Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -1006,6 +1006,11 @@
     Observer.changedInstr(MI);
     return Legalized;
   case TargetOpcode::G_PHI: {
+    // FIXME: add support for when SizeOp0 isn't an exact multiple of
+    // NarrowSize.
+    if (SizeOp0 % NarrowSize != 0)
+      return UnableToLegalize;
+
     unsigned NumParts = SizeOp0 / NarrowSize;
     SmallVector<Register, 2> DstRegs(NumParts);
     SmallVector<SmallVector<Register, 2>, 2> SrcRegs(MI.getNumOperands() / 2);
Index: llvm/test/CodeGen/AArch64/pr48188.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/pr48188.ll
@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -O0 -mtriple=aarch64-unknown-linux-gnu < %s | FileCheck %s
+
+; GlobalISel cannot legalize this phi, so we fall back to SDAG.
+define void @test() nounwind {
+; CHECK-LABEL: test:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    sub sp, sp, #16 // =16
+; CHECK-NEXT:    mov x1, xzr
+; CHECK-NEXT:    mov x0, x1
+; CHECK-NEXT:    str x1, [sp] // 8-byte Folded Spill
+; CHECK-NEXT:    str x0, [sp, #8] // 8-byte Folded Spill
+; CHECK-NEXT:    b .LBB0_1
+; CHECK-NEXT:  .LBB0_1: // %loop
+; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    ldr x0, [sp, #8] // 8-byte Folded Reload
+; CHECK-NEXT:    ldr x1, [sp] // 8-byte Folded Reload
+; CHECK-NEXT:    str x1, [sp] // 8-byte Folded Spill
+; CHECK-NEXT:    str x0, [sp, #8] // 8-byte Folded Spill
+; CHECK-NEXT:    b .LBB0_1
+entry:
+  br label %loop
+
+loop:
+  %p = phi i72 [ 0, %entry ], [ %p, %loop ]
+  br label %loop
+}