Index: include/llvm/CodeGen/GlobalISel/Utils.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/Utils.h
+++ include/llvm/CodeGen/GlobalISel/Utils.h
@@ -89,8 +89,25 @@
                         const char *PassName, StringRef Msg,
                         const MachineInstr &MI);
 
+/// If \p VReg is defined by a G_CONSTANT fits in int64_t
+/// returns it.
 Optional<int64_t> getConstantVRegVal(unsigned VReg,
                                      const MachineRegisterInfo &MRI);
+/// Simple struct used to hold a constant integer value and a virtual
+/// register.
+struct ValueAndVReg {
+  int64_t Value;
+  unsigned VReg;
+};
+/// If \p VReg is defined by a statically evaluable chain of
+/// instructions rooted on a G_CONSTANT (\p LookThroughInstrs == true)
+/// and that constant fits in int64_t, returns its value as well as
+/// the virtual register defined by this G_CONSTANT.
+/// When \p LookThroughInstrs == false, this function behaves like
+/// getConstantVRegVal.
+Optional<ValueAndVReg>
+getConstantVRegValWithLookThrough(unsigned VReg, const MachineRegisterInfo &MRI,
+                                  bool LookThroughInstrs = true);
 const ConstantFP* getConstantFPVRegVal(unsigned VReg,
                                        const MachineRegisterInfo &MRI);
 
Index: lib/CodeGen/GlobalISel/InstructionSelector.cpp
===================================================================
--- lib/CodeGen/GlobalISel/InstructionSelector.cpp
+++ lib/CodeGen/GlobalISel/InstructionSelector.cpp
@@ -49,8 +49,8 @@
     const MachineOperand &MO, int64_t Value,
     const MachineRegisterInfo &MRI) const {
   if (MO.isReg() && MO.getReg())
-    if (auto VRegVal = getConstantVRegVal(MO.getReg(), MRI))
-      return *VRegVal == Value;
+    if (auto VRegVal = getConstantVRegValWithLookThrough(MO.getReg(), MRI))
+      return VRegVal->Value == Value;
   return false;
 }
 
Index: lib/CodeGen/GlobalISel/Utils.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Utils.cpp
+++ lib/CodeGen/GlobalISel/Utils.cpp
@@ -183,18 +183,68 @@
 
 Optional<int64_t> llvm::getConstantVRegVal(unsigned VReg,
                                            const MachineRegisterInfo &MRI) {
-  MachineInstr *MI = MRI.getVRegDef(VReg);
-  if (MI->getOpcode() != TargetOpcode::G_CONSTANT)
+  Optional<ValueAndVReg> ValAndVReg =
+      getConstantVRegValWithLookThrough(VReg, MRI, /*LookThroughInstrs*/ false);
+  assert((!ValAndVReg || ValAndVReg->VReg == VReg) &&
+         "Value found while looking through instrs");
+  if (!ValAndVReg)
+    return None;
+  return ValAndVReg->Value;
+}
+
+Optional<ValueAndVReg> llvm::getConstantVRegValWithLookThrough(
+    unsigned VReg, const MachineRegisterInfo &MRI, bool LookThroughInstrs) {
+  SmallVector<std::pair<unsigned, unsigned>, 4> SeenOpcodes;
+  MachineInstr *MI;
+  while ((MI = MRI.getVRegDef(VReg)) &&
+         MI->getOpcode() != TargetOpcode::G_CONSTANT && LookThroughInstrs) {
+    switch (MI->getOpcode()) {
+    case TargetOpcode::G_TRUNC:
+    case TargetOpcode::G_SEXT:
+    case TargetOpcode::G_ZEXT:
+      SeenOpcodes.push_back(std::make_pair(
+          MI->getOpcode(),
+          MRI.getType(MI->getOperand(0).getReg()).getSizeInBits()));
+      VReg = MI->getOperand(1).getReg();
+      break;
+    case TargetOpcode::COPY:
+      VReg = MI->getOperand(1).getReg();
+      if (TargetRegisterInfo::isPhysicalRegister(VReg))
+        return None;
+      break;
+    default:
+      return None;
+    }
+  }
+  if (!MI || MI->getOpcode() != TargetOpcode::G_CONSTANT ||
+      (!MI->getOperand(1).isImm() && !MI->getOperand(1).isCImm()))
     return None;
 
-  if (MI->getOperand(1).isImm())
-    return MI->getOperand(1).getImm();
+  const MachineOperand &CstVal = MI->getOperand(1);
+  unsigned BitWidth = MRI.getType(MI->getOperand(0).getReg()).getSizeInBits();
+  APInt Val = CstVal.isImm() ? APInt(BitWidth, CstVal.getImm())
+                             : CstVal.getCImm()->getValue();
+  assert(Val.getBitWidth() == BitWidth &&
+         "Value bitwidth doesn't match definition type");
+  while (!SeenOpcodes.empty()) {
+    std::pair<unsigned, unsigned> OpcodeAndSize = SeenOpcodes.pop_back_val();
+    switch (OpcodeAndSize.first) {
+    case TargetOpcode::G_TRUNC:
+      Val = Val.trunc(OpcodeAndSize.second);
+      break;
+    case TargetOpcode::G_SEXT:
+      Val = Val.sext(OpcodeAndSize.second);
+      break;
+    case TargetOpcode::G_ZEXT:
+      Val = Val.zext(OpcodeAndSize.second);
+      break;
+    }
+  }
 
-  if (MI->getOperand(1).isCImm() &&
-      MI->getOperand(1).getCImm()->getBitWidth() <= 64)
-    return MI->getOperand(1).getCImm()->getSExtValue();
+  if (Val.getBitWidth() > 64)
+    return None;
 
-  return None;
+  return ValueAndVReg{Val.getSExtValue(), VReg};
 }
 
 const llvm::ConstantFP* llvm::getConstantFPVRegVal(unsigned VReg,
Index: test/CodeGen/Mips/GlobalISel/llvm-ir/select.ll
===================================================================
--- test/CodeGen/Mips/GlobalISel/llvm-ir/select.ll
+++ test/CodeGen/Mips/GlobalISel/llvm-ir/select.ll
@@ -64,14 +64,12 @@
 define i32 @select_with_negation(i32 %a, i32 %b, i32 %x, i32 %y) {
 ; MIPS32-LABEL: select_with_negation:
 ; MIPS32:       # %bb.0: # %entry
-; MIPS32-NEXT:    lui $1, 65535
-; MIPS32-NEXT:    ori $1, $1, 65535
 ; MIPS32-NEXT:    slt $4, $4, $5
-; MIPS32-NEXT:    xor $1, $4, $1
-; MIPS32-NEXT:    lui $4, 0
-; MIPS32-NEXT:    ori $4, $4, 1
-; MIPS32-NEXT:    and $1, $1, $4
-; MIPS32-NEXT:    movn $7, $6, $1
+; MIPS32-NEXT:    not $4, $4
+; MIPS32-NEXT:    lui $5, 0
+; MIPS32-NEXT:    ori $5, $5, 1
+; MIPS32-NEXT:    and $4, $4, $5
+; MIPS32-NEXT:    movn $7, $6, $4
 ; MIPS32-NEXT:    move $2, $7
 ; MIPS32-NEXT:    jr $ra
 ; MIPS32-NEXT:    nop
Index: test/CodeGen/X86/GlobalISel/ashr-scalar.ll
===================================================================
--- test/CodeGen/X86/GlobalISel/ashr-scalar.ll
+++ test/CodeGen/X86/GlobalISel/ashr-scalar.ll
@@ -28,8 +28,7 @@
 ; X64-LABEL: test_ashr_i64_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movq %rdi, %rax
-; X64-NEXT:    movq $1, %rcx
-; X64-NEXT:    sarq %cl, %rax
+; X64-NEXT:    sarq %rax
 ; X64-NEXT:    retq
   %res = ashr i64 %arg1, 1
   ret i64 %res
@@ -62,8 +61,7 @@
 ; X64-LABEL: test_ashr_i32_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movl $1, %ecx
-; X64-NEXT:    sarl %cl, %eax
+; X64-NEXT:    sarl %eax
 ; X64-NEXT:    retq
   %res = ashr i32 %arg1, 1
   ret i32 %res
@@ -101,8 +99,7 @@
 ; X64-LABEL: test_ashr_i16_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movw $1, %cx
-; X64-NEXT:    sarw %cl, %ax
+; X64-NEXT:    sarw %ax
 ; X64-NEXT:    # kill: def $ax killed $ax killed $eax
 ; X64-NEXT:    retq
   %a = trunc i32 %arg1 to i16
Index: test/CodeGen/X86/GlobalISel/lshr-scalar.ll
===================================================================
--- test/CodeGen/X86/GlobalISel/lshr-scalar.ll
+++ test/CodeGen/X86/GlobalISel/lshr-scalar.ll
@@ -28,8 +28,7 @@
 ; X64-LABEL: test_lshr_i64_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movq %rdi, %rax
-; X64-NEXT:    movq $1, %rcx
-; X64-NEXT:    shrq %cl, %rax
+; X64-NEXT:    shrq %rax
 ; X64-NEXT:    retq
   %res = lshr i64 %arg1, 1
   ret i64 %res
@@ -62,8 +61,7 @@
 ; X64-LABEL: test_lshr_i32_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movl $1, %ecx
-; X64-NEXT:    shrl %cl, %eax
+; X64-NEXT:    shrl %eax
 ; X64-NEXT:    retq
   %res = lshr i32 %arg1, 1
   ret i32 %res
@@ -101,8 +99,7 @@
 ; X64-LABEL: test_lshr_i16_imm1:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movw $1, %cx
-; X64-NEXT:    shrw %cl, %ax
+; X64-NEXT:    shrw %ax
 ; X64-NEXT:    # kill: def $ax killed $ax killed $eax
 ; X64-NEXT:    retq
   %a = trunc i32 %arg1 to i16
Index: test/CodeGen/X86/GlobalISel/shl-scalar.ll
===================================================================
--- test/CodeGen/X86/GlobalISel/shl-scalar.ll
+++ test/CodeGen/X86/GlobalISel/shl-scalar.ll
@@ -27,9 +27,7 @@
 define i64 @test_shl_i64_imm1(i64 %arg1) {
 ; X64-LABEL: test_shl_i64_imm1:
 ; X64:       # %bb.0:
-; X64-NEXT:    movq %rdi, %rax
-; X64-NEXT:    movq $1, %rcx
-; X64-NEXT:    shlq %cl, %rax
+; X64-NEXT:    leaq (%rdi,%rdi), %rax
 ; X64-NEXT:    retq
   %res = shl i64 %arg1, 1
   ret i64 %res
@@ -61,9 +59,8 @@
 define i32 @test_shl_i32_imm1(i32 %arg1) {
 ; X64-LABEL: test_shl_i32_imm1:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movl $1, %ecx
-; X64-NEXT:    shll %cl, %eax
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    leal (%rdi,%rdi), %eax
 ; X64-NEXT:    retq
   %res = shl i32 %arg1, 1
   ret i32 %res
@@ -100,9 +97,8 @@
 define i16 @test_shl_i16_imm1(i32 %arg1) {
 ; X64-LABEL: test_shl_i16_imm1:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %edi, %eax
-; X64-NEXT:    movw $1, %cx
-; X64-NEXT:    shlw %cl, %ax
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    leal (%rdi,%rdi), %eax
 ; X64-NEXT:    # kill: def $ax killed $ax killed $eax
 ; X64-NEXT:    retq
   %a = trunc i32 %arg1 to i16