Index: llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
===================================================================
--- llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -2811,13 +2811,10 @@
       "G_SHUFFLE_VECTOR should have a constant mask operand as G_BUILD_VECTOR");
   // Find the constant indices.
   for (unsigned i = 1, e = MaskDef->getNumOperands(); i < e; ++i) {
-    MachineInstr *ScalarDef = MRI.getVRegDef(MaskDef->getOperand(i).getReg());
-    assert(ScalarDef && "Could not find vreg def of shufflevec index op");
     // Look through copies.
-    while (ScalarDef->getOpcode() == TargetOpcode::COPY) {
-      ScalarDef = MRI.getVRegDef(ScalarDef->getOperand(1).getReg());
-      assert(ScalarDef && "Could not find def of copy operand");
-    }
+    MachineInstr *ScalarDef =
+        getDefIgnoringCopies(MaskDef->getOperand(i).getReg(), MRI);
+    assert(ScalarDef && "Could not find vreg def of shufflevec index op");
     if (ScalarDef->getOpcode() != TargetOpcode::G_CONSTANT) {
       // This be an undef if not a constant.
       assert(ScalarDef->getOpcode() == TargetOpcode::G_IMPLICIT_DEF);
@@ -3229,20 +3226,6 @@
   //
   // cmn z, y
 
-  // Helper lambda to find the def.
-  auto FindDef = [&](Register VReg) {
-    MachineInstr *Def = MRI.getVRegDef(VReg);
-    while (Def) {
-      if (Def->getOpcode() != TargetOpcode::COPY)
-        break;
-      // Copies can be from physical registers. If we hit this, we're done.
-      if (TargetRegisterInfo::isPhysicalRegister(Def->getOperand(1).getReg()))
-        break;
-      Def = MRI.getVRegDef(Def->getOperand(1).getReg());
-    }
-    return Def;
-  };
-
   // Helper lambda to detect the subtract followed by the compare.
   // Takes in the def of the LHS or RHS, and checks if it's a subtract from 0.
   auto IsCMN = [&](MachineInstr *DefMI, const AArch64CC::CondCode &CC) {
@@ -3269,8 +3252,8 @@
   };
 
   // Check if the RHS or LHS of the G_ICMP is defined by a SUB
-  MachineInstr *LHSDef = FindDef(LHS.getReg());
-  MachineInstr *RHSDef = FindDef(RHS.getReg());
+  MachineInstr *LHSDef = getDefIgnoringCopies(LHS.getReg(), MRI);
+  MachineInstr *RHSDef = getDefIgnoringCopies(RHS.getReg(), MRI);
   CmpInst::Predicate P = (CmpInst::Predicate)Predicate.getPredicate();
   const AArch64CC::CondCode CC = changeICMPPredToAArch64CC(P);
 
Index: llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-compare.mir
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-compare.mir
+++ llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-compare.mir
@@ -478,3 +478,30 @@
     %6:gpr(s32) = G_ICMP intpred(eq), %5(s32), %2
     $w0 = COPY %6(s32)
     RET_ReallyLR implicit $w0
+
+...
+---
+name:            test_physreg_copy
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0, $x1
+    ; CHECK-LABEL: name: test_physreg_copy
+    ; CHECK: liveins: $x0, $x1
+    ; CHECK: [[COPY:%[0-9]+]]:gpr64 = COPY $x0
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr64 = COPY $x1
+    ; CHECK: $xzr = SUBSXrr [[COPY]], [[COPY1]], implicit-def $nzcv
+    ; CHECK: [[CSINCWr:%[0-9]+]]:gpr32 = CSINCWr $wzr, $wzr, 1, implicit $nzcv
+    ; CHECK: $w0 = COPY [[CSINCWr]]
+    ; CHECK: RET_ReallyLR implicit $x0
+    %0:gpr(s64) = COPY $x0
+    %1:gpr(s64) = COPY $x1
+    ; When we find the defs of the LHS and RHS of the compare, we walk over
+    ; copies. Make sure that we don't crash when we hit a copy from a physical
+    ; register.
+    %7:gpr(s32) = G_ICMP intpred(eq), %0, %1
+    $w0 = COPY %7(s32)
+    RET_ReallyLR implicit $x0