Index: lib/CodeGen/MachineVerifier.cpp
===================================================================
--- lib/CodeGen/MachineVerifier.cpp
+++ lib/CodeGen/MachineVerifier.cpp
@@ -49,6 +49,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
+#include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 using namespace llvm;
 
 namespace {
@@ -943,6 +944,21 @@
              MI);
     break;
   }
+  case TargetOpcode::COPY: {
+    const MachineOperand &DstOp = MI->getOperand(0);
+    const MachineOperand &SrcOp = MI->getOperand(1);
+    unsigned DstSize =
+        RegisterBankInfo::getSizeInBits(DstOp.getReg(), *MRI, *TRI);
+    unsigned SrcSize =
+        RegisterBankInfo::getSizeInBits(SrcOp.getReg(), *MRI, *TRI);
+    if (DstSize != SrcSize)
+      // Catch only obvious cases not involving subregs for now.
+      if (!DstOp.getSubReg() && !SrcOp.getSubReg()) {
+        report("Copy Instruction is illegal with mismatching sizes", MI);
+        errs() << "Def Size = " << DstSize << ", Src Size = " << SrcSize << "\n";
+      }
+    break;
+  }
   case TargetOpcode::STATEPOINT:
     if (!MI->getOperand(StatepointOpers::IDPos).isImm() ||
         !MI->getOperand(StatepointOpers::NBytesPos).isImm() ||
Index: test/Verifier/test_copy.mir
===================================================================
--- /dev/null
+++ test/Verifier/test_copy.mir
@@ -0,0 +1,30 @@
+#RUN: not llc -o - -global-isel -run-pass=none -verify-machineinstrs %s 2>&1 | FileCheck %s
+# REQUIRES: global-isel, aarch64-registered-target
+--- |
+  ; ModuleID = 'test.ll'
+  source_filename = "test.ll"
+  target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64-unknown-unknown"
+  
+  define i32 @test_copy(i32 %argc) {
+    ret i32 0
+  }
+
+...
+---
+name:            test_copy
+legalized:       true
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+registers:       
+  - { id: 0, class: _, preferred-register: '' }
+liveins:         
+body:             |
+  bb.0:
+    liveins: %w0
+    ; This test is used to catch verifier errors with copys having mismatching sizes
+    ; CHECK: Bad machine code: Copy Instruction is illegal with mismatching sizes
+  
+    %0(s8) = COPY %w0
+...
Index: unittests/CodeGen/CMakeLists.txt
===================================================================
--- unittests/CodeGen/CMakeLists.txt
+++ unittests/CodeGen/CMakeLists.txt
@@ -2,6 +2,7 @@
   AsmPrinter
   CodeGen
   Core
+  GlobalISel
   Support
   )
 
Index: unittests/IR/CMakeLists.txt
===================================================================
--- unittests/IR/CMakeLists.txt
+++ unittests/IR/CMakeLists.txt
@@ -2,6 +2,7 @@
   Analysis
   AsmParser
   Core
+  GlobalISel
   Support
   Passes
   )