Index: llvm/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
===================================================================
--- llvm/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
+++ llvm/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
@@ -230,6 +230,19 @@
 defm "" : Splat<v4f32, "f32x4", F32, splat4, 18>;
 defm "" : Splat<v2f64, "f64x2", F64, splat2, 21>;
 
+// scalar_to_vector leaves high lanes undefined, so can be a splat
+class ScalarSplatPat<ValueType vec_t, ValueType lane_t,
+                     WebAssemblyRegClass reg_t> :
+  Pat<(vec_t (scalar_to_vector (lane_t reg_t:$x))),
+      (!cast<Instruction>("SPLAT_"#vec_t) reg_t:$x)>;
+
+def : ScalarSplatPat<v16i8, i32, I32>;
+def : ScalarSplatPat<v8i16, i32, I32>;
+def : ScalarSplatPat<v4i32, i32, I32>;
+def : ScalarSplatPat<v2i64, i64, I64>;
+def : ScalarSplatPat<v4f32, f32, F32>;
+def : ScalarSplatPat<v2f64, f64, F64>;
+
 //===----------------------------------------------------------------------===//
 // Accessing lanes
 //===----------------------------------------------------------------------===//
Index: llvm/test/CodeGen/WebAssembly/simd-scalar-to-vector.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/WebAssembly/simd-scalar-to-vector.ll
@@ -0,0 +1,27 @@
+; RUN: llc < %s -asm-verbose=false -verify-machineinstrs -disable-wasm-fallthrough-return-opt -wasm-disable-explicit-locals -wasm-keep-registers -mattr=+simd128 | FileCheck %s
+
+; Test that scalar_to_vector is lowered into a splat correctly.
+; TODO: find small test cases that produce scalar_to_vector dag nodes
+; to make this test more readable and comprehensive.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+; CHECK-LABEL: foo:
+; CHECK: i32x4.splat
+define void @foo() {
+entry:
+  %a = load <2 x i16>, <2 x i16>* undef, align 1
+  %b = shufflevector <2 x i16> %a, <2 x i16> undef, <8 x i32> <i32 0, i32 1, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef>
+  %0 = bitcast <8 x i16> %b to <16 x i8>
+  %shuffle.i214 = shufflevector <16 x i8> %0, <16 x i8> <i8 0, i8 0, i8 0, i8 0, i8 0, i8 0, i8 0, i8 0, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef>, <16 x i32> <i32 0, i32 16, i32 1, i32 17, i32 2, i32 18, i32 3, i32 19, i32 4, i32 20, i32 5, i32 21, i32 6, i32 22, i32 7, i32 23>
+  %1 = bitcast <16 x i8> %shuffle.i214 to <8 x i16>
+  %add82 = add <8 x i16> %1, zeroinitializer
+  %2 = select <8 x i1> undef, <8 x i16> undef, <8 x i16> %add82
+  %3 = bitcast <8 x i16> %2 to <16 x i8>
+  %shuffle.i204 = shufflevector <16 x i8> %3, <16 x i8> undef, <16 x i32> <i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14, i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>
+  %4 = bitcast <16 x i8> %shuffle.i204 to <8 x i16>
+  %dst2.0.vec.extract = shufflevector <8 x i16> %4, <8 x i16> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+  store <4 x i16> %dst2.0.vec.extract, <4 x i16>* undef, align 1
+  ret void
+}