Index: lib/Target/X86/X86InstructionSelector.cpp
===================================================================
--- lib/Target/X86/X86InstructionSelector.cpp
+++ lib/Target/X86/X86InstructionSelector.cpp
@@ -85,6 +85,7 @@
                      MachineFunction &MF) const;
   bool selectCondBranch(MachineInstr &I, MachineRegisterInfo &MRI,
                         MachineFunction &MF) const;
+  bool selectImplicitDef(MachineInstr &I, MachineRegisterInfo &MRI) const;
 
   // emit insert subreg instruction and insert it before MachineInstr &I
   bool emitInsertSubreg(unsigned DstReg, unsigned SrcReg, MachineInstr &I,
@@ -334,6 +335,8 @@
     return true;
   if (selectCondBranch(I, MRI, MF))
     return true;
+  if (selectImplicitDef(I, MRI))
+    return true;
 
   return false;
 }
@@ -1128,6 +1131,29 @@
   return true;
 }
 
+bool X86InstructionSelector::selectImplicitDef(MachineInstr &I,
+                                               MachineRegisterInfo &MRI) const {
+
+  if (I.getOpcode() != TargetOpcode::G_IMPLICIT_DEF)
+    return false;
+
+  unsigned DstReg = I.getOperand(0).getReg();
+
+  if (!MRI.getRegClassOrNull(DstReg)) {
+    const LLT DstTy = MRI.getType(DstReg);
+    const TargetRegisterClass *RC = getRegClass(DstTy, DstReg, MRI);
+
+    if (!RBI.constrainGenericRegister(DstReg, *RC, MRI)) {
+      DEBUG(dbgs() << "Failed to constrain " << TII.getName(I.getOpcode())
+                   << " operand\n");
+      return false;
+    }
+  }
+
+  I.setDesc(TII.get(X86::IMPLICIT_DEF));
+  return true;
+}
+
 InstructionSelector *
 llvm::createX86InstructionSelector(const X86TargetMachine &TM,
                                    X86Subtarget &Subtarget,
Index: lib/Target/X86/X86LegalizerInfo.cpp
===================================================================
--- lib/Target/X86/X86LegalizerInfo.cpp
+++ lib/Target/X86/X86LegalizerInfo.cpp
@@ -52,6 +52,9 @@
   const LLT s32 = LLT::scalar(32);
   const LLT s64 = LLT::scalar(64);
 
+  for (auto Ty : {p0, s1, s8, s16, s32})
+    setAction({G_IMPLICIT_DEF, Ty}, Legal);
+
   for (unsigned BinOp : {G_ADD, G_SUB, G_MUL, G_AND, G_OR, G_XOR})
     for (auto Ty : {s8, s16, s32})
       setAction({BinOp, Ty}, Legal);
@@ -120,6 +123,9 @@
   const LLT s32 = LLT::scalar(32);
   const LLT s64 = LLT::scalar(64);
 
+  for (auto Ty : {p0, s1, s8, s16, s32, s64})
+    setAction({G_IMPLICIT_DEF, Ty}, Legal);
+
   for (unsigned BinOp : {G_ADD, G_SUB, G_MUL, G_AND, G_OR, G_XOR})
     for (auto Ty : {s8, s16, s32, s64})
       setAction({BinOp, Ty}, Legal);
Index: lib/Target/X86/X86RegisterBankInfo.cpp
===================================================================
--- lib/Target/X86/X86RegisterBankInfo.cpp
+++ lib/Target/X86/X86RegisterBankInfo.cpp
@@ -211,7 +211,8 @@
 
   switch (MI.getOpcode()) {
   case TargetOpcode::G_LOAD:
-  case TargetOpcode::G_STORE: {
+  case TargetOpcode::G_STORE:
+  case TargetOpcode::G_IMPLICIT_DEF: {
     // we going to try to map 32/64 bit to PMI_FP32/PMI_FP64
     unsigned Size = getSizeInBits(MI.getOperand(0).getReg(), MRI, TRI);
     if (Size != 32 && Size != 64)
Index: test/CodeGen/X86/GlobalISel/legalize-undef.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/legalize-undef.mir
@@ -0,0 +1,29 @@
+# RUN: llc -mtriple=x86_64-linux-gnu -global-isel -run-pass=legalizer %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=X64
+# RUN: llc -mtriple=i386-linux-gnu   -global-isel -run-pass=legalizer %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=X32
+
+---
+name:            test_implicit_def
+# ALL-LABEL: name:  test_implicit_def
+registers:
+# X64:          %0(s1) = G_IMPLICIT_DEF
+# X64-NEXT:     %1(s8) = G_IMPLICIT_DEF
+# X64-NEXT:     %2(s16) = G_IMPLICIT_DEF
+# X64-NEXT:     %3(s32) = G_IMPLICIT_DEF
+# X64-NEXT:     %4(s64) = G_IMPLICIT_DEF
+#
+# X32:          %0(s1) = G_IMPLICIT_DEF
+# X32-NEXT:     %1(s8) = G_IMPLICIT_DEF
+# X32-NEXT:     %2(s16) = G_IMPLICIT_DEF
+# X32-NEXT:     %3(s32) = G_IMPLICIT_DEF
+# X32-NEXT:     %5(s32) = G_IMPLICIT_DEF
+# X32-NEXT:     %6(s32) = G_IMPLICIT_DEF
+# X32-NEXT:     %4(s64) = G_MERGE_VALUES %5(s32), %6(s32)
+body: |
+  bb.0.entry:
+    liveins:
+    %0:_(s1) = G_IMPLICIT_DEF
+    %1:_(s8) = G_IMPLICIT_DEF
+    %2:_(s16) = G_IMPLICIT_DEF
+    %3:_(s32) = G_IMPLICIT_DEF
+    %4:_(s64) = G_IMPLICIT_DEF
+...
Index: test/CodeGen/X86/GlobalISel/regbankselect-X86_64.mir
===================================================================
--- test/CodeGen/X86/GlobalISel/regbankselect-X86_64.mir
+++ test/CodeGen/X86/GlobalISel/regbankselect-X86_64.mir
@@ -181,6 +181,23 @@
     ret i32* @g_int
   }
   
+  define i8 @test_undef() {
+    ret i8 undef
+  }
+
+  define i8 @test_undef2(i8 %a) {
+    %r = add i8 %a, undef
+    ret i8 %r
+  }
+
+  define float @test_undef3() {
+    ret float undef
+  }
+
+  define float @test_undef4(float %a) {
+    %r = fadd float %a, undef
+    ret float %r
+  }  
 ...
 ---
 name:            test_add_i8
@@ -1111,4 +1128,122 @@
     RET 0, implicit %rax
 
 ...
+---
+name:            test_undef
+# CHECK-LABEL: name:  test_undef
+alignment:       4
+legalized:       true
+regBankSelected: false
+# CHECK:       registers:
+# CHECK-NEXT:    - { id: 0, class: gpr, preferred-register: '' }
+registers:
+  - { id: 0, class: _, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# CHECK:           %0(s8) = G_IMPLICIT_DEF
+# CHECK-NEXT:      %al = COPY %0(s8)
+# CHECK-NEXT:      RET 0, implicit %al
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s8) = G_IMPLICIT_DEF
+    %al = COPY %0(s8)
+    RET 0, implicit %al
+
+...
+---
+name:            test_undef2
+# CHECK-LABEL: name:  test_undef2
+alignment:       4
+legalized:       true
+regBankSelected: false
+# CHECK:       registers:
+# CHECK-NEXT:    - { id: 0, class: gpr, preferred-register: '' }
+# CHECK-NEXT:    - { id: 1, class: gpr, preferred-register: '' }
+# CHECK-NEXT:    - { id: 2, class: gpr, preferred-register: '' }
+registers:
+  - { id: 0, class: _, preferred-register: '' }
+  - { id: 1, class: _, preferred-register: '' }
+  - { id: 2, class: _, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# CHECK:           %0(s8) = COPY %edi
+# CHECK-NEXT:      %1(s8) = G_IMPLICIT_DEF
+# CHECK-NEXT:      %2(s8) = G_ADD %0, %1
+# CHECK-NEXT:      %al = COPY %2(s8)
+# CHECK-NEXT:      RET 0, implicit %al
+body:             |
+  bb.1 (%ir-block.0):
+    liveins: %edi
+
+    %0(s8) = COPY %edi
+    %1(s8) = G_IMPLICIT_DEF
+    %2(s8) = G_ADD %0, %1
+    %al = COPY %2(s8)
+    RET 0, implicit %al
+
+...
+---
+name:            test_undef3
+# CHECK-LABEL: name:  test_undef3
+alignment:       4
+legalized:       true
+regBankSelected: false
+# CHECK:       registers:
+# CHECK-NEXT:    - { id: 0, class: gpr, preferred-register: '' }
+registers:
+  - { id: 0, class: _, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# CHECK:           %0(s32) = G_IMPLICIT_DEF
+# CHECK-NEXT:      %xmm0 = COPY %0(s32)
+# CHECK-NEXT:      RET 0, implicit %xmm0
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s32) = G_IMPLICIT_DEF
+    %xmm0 = COPY %0(s32)
+    RET 0, implicit %xmm0
+
+...
+---
+name:            test_undef4
+# CHECK-LABEL: name:  test_undef4
+alignment:       4
+legalized:       true
+regBankSelected: false
+# CHECK:       registers:
+# CHECK-NEXT:    - { id: 0, class: vecr, preferred-register: '' }
+# CHECK-NEXT:    - { id: 1, class: gpr, preferred-register: '' }
+# CHECK-NEXT:    - { id: 2, class: vecr, preferred-register: '' }
+# CHECK-NEXT:    - { id: 3, class: vecr, preferred-register: '' }
+registers:
+  - { id: 0, class: _, preferred-register: '' }
+  - { id: 1, class: _, preferred-register: '' }
+  - { id: 2, class: _, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# CHECK:           %0(s32) = COPY %xmm0
+# CHECK-NEXT:      %1(s32) = G_IMPLICIT_DEF
+# CHECK-NEXT:      %3(s32) = COPY %1(s32)
+# CHECK-NEXT:      %2(s32) = G_FADD %0, %3
+# CHECK-NEXT:      %xmm0 = COPY %2(s32)
+# CHECK-NEXT:      RET 0, implicit %xmm0
+body:             |
+  bb.1 (%ir-block.0):
+    liveins: %xmm0
+
+    %0(s32) = COPY %xmm0
+    %1(s32) = G_IMPLICIT_DEF
+    %2(s32) = G_FADD %0, %1
+    %xmm0 = COPY %2(s32)
+    RET 0, implicit %xmm0
+
+...
 
Index: test/CodeGen/X86/GlobalISel/select-undef.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/select-undef.mir
@@ -0,0 +1,71 @@
+# RUN: llc -mtriple=x86_64-linux-gnu -global-isel -run-pass=instruction-select -verify-machineinstrs %s -o - | FileCheck %s --check-prefix=ALL
+--- |
+
+  define i8 @test() {
+    ret i8 undef
+  }
+
+  define i8 @test2(i8 %a) {
+    %r = add i8 %a, undef
+    ret i8 %r
+  }
+
+...
+---
+name:            test
+# ALL-LABEL: name:  test
+alignment:       4
+legalized:       true
+regBankSelected: true
+# ALL:      registers:
+# ALL-NEXT:   - { id: 0, class: gr8, preferred-register: '' }
+registers:
+  - { id: 0, class: gpr, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# ALL:          %0 = IMPLICIT_DEF
+# ALL-NEXT:     %al = COPY %0
+# ALL-NEXT:     RET 0, implicit %al
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s8) = G_IMPLICIT_DEF
+    %al = COPY %0(s8)
+    RET 0, implicit %al
+
+...
+---
+name:            test2
+# ALL-LABEL: name:  test2
+alignment:       4
+legalized:       true
+regBankSelected: true
+# ALL:      registers:
+# ALL-NEXT:   - { id: 0, class: gr8, preferred-register: '' }
+# ALL-NEXT:   - { id: 1, class: gr8, preferred-register: '' }
+# ALL-NEXT:   - { id: 2, class: gr8, preferred-register: '' }
+registers:
+  - { id: 0, class: gpr, preferred-register: '' }
+  - { id: 1, class: gpr, preferred-register: '' }
+  - { id: 2, class: gpr, preferred-register: '' }
+liveins:
+fixedStack:
+stack:
+constants:
+# ALL:          %0 = COPY %dil
+# ALL-NEXT:     %1 = IMPLICIT_DEF
+# ALL-NEXT:     %2 = ADD8rr %0, %1, implicit-def %eflags
+# ALL-NEXT:     %al = COPY %2
+# ALL-NEXT:     RET 0, implicit %al
+body:             |
+  bb.1 (%ir-block.0):
+    liveins: %edi
+
+    %0(s8) = COPY %edi
+    %1(s8) = G_IMPLICIT_DEF
+    %2(s8) = G_ADD %0, %1
+    %al = COPY %2(s8)
+    RET 0, implicit %al
+
+...
Index: test/CodeGen/X86/GlobalISel/undef.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/undef.ll
@@ -0,0 +1,37 @@
+; 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
+
+define i8 @test() {
+; ALL-LABEL: test:
+; ALL:       # BB#0:
+; ALL-NEXT:    retq
+  ret i8 undef
+}
+
+define i8 @test2(i8 %a) {
+; ALL-LABEL: test2:
+; ALL:       # BB#0:
+; ALL-NEXT:    addb %al, %dil
+; ALL-NEXT:    movl %edi, %eax
+; ALL-NEXT:    retq
+  %r = add i8 %a, undef
+  ret i8 %r
+}
+
+
+define float @test3() {
+; ALL-LABEL: test3:
+; ALL:       # BB#0:
+; ALL-NEXT:    retq
+  ret float undef
+}
+
+define float @test4(float %a) {
+; ALL-LABEL: test4:
+; ALL:       # BB#0:
+; ALL-NEXT:    addss %xmm0, %xmm0
+; ALL-NEXT:    retq
+  %r = fadd float %a, undef
+  ret float %r
+}
+