Index: llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -3777,13 +3777,11 @@
   else if (Root.isCImm())
     Immed = Root.getCImm()->getZExtValue();
   else if (Root.isReg()) {
-    MachineInstr *Def = MRI.getVRegDef(Root.getReg());
-    if (Def->getOpcode() != TargetOpcode::G_CONSTANT)
-      return None;
-    MachineOperand &Op1 = Def->getOperand(1);
-    if (!Op1.isCImm() || Op1.getCImm()->getBitWidth() > 64)
+    auto ValAndVReg =
+        getConstantVRegValWithLookThrough(Root.getReg(), MRI, true);
+    if (!ValAndVReg)
       return None;
-    Immed = Op1.getCImm()->getZExtValue();
+    Immed = ValAndVReg->Value;
   } else
     return None;
   return Immed;
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-cmp.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-cmp.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-cmp.mir
@@ -70,3 +70,47 @@
     RET_ReallyLR implicit $w0
 
 ...
+---
+name:            cmp_imm_lookthrough
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $w0
+    ; CHECK-LABEL: name: cmp_imm_lookthrough
+    ; CHECK: liveins: $w0
+    ; CHECK: [[COPY:%[0-9]+]]:gpr32sp = COPY $w0
+    ; CHECK: $wzr = SUBSWri [[COPY]], 42, 0, implicit-def $nzcv
+    ; CHECK: [[CSINCWr:%[0-9]+]]:gpr32 = CSINCWr $wzr, $wzr, 1, implicit $nzcv
+    ; CHECK: $w0 = COPY [[CSINCWr]]
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:gpr(s32) = COPY $w0
+    %1:gpr(s64) = G_CONSTANT i64 42
+    %2:gpr(s32) = G_TRUNC %1(s64)
+    %5:gpr(s32) = G_ICMP intpred(eq), %0(s32), %2
+    $w0 = COPY %5(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            cmp_imm_lookthrough_bad_trunc
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $w0
+    ; CHECK-LABEL: name: cmp_imm_lookthrough_bad_trunc
+    ; CHECK: liveins: $w0
+    ; CHECK: [[COPY:%[0-9]+]]:gpr32sp = COPY $w0
+    ; CHECK: $wzr = SUBSWri [[COPY]], 0, 0, implicit-def $nzcv
+    ; CHECK: [[CSINCWr:%[0-9]+]]:gpr32 = CSINCWr $wzr, $wzr, 1, implicit $nzcv
+    ; CHECK: $w0 = COPY [[CSINCWr]]
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:gpr(s32) = COPY $w0
+    %1:gpr(s64) = G_CONSTANT i64 68719476736 ; 0x1000000000
+    %2:gpr(s32) = G_TRUNC %1(s64) ; Value truncates to 0
+    %5:gpr(s32) = G_ICMP intpred(eq), %0(s32), %2
+    $w0 = COPY %5(s32)
+    RET_ReallyLR implicit $w0
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-jump-table-brjt.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-jump-table-brjt.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-jump-table-brjt.mir
@@ -57,19 +57,16 @@
   ; CHECK:   successors: %bb.4(0x40000000), %bb.1(0x40000000)
   ; CHECK:   liveins: $w0
   ; CHECK:   [[COPY:%[0-9]+]]:gpr32common = COPY $w0
-  ; CHECK:   [[MOVi32imm:%[0-9]+]]:gpr32 = MOVi32imm 71
-  ; CHECK:   [[MOVi32imm1:%[0-9]+]]:gpr32 = MOVi32imm 0
+  ; CHECK:   [[MOVi32imm:%[0-9]+]]:gpr32 = MOVi32imm 0
   ; CHECK:   [[SUBSWri:%[0-9]+]]:gpr32 = SUBSWri [[COPY]], 4, 0, implicit-def $nzcv
   ; CHECK:   [[SUBREG_TO_REG:%[0-9]+]]:gpr64 = SUBREG_TO_REG 0, [[SUBSWri]], %subreg.sub_32
-  ; CHECK:   [[UBFMXri:%[0-9]+]]:gpr64 = UBFMXri [[SUBREG_TO_REG]], 0, 31
-  ; CHECK:   [[SUBREG_TO_REG1:%[0-9]+]]:gpr64 = SUBREG_TO_REG 0, [[MOVi32imm]], %subreg.sub_32
-  ; CHECK:   [[UBFMXri1:%[0-9]+]]:gpr64 = UBFMXri [[SUBREG_TO_REG1]], 0, 31
-  ; CHECK:   $xzr = SUBSXrr [[UBFMXri]], [[UBFMXri1]], implicit-def $nzcv
+  ; CHECK:   [[UBFMXri:%[0-9]+]]:gpr64common = UBFMXri [[SUBREG_TO_REG]], 0, 31
+  ; CHECK:   $xzr = SUBSXri [[UBFMXri]], 71, 0, implicit-def $nzcv
   ; CHECK:   [[CSINCWr:%[0-9]+]]:gpr32 = CSINCWr $wzr, $wzr, 9, implicit $nzcv
   ; CHECK:   TBNZW [[CSINCWr]], 0, %bb.4
   ; CHECK: bb.1.entry:
   ; CHECK:   successors: %bb.3(0x2aaaaaab), %bb.4(0x2aaaaaab), %bb.2(0x2aaaaaab)
-  ; CHECK:   [[MOVi32imm2:%[0-9]+]]:gpr32 = MOVi32imm 0
+  ; CHECK:   [[MOVi32imm1:%[0-9]+]]:gpr32 = MOVi32imm 0
   ; CHECK:   [[MOVaddrJT:%[0-9]+]]:gpr64 = MOVaddrJT target-flags(aarch64-page) %jump-table.0, target-flags(aarch64-pageoff, aarch64-nc) %jump-table.0
   ; CHECK:   early-clobber %18:gpr64, early-clobber %19:gpr64sp = JumpTableDest32 [[MOVaddrJT]], [[UBFMXri]], %jump-table.0
   ; CHECK:   BR %18
@@ -79,10 +76,10 @@
   ; CHECK:   B %bb.4
   ; CHECK: bb.3.sw.bb1:
   ; CHECK:   successors: %bb.4(0x80000000)
-  ; CHECK:   [[MOVi32imm3:%[0-9]+]]:gpr32 = MOVi32imm 3
-  ; CHECK:   [[MADDWrrr:%[0-9]+]]:gpr32 = MADDWrrr [[COPY]], [[MOVi32imm3]], $wzr
+  ; CHECK:   [[MOVi32imm2:%[0-9]+]]:gpr32 = MOVi32imm 3
+  ; CHECK:   [[MADDWrrr:%[0-9]+]]:gpr32 = MADDWrrr [[COPY]], [[MOVi32imm2]], $wzr
   ; CHECK: bb.4.return:
-  ; CHECK:   [[PHI:%[0-9]+]]:gpr32 = PHI [[MADDWrrr]], %bb.3, [[ADDWri]], %bb.2, [[MOVi32imm1]], %bb.0, [[MOVi32imm2]], %bb.1
+  ; CHECK:   [[PHI:%[0-9]+]]:gpr32 = PHI [[MADDWrrr]], %bb.3, [[ADDWri]], %bb.2, [[MOVi32imm]], %bb.0, [[MOVi32imm1]], %bb.1
   ; CHECK:   $w0 = COPY [[PHI]]
   ; CHECK:   RET_ReallyLR implicit $w0
   bb.1.entry: