Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -274,6 +274,26 @@
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_CONSTANT: {
+    unsigned NarrowSize = NarrowTy.getSizeInBits();
+    int NumParts =
+        MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;
+    const APInt &Cst = MI.getOperand(1).getCImm()->getValue();
+    LLVMContext &Ctx = MIRBuilder.getMF().getFunction()->getContext();
+
+    SmallVector<unsigned, 2> DstRegs;
+    for (int i = 0; i < NumParts; ++i) {
+      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
+      ConstantInt *CI =
+          ConstantInt::get(Ctx, Cst.lshr(NarrowSize * i).trunc(NarrowSize));
+      MIRBuilder.buildConstant(DstReg, *CI);
+      DstRegs.push_back(DstReg);
+    }
+    unsigned DstReg = MI.getOperand(0).getReg();
+    MIRBuilder.buildMerge(DstReg, DstRegs);
+    MI.eraseFromParent();
+    return Legalized;
+  }
   }
 }
 
Index: lib/Target/X86/X86LegalizerInfo.cpp
===================================================================
--- lib/Target/X86/X86LegalizerInfo.cpp
+++ lib/Target/X86/X86LegalizerInfo.cpp
@@ -44,9 +44,11 @@
     return;
 
   const LLT p0 = LLT::pointer(0, 32);
+  const LLT s1 = LLT::scalar(1);
   const LLT s8 = LLT::scalar(8);
   const LLT s16 = LLT::scalar(16);
   const LLT s32 = LLT::scalar(32);
+  const LLT s64 = LLT::scalar(64);
 
   for (unsigned BinOp : {G_ADD, G_SUB})
     for (auto Ty : {s8, s16, s32})
@@ -62,6 +64,13 @@
 
   // Pointer-handling
   setAction({G_FRAME_INDEX, p0}, Legal);
+
+  // Constants
+  for (auto Ty : {s8, s16, s32, p0})
+    setAction({TargetOpcode::G_CONSTANT, Ty}, Legal);
+
+  setAction({TargetOpcode::G_CONSTANT, s1}, WidenScalar);
+  setAction({TargetOpcode::G_CONSTANT, s64}, NarrowScalar);
 }
 
 void X86LegalizerInfo::setLegalizerInfo64bit() {
@@ -70,6 +79,7 @@
     return;
 
   const LLT p0 = LLT::pointer(0, TM.getPointerSize() * 8);
+  const LLT s1 = LLT::scalar(1);
   const LLT s8 = LLT::scalar(8);
   const LLT s16 = LLT::scalar(16);
   const LLT s32 = LLT::scalar(32);
@@ -89,6 +99,12 @@
 
   // Pointer-handling
   setAction({G_FRAME_INDEX, p0}, Legal);
+
+  // Constants
+  for (auto Ty : {s8, s16, s32, s64, p0})
+    setAction({TargetOpcode::G_CONSTANT, Ty}, Legal);
+
+  setAction({TargetOpcode::G_CONSTANT, s1}, WidenScalar);
 }
 
 void X86LegalizerInfo::setLegalizerInfoSSE1() {
Index: test/CodeGen/X86/GlobalISel/legalize-const.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/GlobalISel/legalize-const.mir
@@ -0,0 +1,43 @@
+# RUN: llc -mtriple=i386-linux-gnu   -global-isel -run-pass=legalizer %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=X32
+# RUN: llc -mtriple=x86_64-linux-gnu -global-isel -run-pass=legalizer %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=X64
+
+--- | 
+  define void @constInt_check() {
+    ret void
+  }
+
+...
+---
+name:            constInt_check
+# ALL-LABEL: name:            constInt_check
+registers:       
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+  - { id: 2, class: _ }
+  - { id: 3, class: _ }
+  - { id: 4, class: _ }
+body:             |
+  bb.1 (%ir-block.0):
+    ; ALL: %5(s8) = G_CONSTANT i8 -1
+    ; ALL: %0(s1) = G_TRUNC %5(s8)
+    %0(s1) = G_CONSTANT i1 1
+
+    ; ALL: %1(s8) = G_CONSTANT i8 8    
+    %1(s8) = G_CONSTANT i8 8   
+
+    ; ALL: %2(s16) = G_CONSTANT i16 16
+    %2(s16) = G_CONSTANT i16 16
+
+    ; ALL: %3(s32) = G_CONSTANT i32 32
+    %3(s32) = G_CONSTANT i32 32
+
+    ; X64: %4(s64) = G_CONSTANT i64 64
+    
+    ; X32: %6(s32) = G_CONSTANT i32 64
+    ; X32: %7(s32) = G_CONSTANT i32 0
+    ; X32: %4(s64) = G_MERGE_VALUES %6(s32), %7(s32) 
+    %4(s64) = G_CONSTANT i64 64
+
+    RET 0
+...
+