diff --git a/clang/include/clang/Basic/BuiltinsWebAssembly.def b/clang/include/clang/Basic/BuiltinsWebAssembly.def
--- a/clang/include/clang/Basic/BuiltinsWebAssembly.def
+++ b/clang/include/clang/Basic/BuiltinsWebAssembly.def
@@ -119,10 +119,10 @@
 TARGET_BUILTIN(__builtin_wasm_all_true_i32x4, "iV4i", "nc", "simd128")
 TARGET_BUILTIN(__builtin_wasm_all_true_i64x2, "iV2LLi", "nc", "simd128")
 
-TARGET_BUILTIN(__builtin_wasm_bitmask_i8x16, "iV16Sc", "nc", "simd128")
-TARGET_BUILTIN(__builtin_wasm_bitmask_i16x8, "iV8s", "nc", "simd128")
-TARGET_BUILTIN(__builtin_wasm_bitmask_i32x4, "iV4i", "nc", "simd128")
-TARGET_BUILTIN(__builtin_wasm_bitmask_i64x2, "iV2LLi", "nc", "simd128")
+TARGET_BUILTIN(__builtin_wasm_bitmask_i8x16, "UiV16Sc", "nc", "simd128")
+TARGET_BUILTIN(__builtin_wasm_bitmask_i16x8, "UiV8s", "nc", "simd128")
+TARGET_BUILTIN(__builtin_wasm_bitmask_i32x4, "UiV4i", "nc", "simd128")
+TARGET_BUILTIN(__builtin_wasm_bitmask_i64x2, "UiV2LLi", "nc", "simd128")
 
 TARGET_BUILTIN(__builtin_wasm_abs_f32x4, "V4fV4f", "nc", "simd128")
 TARGET_BUILTIN(__builtin_wasm_abs_f64x2, "V2dV2d", "nc", "simd128")
diff --git a/clang/lib/Headers/wasm_simd128.h b/clang/lib/Headers/wasm_simd128.h
--- a/clang/lib/Headers/wasm_simd128.h
+++ b/clang/lib/Headers/wasm_simd128.h
@@ -804,7 +804,7 @@
   return __builtin_wasm_all_true_i8x16((__i8x16)__a);
 }
 
-static __inline__ int32_t __DEFAULT_FN_ATTRS wasm_i8x16_bitmask(v128_t __a) {
+static __inline__ uint32_t __DEFAULT_FN_ATTRS wasm_i8x16_bitmask(v128_t __a) {
   return __builtin_wasm_bitmask_i8x16((__i8x16)__a);
 }
 
@@ -894,7 +894,7 @@
   return __builtin_wasm_all_true_i16x8((__i16x8)__a);
 }
 
-static __inline__ int32_t __DEFAULT_FN_ATTRS wasm_i16x8_bitmask(v128_t __a) {
+static __inline__ uint32_t __DEFAULT_FN_ATTRS wasm_i16x8_bitmask(v128_t __a) {
   return __builtin_wasm_bitmask_i16x8((__i16x8)__a);
 }
 
@@ -985,7 +985,7 @@
   return __builtin_wasm_all_true_i32x4((__i32x4)__a);
 }
 
-static __inline__ int32_t __DEFAULT_FN_ATTRS wasm_i32x4_bitmask(v128_t __a) {
+static __inline__ uint32_t __DEFAULT_FN_ATTRS wasm_i32x4_bitmask(v128_t __a) {
   return __builtin_wasm_bitmask_i32x4((__i32x4)__a);
 }
 
@@ -1056,7 +1056,7 @@
   return __builtin_wasm_all_true_i64x2((__i64x2)__a);
 }
 
-static __inline__ int32_t __DEFAULT_FN_ATTRS wasm_i64x2_bitmask(v128_t __a) {
+static __inline__ uint32_t __DEFAULT_FN_ATTRS wasm_i64x2_bitmask(v128_t __a) {
   return __builtin_wasm_bitmask_i64x2((__i64x2)__a);
 }