[LLD][ELF][AArch64] Fix ADRP relocations to undefined weak reference.

[peter.smith]

Summary

In the ABI for the 64-bit Arm architecture the section on weak references states:

During linking, the symbol value of an undefined weak reference is:
    - Zero if the relocation type is absolute
    - The address of the place if the relocation type is pc-relative.

The relocations associated with an ADRP are relative so we should resolve the undefined weak reference to the place instead of 0. This matches GNU ld.bfd behaviour.

fixes pr34928 (https://bugs.llvm.org/show_bug.cgi?id=34928)

[grimar]

LGTM, thanks!

ELF/InputSection.cpp

return In.MipsGot->getVA() + In.MipsGot->getSymEntryOffset(File, Sym, A) -
            In.MipsGot->getGp(File);
   case R_MIPS_TLSGD:
     return In.MipsGot->getVA() + In.MipsGot->getGlobalDynOffset(File, Sym) -
            In.MipsGot->getGp(File);
   case R_MIPS_TLSLD:
     return In.MipsGot->getVA() + In.MipsGot->getTlsIndexOffset(File) -
            In.MipsGot->getGp(File);
   case R_AARCH64_PAGE_PC: {
-    uint64_t Val = Sym.isUndefWeak() ? A : Sym.getVA(A);
+    uint64_t Val = Sym.isUndefWeak() ? P + A : Sym.getVA(A);
     return getAArch64Page(Val) - getAArch64Page(P);
   }
   case R_AARCH64_PLT_PAGE_PC: {
-    uint64_t Val = Sym.isUndefWeak() ? A : Sym.getPltVA() + A;
+    uint64_t Val = Sym.isUndefWeak() ? P + A : Sym.getPltVA() + A;
     return getAArch64Page(Val) - getAArch64Page(P);
   }
   case R_RISCV_PC_INDIRECT: {
     if (const Relocation *HiRel = getRISCVPCRelHi20(&Sym, A))
       return getRelocTargetVA(File, HiRel->Type, HiRel->Addend, Sym.getVA(),
                               *HiRel->Sym, HiRel->Expr);
     return 0;
   }

test/ELF/aarch64-undefined-weak.s

 
 // CHECK: Disassembly of section .text:
 // 2162688 = 0x210000
 // CHECK:         210000: {{.*}} b       #4
 // CHECK-NEXT:    210004: {{.*}} bl      #4
 // CHECK-NEXT:    210008: {{.*}} b.eq    #4
 // CHECK-NEXT:    21000c: {{.*}} cbz     x1, #4
 // CHECK-NEXT:    210010: {{.*}} adr     x0, #0
-// CHECK-NEXT:    210014: {{.*}} adrp    x0, #-2162688
+// CHECK-NEXT:    210014: {{.*}} adrp    x0, #0
 // CHECK:         210018: {{.*}} .word   0x00000000
 // CHECK-NEXT:    21001c: {{.*}} .word   0x00000000
 // CHECK-NEXT:    210020: {{.*}} .word   0x00000000
 // CHECK-NEXT:    210024: {{.*}} .short  0x0000
 // CHECK:         $x.2:
 // CHECK-NEXT:    210026: {{.*}} ldr     x8, #0