diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrConv.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrConv.td
index 4926b8fd8725..931f4a913d0f 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrConv.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrConv.td
@@ -26,6 +26,15 @@ def I64_EXTEND_U_I32 : I<(outs I64:$dst), (ins I32:$src),
                          [(set I64:$dst, (zext I32:$src))],
                          "i64.extend_u/i32\t$dst, $src">;
 
+} // defs = [ARGUMENTS]
+
+// Expand a "don't care" extend into zero-extend (chosen over sign-extend
+// somewhat arbitrarily, although it favors popular hardware architectures
+// and is conceptually a simpler operation).
+def : Pat<(i64 (anyext I32:$src)), (I64_EXTEND_U_I32 I32:$src)>;
+
+let Defs = [ARGUMENTS] in {
+
 // Conversion from floating point to integer traps on overflow and invalid.
 let hasSideEffects = 1 in {
 def I32_TRUNC_S_F32 : I<(outs I32:$dst), (ins F32:$src),
diff --git a/llvm/test/CodeGen/WebAssembly/conv.ll b/llvm/test/CodeGen/WebAssembly/conv.ll
index 2674a335a137..76fa38090a1f 100644
--- a/llvm/test/CodeGen/WebAssembly/conv.ll
+++ b/llvm/test/CodeGen/WebAssembly/conv.ll
@@ -214,3 +214,14 @@ define float @f32_demote_f64(double %x) {
   %a = fptrunc double %x to float
   ret float %a
 }
+
+; If the high its are unused, LLVM will optimize sext/zext into anyext, which
+; we need to patterm-match back to a specific instruction.
+
+; CHECK-LABEL: anyext:
+; CHECK: i64.extend_u/i32 $push0=, $0{{$}}
+define i64 @anyext(i32 %x) {
+    %y = sext i32 %x to i64
+    %w = shl i64 %y, 32
+    ret i64 %w
+}