Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -765,6 +765,20 @@
   if (Args.hasArg(OPT_print_map))
     Config->MapFile = "-";
 
+  // With optimizations disabled, SHF_MERGE sections are just concatenated. This
+  // works fine for regular output, but would create a problem for -r as it
+  // would combine sections with different sh_entsize.
+  // One option would be to just copy every SHF_MERGE section as is to the
+  // output. While this would produce a valid ELF file with usable SHF_MERGE
+  // sections, tools like (llvm-)?dwarfdump get confused when they see two
+  // .debug_str.
+  // We could have separate logic for combining SHF_MERGE sections based both on
+  // their name and sh_entsize, but that seems to be more trouble than it is
+  // worth. Instead, we just require at least -O1 as that already has logic for
+  // handling sh_entsize.
+  if (Config->Relocatable)
+    Config->Optimize = std::max(Config->Optimize, 1u);
+
   // --omagic is an option to create old-fashioned executables in which
   // .text segments are writable. Today, the option is still in use to
   // create special-purpose programs such as boot loaders. It doesn't
Index: test/ELF/merge-reloc-O0.s
===================================================================
--- /dev/null
+++ test/ELF/merge-reloc-O0.s
@@ -0,0 +1,48 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o -r -o %t2.o -O0
+# RUN: llvm-readobj -s -section-data %t2.o | FileCheck %s
+
+# We combine just the sections with the same name and sh_entsize.
+
+# CHECK:      Name: .foo
+# CHECK-NEXT: Type: SHT_PROGBITS
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT:   SHF_MERGE
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size: 16
+# CHECK-NEXT: Link:
+# CHECK-NEXT: Info:
+# CHECK-NEXT: AddressAlignment: 8
+# CHECK-NEXT: EntrySize: 8
+# CHECK-NEXT: SectionData (
+# CHECK-NEXT:   0000: 41000000 00000000 42000000 00000000
+# CHECK-NEXT: )
+
+# CHECK:      Name: .foo
+# CHECK-NEXT: Type: SHT_PROGBITS
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT:   SHF_MERGE
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size: 8
+# CHECK-NEXT: Link:
+# CHECK-NEXT: Info:
+# CHECK-NEXT: AddressAlignment: 4
+# CHECK-NEXT: EntrySize: 4
+# CHECK-NEXT: SectionData (
+# CHECK-NEXT:   0000: 41000000 42000000
+# CHECK-NEXT: )
+
+        .section .foo, "aM",@progbits,8,unique,0
+        .quad 0x41
+        .section .foo, "aM",@progbits,8,unique,1
+        .quad 0x42
+        .section .foo, "aM",@progbits,4,unique,2
+        .long 0x41
+        .long 0x42