[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 +2206,17 @@ 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

@@ -963,6 +963,8 @@ 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.