Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -292,14 +292,15 @@
     switch (Sec.sh_type) {
     case SHT_GROUP:
       Sections[I] = &InputSection<ELFT>::Discarded;
-      if (ComdatGroups.insert(CachedHashStringRef(
-                                  getShtGroupSignature(ObjSections, Sec)))
+      if (ComdatGroups
+              .insert(
+                  CachedHashStringRef(getShtGroupSignature(ObjSections, Sec)))
               .second)
         continue;
       for (uint32_t SecIndex : getShtGroupEntries(Sec)) {
         if (SecIndex >= Size)
-          fatal(toString(this) + ": invalid section index in group: " +
-                Twine(SecIndex));
+          fatal(toString(this) +
+                ": invalid section index in group: " + Twine(SecIndex));
         Sections[SecIndex] = &InputSection<ELFT>::Discarded;
       }
       break;
@@ -326,6 +327,27 @@
       IS->DependentSection = Sections[I];
     }
   }
+
+  // Handle SHT_GROUP by giving all members of the selected group a GC link back
+  // to the group section, and discarding the rest. This is done after the main
+  // loop because it needs group member sections to be initialized.
+  I = -1;
+  for (const Elf_Shdr &Sec : ObjSections) {
+    ++I;
+    if (Sec.sh_type != SHT_GROUP)
+      continue;
+
+    auto GroupSections = getShtGroupEntries(Sec);
+    for (uint32_t SecIndex : GroupSections) {
+      if (SecIndex >= Size)
+        fatal(toString(this) +
+              ": invalid section index in group: " + Twine(SecIndex));
+      if (!Sections[SecIndex] ||
+          Sections[SecIndex] == &InputSection<ELFT>::Discarded)
+        continue;
+      Sections[SecIndex]->GroupSections = GroupSections;
+    }
+  }
 }
 
 template <class ELFT>
Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -92,6 +92,8 @@
   uint32_t Link;
   uint32_t Info;
 
+  ArrayRef<typename ELFT::Word> GroupSections;
+
   InputSectionBase()
       : InputSectionData(Regular, "", ArrayRef<uint8_t>(), false), Repl(this) {
     NumRelocations = 0;
Index: ELF/MarkLive.cpp
===================================================================
--- ELF/MarkLive.cpp
+++ ELF/MarkLive.cpp
@@ -89,6 +89,14 @@
   }
   if (Sec.DependentSection)
     Fn({Sec.DependentSection, 0});
+  for (auto SecIdx : Sec.GroupSections) {
+    InputSectionBase<ELFT> *SameGroupSection =
+        Sec.getFile()->getSections()[SecIdx];
+    if (SameGroupSection &&
+        SameGroupSection != &InputSection<ELFT>::Discarded &&
+        SameGroupSection != &Sec)
+      Fn({SameGroupSection, 0});
+  }
 }
 
 // The .eh_frame section is an unfortunate special case.
@@ -160,6 +168,10 @@
 // 1) Sections used by the loader (.init, .fini, .ctors, .dtors or .jcr)
 // 2) Non-allocatable sections which typically contain debugging information
 template <class ELFT> static bool isReserved(InputSectionBase<ELFT> *Sec) {
+  // Sections that are part of a group follow the usual GC rules.
+  if (Sec->GroupSections.size() > 0)
+    return false;
+
   switch (Sec->Type) {
   case SHT_FINI_ARRAY:
   case SHT_INIT_ARRAY:
Index: test/ELF/comdat-gc.s
===================================================================
--- /dev/null
+++ test/ELF/comdat-gc.s
@@ -0,0 +1,38 @@
+// Test that GC respects section groups.
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: ld.lld -shared -gc-sections %t.o -o %t
+// RUN: llvm-objdump -t %t | FileCheck --check-prefix=SYM %s
+// RUN: llvm-objdump -section-headers %t | FileCheck --check-prefix=SEC %s
+
+	.text
+	.section	.bss.x,"aGw",@nobits,x,comdat
+	.globl	x
+x:
+	.quad	0
+	.section	.bss.y,"aGw",@nobits,x,comdat
+y:
+	.quad	0
+// "x" is a GC root (external global in a
+// shared library), "y" is in the same section group as "x" and therefore must
+// not be discarded.
+// SYM-DAG: .bss{{.*}} y{{$}}
+
+	.section	zzz_no_comdat,"aw",@nobits
+z1:
+	.quad	0
+// A section with a name that is a valid C identifier, not part of any section
+// group => GC root.
+// SYM-DAG: zzz_no_comdat{{.*}} z1{{$}}
+
+	.section	zzz_comdat,"aGw",@nobits,z2,comdat
+z2:
+	.quad	0
+// No special treatment if the section is in a section group.
+// SEC-NOT: zzz_comdat
+
+	.section	.ctors,"aGw",@progbits,z3,comdat
+z3:
+	.quad	0
+// No special treatment if the section is in a section group, even for magic
+// sections like .ctors.
+// SEC-NOT: .ctors