Index: lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- lib/Target/X86/X86InstrInfo.cpp
+++ lib/Target/X86/X86InstrInfo.cpp
@@ -926,6 +926,14 @@
   const MachineOperand &Dest = MI.getOperand(0);
   const MachineOperand &Src = MI.getOperand(1);
 
+  // Ideally, operations with undef should be folded before we get here, but we
+  // can't guarantee it. Bail out because optimizing undefs is a waste of time.
+  if (Src.isUndef())
+    return nullptr;
+  if (MI.getNumOperands() > 2)
+    if (MI.getOperand(2).isReg() && MI.getOperand(2).isUndef())
+      return nullptr;
+
   MachineInstr *NewMI = nullptr;
   // FIXME: 16-bit LEA's are really slow on Athlons, but not bad on P4's.  When
   // we have better subtarget support, enable the 16-bit LEA generation here.
Index: test/CodeGen/X86/GlobalISel/undef.ll
===================================================================
--- test/CodeGen/X86/GlobalISel/undef.ll
+++ test/CodeGen/X86/GlobalISel/undef.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=x86_64-linux-gnu            -global-isel -verify-machineinstrs < %s -o - | FileCheck %s --check-prefix=ALL
+; RUN: llc -mtriple=x86_64-linux-gnu -global-isel -verify-machineinstrs < %s -o - | FileCheck %s --check-prefix=ALL
 
 define i8 @test() {
 ; ALL-LABEL: test:
@@ -8,8 +8,8 @@
   ret i8 undef
 }
 
-define i8 @test2(i8 %a) {
-; ALL-LABEL: test2:
+define i8 @add_undef_i8(i8 %a) {
+; ALL-LABEL: add_undef_i8:
 ; ALL:       # %bb.0:
 ; ALL-NEXT:    movl %edi, %eax
 ; ALL-NEXT:    addb %al, %al
@@ -19,6 +19,26 @@
   ret i8 %r
 }
 
+define i16 @add_undef_i16(i16 %a) {
+; ALL-LABEL: add_undef_i16:
+; ALL:       # %bb.0:
+; ALL-NEXT:    movl %edi, %eax
+; ALL-NEXT:    addw %ax, %ax
+; ALL-NEXT:    # kill: def $ax killed $ax killed $eax
+; ALL-NEXT:    retq
+  %r = add i16 %a, undef
+  ret i16 %r
+}
+
+define i32 @add_undef_i32(i32 %a) {
+; ALL-LABEL: add_undef_i32:
+; ALL:       # %bb.0:
+; ALL-NEXT:    movl %edi, %eax
+; ALL-NEXT:    addl %eax, %eax
+; ALL-NEXT:    retq
+  %r = add i32 %a, undef
+  ret i32 %r
+}
 
 define float @test3() {
 ; ALL-LABEL: test3: