &Streamer) const {
   int64_t FinalOffset = Offset + MV.getConstant();

llvm/lib/Target/X86/X86TargetObjectFile.h

@@ -36,16 +36,22 @@ namespace llvm {
                                             MCStreamer &Streamer) const override;
   };
 
-  /// This implementation is used for X86 ELF targets that don't
-  /// have a further specialization.
+  /// This implementation is used for X86 ELF targets that don't have a further
+  /// specialization (and as a base class for X86_64, which does).
   class X86ELFTargetObjectFile : public TargetLoweringObjectFileELF {
   public:
     X86ELFTargetObjectFile() {
       PLTRelativeVariantKind = MCSymbolRefExpr::VK_PLT;
-      SupportIndirectSymViaGOTPCRel = true;
     }
     /// Describe a TLS variable address within debug info.
     const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
+  };
+
+  /// This implementation is used for X86_64 ELF targets, and defers to
+  /// X86ELFTargetObjectFile for commonalities with 32-bit targets.
+  class X86_64ELFTargetObjectFile : public X86ELFTargetObjectFile {
+  public:
+    X86_64ELFTargetObjectFile() { SupportIndirectSymViaGOTPCRel = true; }
 
     const MCExpr *
     getIndirectSymViaGOTPCRel(const GlobalValue *GV, const MCSymbol *Sym,

llvm/test/MC/ELF/rtti-proxy-i686.ll

@@ -0,0 +1,20 @@
+; REQUIRES: x86-registered-target
+
+;; Validate that we produce RTTI proxies for 32-bit x86.
+; RUN: llc %s -mtriple=i686-elf -o - | FileCheck %s
+
+;; Validate that we produce a valid object file.
+; RUN: llc %s -mtriple=i686-elf --filetype=obj -o %t.o
+; RUN: llvm-readobj --relocs %t.o | FileCheck --check-prefix=RELOCS %s
+
+@vtable = dso_local unnamed_addr constant i32 trunc (i64 sub (i64 ptrtoint (ptr @rtti.proxy to i64), i64 ptrtoint (ptr @vtable to i64)) to i32), align 4
+@rtti = external global i8, align 8
+@rtti.proxy = linkonce_odr hidden unnamed_addr constant ptr @rtti
+
+; CHECK-LABEL: vtable:
+; CHECK-NEXT:    .long   rtti.proxy-vtable
+
+; CHECK-LABEL: rtti.proxy:
+; CHECK-NEXT:    .long   rtti
+
+; RELOCS: R_386_32 rtti