[llvm] Support indirect symbol replacement with GOTPCREL for x86_64 ELF

There's an existing check in LLVM that can replace an offset to a
DSO-local symbol that is just a pointer to another symbol with a
GOTPCREL reloc. This is exactly what the RTTI proxy in the relative
vtables ABI is. This feature is supported for different macho platforms,
but not for ELF. This extends that support.

llvm/lib/Target/X86/X86TargetObjectFile.cpp

@@ -56,3 +56,13 @@ const MCExpr *X86ELFTargetObjectFile::getDebugThreadLocalSymbol(
     const MCSymbol *Sym) const {
   return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_DTPOFF, getContext());
 }
+
+const MCExpr *X86ELFTargetObjectFile::getIndirectSymViaGOTPCRel(
+    const GlobalValue *GV, const MCSymbol *Sym, const MCValue &MV,
+    int64_t Offset, MachineModuleInfo *MMI, MCStreamer &Streamer) const {
+  int64_t FinalOffset = Offset + MV.getConstant();
+  const MCExpr *Res =
+      MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_GOTPCREL, getContext());
+  const MCExpr *Off = MCConstantExpr::create(FinalOffset, getContext());
+  return MCBinaryExpr::createAdd(Res, Off, getContext());
+}

llvm/lib/Target/X86/X86TargetObjectFile.h

@@ -42,9 +42,16 @@ namespace llvm {
   public:
     X86ELFTargetObjectFile() {
       PLTRelativeVariantKind = MCSymbolRefExpr::VK_PLT;
+      SupportIndirectSymViaGOTPCRel = true;
     }
     /// Describe a TLS variable address within debug info.
     const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
+
+    const MCExpr *
+    getIndirectSymViaGOTPCRel(const GlobalValue *GV, const MCSymbol *Sym,
+                              const MCValue &MV, int64_t Offset,
+                              MachineModuleInfo *MMI,
+                              MCStreamer &Streamer) const override;
   };
 
 } // end namespace llvm

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

@@ -0,0 +1,27 @@
+; RUN: llc %s -mtriple=x86_64-unknown-fuchsia  -o - | FileCheck %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
+@vtable_with_offset = dso_local unnamed_addr constant [2 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (ptr @rtti.proxy to i64), i64 ptrtoint (ptr @vtable_with_offset to i64)) to i32)], align 4
+@vtable_with_negative_offset = dso_local unnamed_addr constant [2 x i32] [
+  i32 trunc (
+    i64 sub (
+      i64 ptrtoint (ptr @rtti.proxy to i64),
+      i64 ptrtoint (ptr getelementptr inbounds ([2 x i32], ptr @vtable_with_negative_offset, i32 0, i32 1) to i64)
+    )
+    to i32),
+  i32 0
+], align 4
+@rtti = external global i8, align 8
+@rtti.proxy = linkonce_odr hidden unnamed_addr constant ptr @rtti
+
+; CHECK-NOT: rtti.proxy
+; CHECK-LABEL: vtable:
+; CHECK-NEXT:    .{{word|long}}   rtti@GOTPCREL+0{{$}}
+
+; CHECK-LABEL: vtable_with_offset:
+; CHECK-NEXT:    .{{word|long}}   0
+; CHECK-NEXT:    .{{word|long}}   rtti@GOTPCREL+4{{$}}
+
+; CHECK-LABEL: vtable_with_negative_offset:
+; CHECK-NEXT:    .{{word|long}}   rtti@GOTPCREL-4{{$}}
+; CHECK-NEXT:    .{{word|long}}   0