Index: llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp
+++ llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp
@@ -7,8 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPUMCTargetDesc.h"
+#include "MCTargetDesc/AMDGPUFixupKinds.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/MC/MCELFObjectWriter.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCObjectWriter.h"
@@ -80,6 +82,15 @@
     return ELF::R_AMDGPU_ABS64;
   }
 
+  if (Fixup.getTargetKind() == AMDGPU::fixup_si_sopp_br) {
+    const auto *SymA = Target.getSymA();
+    assert(SymA);
+
+    Ctx.reportError(Fixup.getLoc(),
+                    Twine("undefined label '") + SymA->getSymbol().getName() + "'");
+    return ELF::R_AMDGPU_NONE;
+  }
+
   llvm_unreachable("unhandled relocation type");
 }
 
Index: llvm/test/MC/AMDGPU/labels-branch-err.s
===================================================================
--- /dev/null
+++ llvm/test/MC/AMDGPU/labels-branch-err.s
@@ -0,0 +1,4 @@
+//  RUN: not llvm-mc -arch=amdgcn -filetype=obj -o /dev/null %s 2>&1 | FileCheck -check-prefix=ERROR %s
+//  ERROR: error: undefined label 'undef_label'
+
+s_branch undef_label