Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -938,6 +938,12 @@
     Observer.changedInstr(MI);
     return Legalized;
   }
+  case TargetOpcode::G_IMPLICIT_DEF: {
+    Observer.changingInstr(MI);
+    widenScalarDst(MI, WideTy);
+    Observer.changedInstr(MI);
+    return Legalized;
+  }
   case TargetOpcode::G_BRCOND:
     Observer.changingInstr(MI);
     widenScalarSrc(MI, WideTy, 0, TargetOpcode::G_ANYEXT);
Index: test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
===================================================================
--- test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
+++ test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
@@ -1,5 +1,5 @@
 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
-# RUN: llc -march=aarch64 -run-pass=legalizer %s -o - | FileCheck %s
+# RUN: llc -march=aarch64 -run-pass=legalizer -O0 %s -o - | FileCheck %s
 ---
 name:            test_implicit_def
 body: |
@@ -16,3 +16,21 @@
     %1:_(s64) = G_TRUNC %0(s128)
     $x0 = COPY %1(s64)
 ...
+
+---
+name: test_implicit_def_s3
+body: |
+  bb.0:
+    liveins:
+
+    ; CHECK-LABEL: name: test_implicit_def_s3
+    ; CHECK: [[DEF:%[0-9]+]]:_(s8) = G_IMPLICIT_DEF
+    ; CHECK: [[TRUNC:%[0-9]+]]:_(s3) = G_TRUNC [[DEF]](s8)
+    ; CHECK: [[SEXT:%[0-9]+]]:_(s64) = G_SEXT [[TRUNC]](s3)
+    ; CHECK: $x0 = COPY [[SEXT]](s64)
+    %0:_(s3) = G_IMPLICIT_DEF
+    %1:_(s64) = G_SEXT %0
+    $x0 = COPY %1(s64)
+...
+
+# FIXME: s2 not correctly handled