Index: lib/ReaderWriter/ELF/ARM/ARMRelocationHandler.cpp
===================================================================
--- lib/ReaderWriter/ELF/ARM/ARMRelocationHandler.cpp
+++ lib/ReaderWriter/ELF/ARM/ARMRelocationHandler.cpp
@@ -613,6 +613,9 @@
   case R_ARM_IRELATIVE:
     // Runtime only relocations. Ignore here.
     break;
+  case R_ARM_V4BX:
+    // TODO implement
+    break;
   default:
     return make_unhandled_reloc_error();
   }
Index: lib/ReaderWriter/ELF/ELFFile.cpp
===================================================================
--- lib/ReaderWriter/ELF/ELFFile.cpp
+++ lib/ReaderWriter/ELF/ELFFile.cpp
@@ -38,13 +38,15 @@
                               const Elf_Sym *targetSym) {
   // Return the atom for targetSym if we can do so.
   Atom *target = _symbolToAtomMapping.lookup(targetSym);
-  if (target->definition() != Atom::definitionRegular)
-    return target;
-  Atom::Scope scope = llvm::cast<DefinedAtom>(target)->scope();
-  if (scope == DefinedAtom::scopeTranslationUnit)
-    return target;
-  if (!redirectReferenceUsingUndefAtom(sourceSym, targetSym))
-    return target;
+  if (target) {
+    if (target->definition() != Atom::definitionRegular)
+      return target;
+    Atom::Scope scope = llvm::cast<DefinedAtom>(target)->scope();
+    if (scope == DefinedAtom::scopeTranslationUnit)
+      return target;
+    if (!redirectReferenceUsingUndefAtom(sourceSym, targetSym))
+      return target;
+  }
 
   // Otherwise, create a new undefined symbol and returns it.
   StringRef targetName = target->name();
Index: test/elf/ARM/rel-v4bx.test
===================================================================
--- /dev/null
+++ test/elf/ARM/rel-v4bx.test
@@ -0,0 +1,76 @@
+# REQUIRES: arm
+
+# Check handling of R_ARM_V4BX relocation.
+# RUN: yaml2obj -format=elf %s > %t-o.o
+# RUN: lld -flavor gnu -target arm -m armelf_linux_eabi \
+# RUN: --noinhibit-exec %t-o.o -o %t
+# RUN: llvm-objdump -d -triple=armv4t %t | FileCheck %s
+
+# CHECK: Disassembly of section .text:
+# CHECK: main:
+# CHECK:   400114:       03 30 8f e0     add     r3, pc, r3
+# CHECK:   400118:       02 20 93 e7     ldr     r2, [r3, r2]
+# CHECK:   40011c:       00 00 52 e3     cmp     r2, #0
+# The following instruction creates a RM_ARM_V4BX relocation that
+# allows the linker to replace the bx instruction to a ARMv4
+# compatible one.  Currently lld does not change it.
+# CHECK:   400120:       1e ff 2f 01     bxeq    lr 
+
+---
+FileHeader:      
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:        
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         03308FE0022093E7000052E31EFF2F01
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Flags:           [ SHF_INFO_LINK ]
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:     
+      - Offset:          0x000000000000000C
+        Symbol:          ''
+        Type:            R_ARM_V4BX
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .ARM.attributes
+    Type:            SHT_ARM_ATTRIBUTES
+    AddressAlign:    0x0000000000000001
+    Content:         4115000000616561626900010B000000060208010901
+Symbols:         
+  Local:           
+    - Name:            .text
+      Type:            STT_SECTION
+      Section:         .text
+    - Name:            .data
+      Type:            STT_SECTION
+      Section:         .data
+    - Name:            .bss
+      Type:            STT_SECTION
+      Section:         .bss
+    - Name:            '$a'
+      Section:         .text
+    - Name:            .ARM.attributes
+      Type:            STT_SECTION
+      Section:         .ARM.attributes
+  Global:          
+    - Name:            main
+      Type:            STT_FUNC
+      Section:         .text
+...