Index: llvm/trunk/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
===================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyInstrSIMD.td
@@ -30,7 +30,7 @@
 // Constrained immediate argument types
 foreach SIZE = [8, 16] in
 def ImmI#SIZE : ImmLeaf<i32,
-  "return ((uint64_t)Imm & ((1UL << "#SIZE#") - 1)) == (uint64_t)Imm;"
+  "return -(1 << ("#SIZE#" - 1)) <= Imm && Imm < (1 << ("#SIZE#" - 1));"
 >;
 foreach SIZE = [2, 4, 8, 16, 32] in
 def LaneIdx#SIZE : ImmLeaf<i32, "return 0 <= Imm && Imm < "#SIZE#";">;
Index: llvm/trunk/test/CodeGen/WebAssembly/simd-build-vector.ll
===================================================================
--- llvm/trunk/test/CodeGen/WebAssembly/simd-build-vector.ll
+++ llvm/trunk/test/CodeGen/WebAssembly/simd-build-vector.ll
@@ -23,12 +23,12 @@
 
 ; CHECK-LABEL: different_const_one_replaced_i8x16:
 ; CHECK-NEXT:  .functype       different_const_one_replaced_i8x16 (i32) -> (v128)
-; CHECK-NEXT:  v128.const      $push[[L0:[0-9]+]]=, 1, 2, 3, 4, 5, 0, 7, 8
+; CHECK-NEXT:  v128.const      $push[[L0:[0-9]+]]=, 1, -2, 3, -4, 5, 0, 7, -8
 ; CHECK-NEXT:  i16x8.replace_lane      $push[[L1:[0-9]+]]=, $pop[[L0]], 5, $0
 ; CHECK-NEXT:  return          $pop[[L1]]
 define <8 x i16> @different_const_one_replaced_i8x16(i16 %x) {
   %v = insertelement
-    <8 x i16> <i16 1, i16 2, i16 3, i16 4, i16 5, i16 6, i16 7, i16 8>,
+    <8 x i16> <i16 1, i16 -2, i16 3, i16 -4, i16 5, i16 -6, i16 7, i16 -8>,
     i16 %x,
     i32 5
   ret <8 x i16> %v