Index: lib/Target/X86/X86InstructionSelector.cpp
===================================================================
--- lib/Target/X86/X86InstructionSelector.cpp
+++ lib/Target/X86/X86InstructionSelector.cpp
@@ -18,13 +18,13 @@
 #include "X86RegisterInfo.h"
 #include "X86Subtarget.h"
 #include "X86TargetMachine.h"
+#include "llvm/CodeGen/GlobalISel/InstructionSelector.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
-#include "llvm/CodeGen/GlobalISel/InstructionSelector.h"
 #include "llvm/IR/Type.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -65,6 +65,8 @@
                          MachineFunction &MF) const;
   bool selectFrameIndex(MachineInstr &I, MachineRegisterInfo &MRI,
                         MachineFunction &MF) const;
+  bool selectConstant(MachineInstr &I, MachineRegisterInfo &MRI,
+                      MachineFunction &MF) const;
 
   const X86Subtarget &STI;
   const X86InstrInfo &TII;
@@ -203,6 +205,8 @@
     return true;
   if (selectFrameIndex(I, MRI, MF))
     return true;
+  if (selectConstant(I, MRI, MF))
+    return true;
 
   return selectImpl(I);
 }
@@ -458,6 +462,53 @@
   return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
 }
 
+bool X86InstructionSelector::selectConstant(MachineInstr &I,
+                                            MachineRegisterInfo &MRI,
+                                            MachineFunction &MF) const {
+  if (I.getOpcode() != TargetOpcode::G_CONSTANT)
+    return false;
+
+  const unsigned DefReg = I.getOperand(0).getReg();
+  LLT Ty = MRI.getType(DefReg);
+
+  assert(Ty.isScalar() && "invalid element type.");
+
+  uint64_t Val = 0;
+  if (I.getOperand(1).isCImm()) {
+    Val = I.getOperand(1).getCImm()->getZExtValue();
+    I.getOperand(1).ChangeToImmediate(Val);
+  } else if (I.getOperand(1).isImm()) {
+    Val = I.getOperand(1).getImm();
+  } else
+    llvm_unreachable("Unsupported operand type.");
+
+  unsigned NewOpc;
+  switch (Ty.getSizeInBits()) {
+  case 8:
+    NewOpc = X86::MOV8ri;
+    break;
+  case 16:
+    NewOpc = X86::MOV16ri;
+    break;
+  case 32:
+    NewOpc = X86::MOV32ri;
+    break;
+  case 64: {
+    // TODO: in case isUInt<32>(Val), X86::MOV32ri can be used
+    if (isInt<32>(Val))
+      NewOpc = X86::MOV64ri32;
+    else
+      NewOpc = X86::MOV64ri;
+    break;
+  }
+  default:
+    llvm_unreachable("Can't select G_CONSTANT, unsupported type.");
+  }
+
+  I.setDesc(TII.get(NewOpc));
+  return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
+}
+
 InstructionSelector *
 llvm::createX86InstructionSelector(X86Subtarget &Subtarget,
                                    X86RegisterBankInfo &RBI) {
Index: test/CodeGen/X86/GlobalISel/X86-regbankselect.mir
===================================================================
--- test/CodeGen/X86/GlobalISel/X86-regbankselect.mir
+++ test/CodeGen/X86/GlobalISel/X86-regbankselect.mir
@@ -102,6 +102,10 @@
     ret double* %p1
   }
 
+  define void @constInt_check() {
+    ret void
+  }
+
 ...
 ---
 name:            test_add_i8
@@ -604,4 +608,27 @@
     RET 0, implicit %rax
 
 ...
+---
+name:            constInt_check
+alignment:       4
+legalized:       true
+# CHECK-LABEL: name:            constInt_check
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gpr }
+# CHECK-NEXT:  - { id: 1, class: gpr }
+# CHECK-NEXT:  - { id: 2, class: gpr }
+# CHECK-NEXT:  - { id: 3, class: gpr }
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+  - { id: 2, class: _ }
+  - { id: 3, class: _ }
+body:             |
+  bb.0 (%ir-block.0):
+    %0(s8) = G_CONSTANT i8 8
+    %1(s16) = G_CONSTANT i16 16
+    %2(s32) = G_CONSTANT i32 32
+    %3(s64) = G_CONSTANT i64 64
+    RET 0
 
+...
Index: test/CodeGen/X86/GlobalISel/constant.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/constant.ll
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=x86_64-linux-gnu -global-isel < %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=X64
+
+define i8 @const_i8() {
+; ALL-LABEL: const_i8:
+; ALL:       # BB#0:
+; ALL-NEXT:    movb $2, %al
+; ALL-NEXT:    retq
+  ret i8 2
+}
+
+define i16 @const_i16() {
+; ALL-LABEL: const_i16:
+; ALL:       # BB#0:
+; ALL-NEXT:    movw $3, %ax
+; ALL-NEXT:    retq
+  ret i16 3
+}
+
+define i32 @const_i32() {
+; ALL-LABEL: const_i32:
+; ALL:       # BB#0:
+; ALL-NEXT:    movl $4, %eax
+; ALL-NEXT:    retq
+  ret i32 4
+}
+
+define i64 @const_i64() {
+; ALL-LABEL: const_i64:
+; ALL:       # BB#0:
+; ALL-NEXT:    movabsq $68719476720, %rax # imm = 0xFFFFFFFF0
+; ALL-NEXT:    retq
+  ret i64 68719476720
+}
+
+;i64 value fit into u32
+define i64 @const_i64_u32() {
+; ALL-LABEL: const_i64_u32:
+; ALL:       # BB#0:
+; ALL-NEXT:    movq $1879048192, %rax # imm = 0x70000000
+; ALL-NEXT:    retq
+  ret i64 1879048192
+}
+
+;i64 value fit into i32
+define i64 @const_i64_i32() {
+; ALL-LABEL: const_i64_i32:
+; ALL:       # BB#0:
+; ALL-NEXT:    movq $-1, %rax
+; ALL-NEXT:    retq
+  ret i64 -1
+}
+
+
Index: test/CodeGen/X86/GlobalISel/select-constant.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/select-constant.mir
@@ -0,0 +1,143 @@
+# RUN: llc -mtriple=x86_64-linux-gnu -global-isel -run-pass=instruction-select %s -o - | FileCheck %s --check-prefix=CHECK
+
+--- |
+  define i8 @const_i8() {
+    ret i8 2
+  }
+
+  define i16 @const_i16() {
+    ret i16 3
+  }
+
+  define i32 @const_i32() {
+    ret i32 4
+  }
+
+  define i64 @const_i64() {
+    ret i64 68719476720
+  }
+
+  define i64 @const_i64_u32() {
+    ret i64 1879048192
+  }
+
+  define i64 @const_i64_i32() {
+    ret i64 -1
+  }
+
+...
+---
+name:            const_i8
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i8
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr8 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV8ri 2
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s8) = G_CONSTANT i8 2
+    %al = COPY %0(s8)
+    RET 0, implicit %al
+
+...
+---
+name:            const_i16
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i16
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr16 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV16ri 3
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s16) = G_CONSTANT i16 3
+    %ax = COPY %0(s16)
+    RET 0, implicit %ax
+
+...
+---
+name:            const_i32
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i32
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr32 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV32ri 4
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s32) = G_CONSTANT i32 4
+    %eax = COPY %0(s32)
+    RET 0, implicit %eax
+
+...
+---
+name:            const_i64
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i64
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr64 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV64ri 68719476720
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s64) = G_CONSTANT i64 68719476720
+    %rax = COPY %0(s64)
+    RET 0, implicit %rax
+
+...
+---
+name:            const_i64_u32
+alignment:       4
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i64_u32
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr64 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV64ri32 1879048192
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s64) = G_CONSTANT i64 1879048192
+    %rax = COPY %0(s64)
+    RET 0, implicit %rax
+
+...
+---
+name:            const_i64_i32
+legalized:       true
+regBankSelected: true
+selected:        false
+# CHECK-LABEL: name:            const_i64_i32
+# CHECK: registers:
+# CHECK-NEXT:  - { id: 0, class: gr64 }
+registers:
+  - { id: 0, class: gpr }
+# CHECK:  body:
+# CHECK:    %0 = MOV64ri32 -1
+body:             |
+  bb.1 (%ir-block.0):
+    %0(s64) = G_CONSTANT i64 -1
+    %rax = COPY %0(s64)
+    RET 0, implicit %rax
+
+...