[llvm-objcopy] Remove empty SHT_GROUP sections (llvm#97141)

Currently `llvm-objcopy/llvm-strip` in `--strip-debug` mode doesn't
remove such sections. This behavior can lead to incompatibilities with
GNU binutils (for examples ld.bfd before https://sourceware.org/PR20520
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 llvm#97139

Author: chestnykh

llvm/lib/ObjCopy/ELF/ELFObject.cpp

@@ -2239,8 +2239,17 @@ 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 altogether.
   Sections.erase(Iter, std::end(Sections));
+
+  // Finally erase empty SHT_GROUP sections.
+  llvm::erase_if(Sections, [](const SecPtr &Sec) {
+    if (auto GroupSec = dyn_cast<GroupSection>(Sec.get()))
+      return GroupSec->getMembersCount() == 0;
+
+    return false;
+  });
+
   return Error::success();
 }
 

llvm/lib/ObjCopy/ELF/ELFObject.h

@@ -964,6 +964,8 @@ class GroupSection : public SectionBase {
       const DenseMap<SectionBase *, SectionBase *> &FromTo) override;
   void onRemove() override;
 
+  size_t getMembersCount() const { return GroupMembers.size(); }
+
   static bool classof(const SectionBase *S) {
     return S->OriginalType == ELF::SHT_GROUP;
   }

llvm/test/tools/llvm-objcopy/ELF/remove-section-in-group.test

@@ -1,6 +1,6 @@
 ## This checks that the group section is shrunk when its member is removed.
 
-# RUN: yaml2obj %s -o - \
+# RUN: yaml2obj --docnum=1 %s -o - \
 # RUN:   | llvm-objcopy -R .foo - - \
 # RUN:   | obj2yaml - \
 # RUN:   | FileCheck %s
@@ -35,3 +35,51 @@ Symbols:
   - Name:     foo_bar_grp
     Section:  .group
     Binding:  STB_GLOBAL
+
+# RUN: yaml2obj --docnum=2 %s -o %t
+# RUN: llvm-objcopy --remove-section=.debug_macro %t
+# RUN: llvm-readelf --section-groups %t | FileCheck %s --check-prefix=GROUP-REMOVED
+
+--- !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
+
+# GROUP-REMOVED: There are no section groups in this file.
+
+# RUN: yaml2obj --docnum=3 %s -o %t
+# RUN: llvm-objcopy --remove-section=.group %t
+# RUN: llvm-readelf --section-groups %t | FileCheck %s --check-prefix=EMPTY-GROUP-REMOVED
+
+--- !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
+Symbols:
+  - Name:     foo_grp
+    Section:  .group
+
+# EMPTY-GROUP-REMOVED: There are no section groups in this file.