[llvm-strip] Remove empty SHT_GROUP sections.

Currently `llvm-strip` in `--strip-debug` mode doesn't remove
such sections. This behavior can lead to incompatibilities
with GNU binutils (for examples ld.bfd cannot process
the object file contains empty .group section).
The ELF object that contains group section with `.debug_*`
sections inside can be obtained by `gcc -g3`.
Fix #97139

Author: chestnykh

llvm/lib/ObjCopy/ELF/ELFObject.cpp

@@ -2206,8 +2206,16 @@ Error Object::removeSections(
   // Transfer removed sections into the Object RemovedSections container for use
   // later.
   std::move(Iter, Sections.end(), std::back_inserter(RemovedSections));
-  // Now finally get rid of them all together.
+  // Now get rid of them all together.
   Sections.erase(Iter, std::end(Sections));
+
+  // Finally iterate over all sections and erase empty SHT_GROUP sections.
+  for (auto Iter = Sections.begin(); Iter != Sections.end(); ++Iter) {
+    if (auto GroupSec = dyn_cast<GroupSection>(Iter->get())) {
+      if (GroupSec->getMembersCount() == 0)
+        Sections.erase(Iter);
+    }
+  }
   return Error::success();
 }
 

llvm/lib/ObjCopy/ELF/ELFObject.h

@@ -963,6 +963,8 @@ class GroupSection : public SectionBase {
       const DenseMap<SectionBase *, SectionBase *> &FromTo) override;
   void onRemove() override;
 
+  size_t getMembersCount() { return GroupMembers.size(); }
+
   static bool classof(const SectionBase *S) {
     return S->OriginalType == ELF::SHT_GROUP;
   }

llvm/test/tools/llvm-objcopy/ELF/strip-empty-group.test

@@ -0,0 +1,31 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-strip --strip-debug %t -o %t1
+# RUN: llvm-readelf --section-groups %t1 | FileCheck %s
+# RUN: llvm-strip --strip-unneeded %t -o %t2
+# RUN: llvm-readelf --section-groups %t2 | FileCheck %s
+# RUN: llvm-strip --remove-section=.debug_macro %t -o %t3
+# RUN: llvm-readelf --section-groups %t3 | FileCheck %s
+# RUN: llvm-strip --remove-section=.debug_* %t -o %t4
+# RUN: llvm-readelf --section-groups %t4 | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:      ELFCLASS64
+  Data:       ELFDATA2LSB
+  Type:       ET_REL
+  Machine:    EM_X86_64
+Sections:
+  - Name:     .group
+    Type:     SHT_GROUP
+    Info:     foo_grp
+    Members:
+      - SectionOrType:  GRP_COMDAT
+      - SectionOrType:  .debug_macro
+  - Name:     .debug_macro
+    Type:     SHT_PROGBITS
+    Flags:    [ SHF_GROUP ]
+Symbols:
+  - Name:     foo_grp
+    Section:  .group
+
+# CHECK: There are no section groups in this file.