Index: lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
===================================================================
--- lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -118,6 +118,11 @@
     ConstantFP *Val = cast<ConstantFP>(Constant::getNullValue(
         Type::getDoubleTy(MF.getFunction().getContext())));
     MI->addOperand(MachineOperand::CreateFPImm(Val));
+  } else if (RegClass == &WebAssembly::V128RegClass) {
+    // TODO: make splat instead of constant
+    MI->setDesc(TII->get(WebAssembly::CONST_V128_v16i8));
+    for (int I = 0; I < 16; ++I)
+      MI->addOperand(MachineOperand::CreateImm(0));
   } else {
     llvm_unreachable("Unexpected reg class");
   }
Index: test/CodeGen/WebAssembly/implicit-def.ll
===================================================================
--- test/CodeGen/WebAssembly/implicit-def.ll
+++ test/CodeGen/WebAssembly/implicit-def.ll
@@ -1,50 +1,133 @@
-; RUN: llc -o - %s -asm-verbose=false -wasm-keep-registers | FileCheck %s
+; RUN: llc -o - %s -asm-verbose=false -wasm-keep-registers -disable-wasm-fallthrough-return-opt -mattr=+simd128 | FileCheck %s
+
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
 ; Test that stackified IMPLICIT_DEF instructions are converted into
-; CONST_I32 to provide an explicit push.
-
-; CHECK:      br_if 2,
-; CHECK:      i32.const $push[[L0:[0-9]+]]=, 0{{$}}
-; CHECK-NEXT: return $pop[[L0]]{{$}}
-define i1 @f() {
-  %a = xor i1 0, 0
-  switch i1 %a, label %C [
-    i1 0, label %A
-    i1 1, label %B
-  ]
-
-A:
-  %b = xor i1 0, 0
+; CONST_XXX instructions to provide an explicit push.
+
+; CHECK-LABEL: implicit_def_i32:
+; CHECK: .LBB{{[0-9]+}}_4:{{$}}
+; CHECK-NEXT: end_block{{$}}
+; CHECK-NEXT: i32.const $push[[R:[0-9]+]]=, 0{{$}}
+; CHECK-NEXT: return $pop[[R]]{{$}}
+; CHECK-NEXT: end_function{{$}}
+define i32 @implicit_def_i32() {
+  br i1 undef, label %A, label %X
+
+A:                                                ; preds = %0
+  %d = icmp slt i1 0, 0
+  br i1 %d, label %C, label %B
+
+B:                                                ; preds = %A
+  br label %C
+
+C:                                                ; preds = %B, %A
+  %h = phi i32 [ undef, %A ], [ 0, %B ]
+  br label %X
+
+X:                                                ; preds = %0, C
+  %i = phi i32 [ 1, %0 ], [ %h, %C ]
+  ret i32 %i
+}
+
+; CHECK-LABEL: implicit_def_i64:
+; CHECK: .LBB{{[0-9]+}}_4:{{$}}
+; CHECK-NEXT: end_block{{$}}
+; CHECK-NEXT: i64.const $push[[R:[0-9]+]]=, 0{{$}}
+; CHECK-NEXT: return $pop[[R]]{{$}}
+; CHECK-NEXT: end_function{{$}}
+define i64 @implicit_def_i64() {
+  br i1 undef, label %A, label %X
+
+A:                                                ; preds = %0
+  %d = icmp slt i1 0, 0
+  br i1 %d, label %C, label %B
+
+B:                                                ; preds = %A
+  br label %C
+
+C:                                                ; preds = %B, %A
+  %h = phi i64 [ undef, %A ], [ 0, %B ]
   br label %X
 
-B:
-  %c = xor i1 0, 0
-  br i1 %c, label %D, label %X
+X:                                                ; preds = %0, C
+  %i = phi i64 [ 1, %0 ], [ %h, %C ]
+  ret i64 %i
+}
+
+; CHECK-LABEL: implicit_def_f32:
+; CHECK: .LBB{{[0-9]+}}_4:{{$}}
+; CHECK-NEXT: end_block{{$}}
+; CHECK-NEXT: f32.const $push[[R:[0-9]+]]=, 0x0p0{{$}}
+; CHECK-NEXT: return $pop[[R]]{{$}}
+; CHECK-NEXT: end_function{{$}}
+define float @implicit_def_f32() {
+  br i1 undef, label %A, label %X
 
-C:
-  %d = icmp slt i32 0, 0
-  br i1 %d, label %G, label %F
+A:                                                ; preds = %0
+  %d = icmp slt i1 0, 0
+  br i1 %d, label %C, label %B
 
-D:
-  %e = xor i1 0, 0
-  br i1 %e, label %E, label %X
+B:                                                ; preds = %A
+  br label %C
 
-E:
-  %f = xor i1 0, 0
+C:                                                ; preds = %B, %A
+  %h = phi float [ undef, %A ], [ 0.0, %B ]
   br label %X
 
-F:
-  %g = xor i1 0, 0
-  br label %G
+X:                                                ; preds = %0, C
+  %i = phi float [ 1.0, %0 ], [ %h, %C ]
+  ret float %i
+}
+
+; CHECK-LABEL: implicit_def_f64:
+; CHECK: .LBB{{[0-9]+}}_4:{{$}}
+; CHECK-NEXT: end_block{{$}}
+; CHECK-NEXT: f64.const $push[[R:[0-9]+]]=, 0x0p0{{$}}
+; CHECK-NEXT: return $pop[[R]]{{$}}
+; CHECK-NEXT: end_function{{$}}
+define double @implicit_def_f64() {
+  br i1 undef, label %A, label %X
+
+A:                                                ; preds = %0
+  %d = icmp slt i1 0, 0
+  br i1 %d, label %C, label %B
+
+B:                                                ; preds = %A
+  br label %C
 
-G:
-  %h = phi i1 [ undef, %C ], [ false, %F ]
+C:                                                ; preds = %B, %A
+  %h = phi double [ undef, %A ], [ 0.0, %B ]
   br label %X
 
-X:
-  %i = phi i1 [ true, %A ], [ true, %B ], [ true, %D ], [ true, %E ], [ %h, %G ]
-  ret i1 %i
+X:                                                ; preds = %0, C
+  %i = phi double [ 1.0, %0 ], [ %h, %C ]
+  ret double %i
 }
 
+; CHECK-LABEL: implicit_def_v4i32:
+; CHECK: .LBB{{[0-9]+}}_4:{{$}}
+; CHECK-NEXT: end_block{{$}}
+; CHECK-NEXT: v128.const $push[[R:[0-9]+]]=, 0, 0, 0, 0, 0, 0, 0, 0,
+; CHECK-SAME:                                0, 0, 0, 0, 0, 0, 0, 0{{$}}
+; CHECK-NEXT: return $pop[[R]]{{$}}
+; CHECK-NEXT: end_function{{$}}
+define <4 x i32> @implicit_def_v4i32() {
+  br i1 undef, label %A, label %X
+
+A:                                                ; preds = %0
+  %d = icmp slt i1 0, 0
+  br i1 %d, label %C, label %B
+
+B:                                                ; preds = %A
+  br label %C
+
+C:                                                ; preds = %B, %A
+  %h = phi <4 x i32> [ undef, %A ], [ <i32 0, i32 0, i32 0, i32 0>, %B ]
+  br label %X
+
+X:                                                ; preds = %0, C
+  %i = phi <4 x i32> [ <i32 1, i32 1, i32 1, i32 1>, %0 ], [ %h, %C ]
+  ret <4 x i32> %i
+}