Index: llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
===================================================================
--- llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -92,7 +92,9 @@
     case FK_Data_4:
       switch (Modifier) {
       default:
-        llvm_unreachable("Unsupported Modifier");
+        Ctx.reportError(Fixup.getLoc(),
+                        "invalid fixup for 4-byte pc-relative data relocation");
+        return ELF::R_ARM_NONE;
       case MCSymbolRefExpr::VK_None: {
         if (const MCSymbolRefExpr *SymRef = Target.getSymA()) {
           // For GNU AS compatibility expressions such as
@@ -162,21 +164,27 @@
   case FK_Data_1:
     switch (Modifier) {
     default:
-      llvm_unreachable("unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(),
+                      "invalid fixup for 1-byte data relocation");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_ABS8;
     }
   case FK_Data_2:
     switch (Modifier) {
     default:
-      llvm_unreachable("unsupported modifier");
+      Ctx.reportError(Fixup.getLoc(),
+                      "invalid fixup for 2-byte data relocation");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_ABS16;
     }
   case FK_Data_4:
     switch (Modifier) {
     default:
-      llvm_unreachable("Unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(),
+                      "invalid fixup for 4-byte data relocation");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_ARM_NONE:
       return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_GOT:
@@ -218,7 +226,8 @@
   case ARM::fixup_arm_movt_hi16:
     switch (Modifier) {
     default:
-      llvm_unreachable("Unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(), "invalid fixup for ARM MOVT instruction");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_MOVT_ABS;
     case MCSymbolRefExpr::VK_ARM_SBREL:
@@ -227,7 +236,8 @@
   case ARM::fixup_arm_movw_lo16:
     switch (Modifier) {
     default:
-      llvm_unreachable("Unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(), "invalid fixup for ARM MOVW instruction");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_MOVW_ABS_NC;
     case MCSymbolRefExpr::VK_ARM_SBREL:
@@ -236,7 +246,9 @@
   case ARM::fixup_t2_movt_hi16:
     switch (Modifier) {
     default:
-      llvm_unreachable("Unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(),
+                      "invalid fixup for Thumb MOVT instruction");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_THM_MOVT_ABS;
     case MCSymbolRefExpr::VK_ARM_SBREL:
@@ -245,7 +257,9 @@
   case ARM::fixup_t2_movw_lo16:
     switch (Modifier) {
     default:
-      llvm_unreachable("Unsupported Modifier");
+      Ctx.reportError(Fixup.getLoc(),
+                      "invalid fixup for Thumb MOVW instruction");
+      return ELF::R_ARM_NONE;
     case MCSymbolRefExpr::VK_None:
       return ELF::R_ARM_THM_MOVW_ABS_NC;
     case MCSymbolRefExpr::VK_ARM_SBREL:
Index: llvm/test/MC/ARM/error-location.s
===================================================================
--- llvm/test/MC/ARM/error-location.s
+++ llvm/test/MC/ARM/error-location.s
@@ -1,4 +1,5 @@
 @ RUN: not llvm-mc -triple armv7a--none-eabi -filetype obj < %s -o /dev/null 2>&1 | FileCheck %s
+@ RUN: not llvm-mc -triple thumbv7a--none-eabi -filetype obj < %s -o /dev/null 2>&1 | FileCheck %s
 
 @ Note: These errors are not always emitted in the order in which the relevant
 @ source appears, this file is carefully ordered so that that is the case.
@@ -16,7 +17,24 @@
 @ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: Cannot represent a difference across sections
   .word x_a - y_a
 
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for {{ARM|Thumb}} MOVT instruction
+  movt r9, :upper16: bar(PREL31)
 
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for {{ARM|Thumb}} MOVW instruction
+  movw r9, :lower16: bar(PREL31)
+
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for 1-byte data relocation
+  .byte f30(PLT)
+
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for 2-byte data relocation
+  .hword f30(PLT)
+
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for 4-byte data relocation
+  .word f30(PLT)
+
+@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: invalid fixup for 4-byte pc-relative data relocation
+  .word x_a(PLT) - label1
+label1:
 
 w:
   .word 0