[llvm] Support indirect symbol replacement with R_ARM_GOT_PREL (#81916)

R_ARM_GOT_PREL is equivalent to GOTPCREL on other architectures, so we
can use it for indirect symbol replacement the same way. This is the
equivalent of #67754 for x86_64
and #78003 for AArch64 and
64-bit RISC-V.

Author: smeenai

llvm/lib/Target/ARM/ARMTargetObjectFile.cpp

@@ -16,6 +16,7 @@
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCTargetOptions.h"
+#include "llvm/MC/MCValue.h"
 #include "llvm/MC/SectionKind.h"
 #include "llvm/Target/TargetMachine.h"
 #include <cassert>
@@ -56,6 +57,16 @@ void ARMElfTargetObjectFile::Initialize(MCContext &Ctx,
 
 MCRegister ARMElfTargetObjectFile::getStaticBase() const { return ARM::R9; }
 
+const MCExpr *ARMElfTargetObjectFile::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_ARM_GOT_PREL, getContext());
+  const MCExpr *Off = MCConstantExpr::create(FinalOffset, getContext());
+  return MCBinaryExpr::createAdd(Res, Off, getContext());
+}
+
 const MCExpr *ARMElfTargetObjectFile::
 getIndirectSymViaRWPI(const MCSymbol *Sym) const {
   return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_ARM_SBREL,

llvm/lib/Target/ARM/ARMTargetObjectFile.h

@@ -19,12 +19,19 @@ class ARMElfTargetObjectFile : public TargetLoweringObjectFileELF {
 public:
   ARMElfTargetObjectFile() {
     PLTRelativeVariantKind = MCSymbolRefExpr::VK_ARM_PREL31;
+    SupportIndirectSymViaGOTPCRel = true;
   }
 
   void Initialize(MCContext &Ctx, const TargetMachine &TM) override;
 
   MCRegister getStaticBase() const override;
 
+  const MCExpr *getIndirectSymViaGOTPCRel(const GlobalValue *GV,
+                                          const MCSymbol *Sym,
+                                          const MCValue &MV, int64_t Offset,
+                                          MachineModuleInfo *MMI,
+                                          MCStreamer &Streamer) const override;
+
   const MCExpr *getIndirectSymViaRWPI(const MCSymbol *Sym) const override;
 
   const MCExpr *getTTypeGlobalReference(const GlobalValue *GV,

llvm/test/MC/ELF/rtti-proxy-arm-got-prel.ll

@@ -0,0 +1,45 @@
+; REQUIRES: arm-registered-target
+
+;; Verify that the generated assembly omits the RTTI proxy and uses the correct
+;; relocations and addends.
+; RUN: llc %s -mtriple=armv7 -o - | FileCheck %s
+
+;; Verify that the generated object uses the correct relocations and addends.
+; RUN: llc %s -filetype=obj -mtriple=thumbv7 -o %t.o
+; RUN: llvm-readelf --relocs --hex-dump=.rodata %t.o | FileCheck --check-prefix=OBJ %s
+
+@vtable = dso_local unnamed_addr constant i32 sub (i32 ptrtoint (ptr @rtti.proxy to i32), i32 ptrtoint (ptr @vtable to i32)), align 4
+
+@vtable_with_offset = dso_local unnamed_addr constant [2 x i32] [i32 0, i32 sub (i32 ptrtoint (ptr @rtti.proxy to i32), i32 ptrtoint (ptr @vtable_with_offset to i32))], align 4
+
+@vtable_with_negative_offset = dso_local unnamed_addr constant [2 x i32] [
+  i32 sub (
+    i32 ptrtoint (ptr @rtti.proxy to i32),
+    i32 ptrtoint (ptr getelementptr inbounds ([2 x i32], ptr @vtable_with_negative_offset, i32 0, i32 1) to i32)
+  ),
+  i32 0
+], align 4
+
+@rtti = external global i8, align 4
+@rtti.proxy = linkonce_odr hidden unnamed_addr constant ptr @rtti
+
+; CHECK-NOT: rtti.proxy
+
+; CHECK-LABEL: vtable:
+; CHECK-NEXT:    .long   rtti(GOT_PREL)+0{{$}}
+
+; CHECK-LABEL: vtable_with_offset:
+; CHECK-NEXT:    .long   0
+; CHECK-NEXT:    .long   rtti(GOT_PREL)+4{{$}}
+
+; CHECK-LABEL: vtable_with_negative_offset:
+; CHECK-NEXT:    .long   rtti(GOT_PREL)-4{{$}}
+; CHECK-NEXT:    .long   0
+
+; OBJ-LABEL: Relocation section '.rel.rodata' at offset [[#%#x,]] contains 3 entries:
+; OBJ:       {{^}}00000000 [[#]] R_ARM_GOT_PREL [[#]] rtti{{$}}
+; OBJ-NEXT:  {{^}}00000008 [[#]] R_ARM_GOT_PREL [[#]] rtti{{$}}
+; OBJ-NEXT:  {{^}}0000000c [[#]] R_ARM_GOT_PREL [[#]] rtti{{$}}
+
+; OBJ-LABEL: Hex dump of section '.rodata':
+; OBJ-NEXT:  0x00000000 00000000 00000000 04000000 fcffffff