Index: lib/Target/ARM/AsmParser/ARMAsmParser.cpp
===================================================================
--- lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -1580,7 +1580,8 @@
       (Value >= 0x010000 && Value <= 0xff0000) ||
       (Value >= 0x01000000 && Value <= 0xff000000) ||
       (Value >= 0x01ff && Value <= 0xffff && (Value & 0xff) == 0xff) ||
-      (Value >= 0x01ffff && Value <= 0xffffff && (Value & 0xffff) == 0xffff);
+      (Value >= 0x01ffff && Value <= 0xffffff && (Value & 0xffff) == 0xffff) ||
+      (Value == 0xffffffff);
   }
   bool isNEONi32vmovNeg() const {
     if (!isImm()) return false;
@@ -2284,8 +2285,34 @@
       Value = (Value >> 8) | ((Value & 0xff) ? 0xc00 : 0x200);
     else if (Value > 0xffff && Value <= 0xffffff)
       Value = (Value >> 16) | ((Value & 0xff) ? 0xd00 : 0x400);
-    else if (Value > 0xffffff)
+    else if (Value > 0xffffff && Value != 0xffffffff)
       Value = (Value >> 24) | 0x600;
+    else if (Value == 0xffffffff) {
+      // FIXME: This is compatability workaround for GNU assembler:
+      // gcc sometimes emits "vmov.i32 dN, 0xffffffff" instructions,
+      // even they are illegal gcc-as eats them and replaces
+      // with "vmov.i8 dN, 0xff".
+      switch (Inst.getOpcode()) {
+      case ARM::VMOVv2i32:
+        Inst.setOpcode(ARM::VMOVv8i8);
+        Value = 0xeff;
+        break;
+      case ARM::VMOVv4i32:
+        Inst.setOpcode(ARM::VMOVv16i8);
+        Value = 0xeff;
+        break;
+      case ARM::VMVNv4i32:
+        Inst.setOpcode(ARM::VMOVv2i32);
+        Value = 0;
+        break;
+      case ARM::VMVNv2i32:
+        Inst.setOpcode(ARM::VMOVv2i32);
+        Value = 0;
+        break;
+      default:
+        llvm_unreachable("Unexpected instruction opcode.");
+      }
+    }
     Inst.addOperand(MCOperand::CreateImm(Value));
   }
 
Index: test/MC/ARM/vmov-i32-allones.s
===================================================================
--- /dev/null
+++ test/MC/ARM/vmov-i32-allones.s
@@ -0,0 +1,12 @@
+@ PR18921, "vmov" part.
+@ RUN: llvm-mc -triple=armv7-linux-gnueabi -show-encoding < %s | FileCheck %s
+.text
+
+@ CHECK: vmov.i8 d2, #0xff @ encoding: [0x1f,0x2e,0x87,0xf3]
+@ CHECK: vmov.i8 q2, #0xff @ encoding: [0x5f,0x4e,0x87,0xf3]
+@ CHECK: vmov.i32 d2, #0x0  @ encoding: [0x10,0x20,0x80,0xf2]
+@ CHECK: vmov.i32 q2, #0x0  @ encoding: [0x10,0x40,0x80,0xf2]
+        vmov.i32        d2, #0xffffffff
+        vmov.i32        q2, #0xffffffff
+        vmvn.i32        d2, #0xffffffff
+        vmvn.i32        q2, #0xffffffff