Index: llvm/trunk/test/TableGen/GlobalISelEmitterSkippedPatterns.td
===================================================================
--- llvm/trunk/test/TableGen/GlobalISelEmitterSkippedPatterns.td
+++ llvm/trunk/test/TableGen/GlobalISelEmitterSkippedPatterns.td
@@ -0,0 +1,45 @@
+// RUN: llvm-tblgen -warn-on-skipped-patterns -gen-global-isel -I %p/../../include %s -o /dev/null 2>&1 | FileCheck %s
+include "llvm/Target/Target.td"
+
+//===- Boiler plate target code -===//
+def MyTargetISA : InstrInfo;
+def MyTarget : Target { let InstructionSet = MyTargetISA; }
+
+let TargetPrefix = "mytarget" in {
+def int_mytarget_nop : Intrinsic<[llvm_i32_ty], [llvm_i32_ty], [IntrNoMem]>;
+}
+
+def R0 : Register<"r0"> { let Namespace = "MyTarget"; }
+def GPR32 : RegisterClass<"MyTarget", [i32], 32, (add R0)>;
+
+class I<dag OOps, dag IOps, list<dag> Pat>
+  : Instruction {
+  let Namespace = "MyTarget";
+  let OutOperandList = OOps;
+  let InOperandList = IOps;
+  let Pattern = Pat;
+}
+
+def complex : Operand<i32>, ComplexPattern<i32, 2, "SelectComplexPattern", []> {
+  let MIOperandInfo = (ops i32imm, i32imm);
+}
+
+def gi_complex :
+    GIComplexOperandMatcher<s32, "selectComplexPattern">,
+    GIComplexPatternEquiv<complex>;
+def complex_rr : Operand<i32>, ComplexPattern<i32, 2, "SelectComplexPatternRR", []> {
+  let MIOperandInfo = (ops GPR32, GPR32);
+}
+
+def gi_complex_rr :
+    GIComplexOperandMatcher<s32, "selectComplexPatternRR">,
+    GIComplexPatternEquiv<complex_rr>;
+
+def INSN : I<(outs GPR32:$dst), (ins GPR32:$src1, complex:$src2), []>;
+
+//===- Bail out when we define a variable twice wrt complex suboperands. -===//
+
+// CHECK: warning: Skipped pattern: Complex suboperand referenced more than once (Operand: x)
+def : Pat<(add (complex_rr GPR32:$x, GPR32:$y),
+               (complex_rr GPR32:$x, GPR32:$z)),
+               (INSN GPR32:$z, complex:$y)>;
Index: llvm/trunk/utils/TableGen/GlobalISelEmitter.cpp
===================================================================
--- llvm/trunk/utils/TableGen/GlobalISelEmitter.cpp
+++ llvm/trunk/utils/TableGen/GlobalISelEmitter.cpp
@@ -881,12 +881,19 @@
 
   void defineOperand(StringRef SymbolicName, OperandMatcher &OM);
 
-  void defineComplexSubOperand(StringRef SymbolicName, Record *ComplexPattern,
-                               unsigned RendererID, unsigned SubOperandID) {
-    assert(ComplexSubOperands.count(SymbolicName) == 0 && "Already defined");
+  Error defineComplexSubOperand(StringRef SymbolicName, Record *ComplexPattern,
+                                unsigned RendererID, unsigned SubOperandID) {
+    if (ComplexSubOperands.count(SymbolicName))
+      return failedImport(
+          "Complex suboperand referenced more than once (Operand: " +
+          SymbolicName + ")");
+
     ComplexSubOperands[SymbolicName] =
         std::make_tuple(ComplexPattern, RendererID, SubOperandID);
+
+    return Error::success();
   }
+
   Optional<DefinedComplexPatternSubOperand>
   getComplexSubOperand(StringRef SymbolicName) const {
     const auto &I = ComplexSubOperands.find(SymbolicName);
@@ -3421,9 +3428,12 @@
 
       for (unsigned i = 0, e = SrcChild->getNumChildren(); i != e; ++i) {
         auto *SubOperand = SrcChild->getChild(i);
-        if (!SubOperand->getName().empty())
-          Rule.defineComplexSubOperand(SubOperand->getName(),
-                                       SrcChild->getOperator(), RendererID, i);
+        if (!SubOperand->getName().empty()) {
+          if (auto Error = Rule.defineComplexSubOperand(SubOperand->getName(),
+                                                        SrcChild->getOperator(),
+                                                        RendererID, i))
+            return Error;
+        }
       }
 
       return Error::success();