[ELF] Don't emit .relr.dyn section if there are no relocs

[phosek]

Summary

This resolves PR38875.

[grimar]

This is LGTM anyways but I have a question/suggestion though.

lld/test/ELF/empty-pack-dyn-relocs.s

1

Can it be x86? I think relr can perhaps be a generic feature and for those, we use x86 target by default.

[ruiu]

LGTM but please change the "REQUIRES:" line as George suggested.

lld/ELF/SyntheticSections.h

struct RelativeReloc {
 
   const InputSectionBase *InputSec;
   uint64_t OffsetInSec;
 };
 
 class RelrBaseSection : public SyntheticSection {
 public:
   RelrBaseSection();
+  bool empty() const override { return Relocs.empty(); }
   std::vector<RelativeReloc> Relocs;
 };
 
 // RelrSection is used to encode offsets for relative relocations.
 // Proposal for adding SHT_RELR sections to generic-abi is here:
 //   https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg
 // For more details, see the comment in RelrSection::updateAllocSize().
 template <class ELFT> class RelrSection final : public RelrBaseSection {

lld/test/ELF/empty-pack-dyn-relocs.s

+// REQUIRES: aarch64
+
+// RUN: llvm-mc -filetype=obj -triple=aarch64-unknown-linux %s -o %t.o
+// RUN: ld.lld -pie --pack-dyn-relocs=relr %t.o -o %t
+// RUN: llvm-readobj -sections %t | FileCheck %s
+
+.global _start
+_start:
+  nop
+
+# CHECK-NOT: Name: .relr.dyn