Index: llvm/trunk/lib/Target/X86/X86LegalizerInfo.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/X86LegalizerInfo.cpp
+++ llvm/trunk/lib/Target/X86/X86LegalizerInfo.cpp
@@ -163,6 +163,9 @@
   const LLT s128 = LLT::scalar(128);
 
   setAction({G_IMPLICIT_DEF, s64}, Legal);
+  // Need to have that, as tryFoldImplicitDef will create this pattern:
+  // s128 = EXTEND (G_IMPLICIT_DEF s32/s64) -> s128 = G_IMPLICIT_DEF
+  setAction({G_IMPLICIT_DEF, s128}, Legal);
 
   setAction({G_PHI, s64}, Legal);
 
Index: llvm/trunk/test/CodeGen/X86/GlobalISel/select-undef.mir
===================================================================
--- llvm/trunk/test/CodeGen/X86/GlobalISel/select-undef.mir
+++ llvm/trunk/test/CodeGen/X86/GlobalISel/select-undef.mir
@@ -11,6 +11,9 @@
     ret i8 %r
   }
 
+  define float @test3() {
+    ret float undef
+  }
 ...
 ---
 name:            test
@@ -64,3 +67,22 @@
     RET 0, implicit $al
 
 ...
+---
+name:            test3
+alignment:       4
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+registers:
+  - { id: 1, class: vecr }
+body:             |
+  bb.1 (%ir-block.0):
+    ; ALL-LABEL: name: test3
+    ; ALL: [[DEF:%[0-9]+]]:vr128 = IMPLICIT_DEF
+    ; ALL: $xmm0 = COPY [[DEF]]
+    ; ALL: RET 0, implicit $xmm0
+    %1:vecr(s128) = G_IMPLICIT_DEF
+    $xmm0 = COPY %1:vecr(s128)
+    RET 0, implicit $xmm0
+
+...