diff --git a/js/src/builtin/SIMD.cpp b/js/src/builtin/SIMD.cpp
index 91aa10a5878..b7efc67d9f1 100644
--- a/js/src/builtin/SIMD.cpp
+++ b/js/src/builtin/SIMD.cpp
@@ -13,6 +13,7 @@
 
 #include "builtin/SIMD.h"
 
+#include "mozilla/IntegerTypeTraits.h"
 #include "jsapi.h"
 #include "jsfriendapi.h"
 
@@ -147,12 +148,17 @@ static bool SignMask(JSContext *cx, unsigned argc, Value *vp)
         return false;
     }
 
-    Elem *data = reinterpret_cast<Elem *>(typedObj.typedMem());
-    int32_t mx = data[0] < 0.0 ? 1 : 0;
-    int32_t my = data[1] < 0.0 ? 1 : 0;
-    int32_t mz = data[2] < 0.0 ? 1 : 0;
-    int32_t mw = data[3] < 0.0 ? 1 : 0;
-    int32_t result = mx | my << 1 | mz << 2 | mw << 3;
+    // Load the data as integer so that we treat the sign bit consistently,
+    // since -0.0 is not less than zero, but it still has the sign bit set.
+    typedef typename mozilla::SignedStdintTypeForSize<sizeof(Elem)>::Type Int;
+    static_assert(SimdType::lanes * sizeof(Int) <= jit::Simd128DataSize,
+                  "signMask access should respect the bounds of the type");
+    const Elem *elems = reinterpret_cast<const Elem *>(typedObj.typedMem());
+    int32_t result = 0;
+    for (unsigned i = 0; i < SimdType::lanes; ++i) {
+        Int x = mozilla::BitwiseCast<Int>(elems[i]);
+        result |= (x < 0) << i;
+    }
     args.rval().setInt32(result);
     return true;
 }
diff --git a/js/src/jit-test/tests/asm.js/testSIMD.js b/js/src/jit-test/tests/asm.js/testSIMD.js
index bbe56a78910..db8848bb8bd 100644
--- a/js/src/jit-test/tests/asm.js/testSIMD.js
+++ b/js/src/jit-test/tests/asm.js/testSIMD.js
@@ -188,7 +188,7 @@ assertEq(asmLink(asmCompile('glob', USE_ASM + I32 + 'function f() { var x=i4(0,-
 
 assertEq(asmLink(asmCompile('glob', USE_ASM + F32 + FROUND + 'var Infinity = glob.Infinity; function f() { var x=f4(0,0,0,0); x=f4(f32(1), f32(-13.37), f32(42), f32(-Infinity)); return x.signMask | 0 } return f'), this)(), 0b1010);
 assertEq(asmLink(asmCompile('glob', USE_ASM + F32 + FROUND + 'var Infinity = glob.Infinity; function f() { var x=f4(0,0,0,0); x=f4(f32(-1), f32(0), f32(-0.000001), f32(Infinity)); return x.signMask | 0 } return f'), this)(), 0b0101);
-assertEq(asmLink(asmCompile('glob', USE_ASM + F32 + FROUND + 'var NaN = glob.NaN; function f() { var x=f4(0,0,0,0); x=f4(f32(-1), f32(NaN), f32(3.), f32(4.)); return x.signMask | 0 } return f'), this)(), 0b0001);
+assertEq(asmLink(asmCompile('glob', USE_ASM + F32 + FROUND + 'var NaN = glob.NaN; function f() { var x=f4(0,0,0,0); x=f4(f32(-1), f32(NaN), f32(-0), f32(0)); return x.signMask | 0 } return f'), this)(), 0b0101);
 
 // 1.3.3. Variable assignments
 assertAsmTypeFail('glob', USE_ASM + I32 + I32A + "function f() {var x=i4(1,2,3,4); x=i4();} return f");
diff --git a/js/src/tests/ecma_7/SIMD/signmask.js b/js/src/tests/ecma_7/SIMD/signmask.js
new file mode 100644
index 00000000000..88adbc43dfb
--- /dev/null
+++ b/js/src/tests/ecma_7/SIMD/signmask.js
@@ -0,0 +1,33 @@
+// |reftest| skip-if(!this.hasOwnProperty("SIMD"))
+
+var float32x4 = SIMD.float32x4;
+var int32x4 = SIMD.int32x4;
+
+function test_float32x4() {
+  var v, w;
+  for ([v, w] of [[float32x4(-1, 20, 30, 4), 0b0001],
+                  [float32x4(9.999, 2.1234, 30.4443, -4), 0b1000],
+                  [float32x4(0, -Infinity, +Infinity, -0), 0b1010]])
+  {
+      assertEq(v.signMask, w);
+  }
+
+  if (typeof reportCompare === "function")
+    reportCompare(true, true);
+}
+
+function test_int32x4() {
+  var v, w;
+  for ([v, w] of [[int32x4(-1, 20, 30, 4), 0b0001],
+                  [int32x4(10, 2, 30.2, -4), 0b1000],
+                  [int32x4(0, 0x80000000, 0x7fffffff, -0), 0b0010]])
+  {
+      assertEq(v.signMask, w);
+  }
+
+  if (typeof reportCompare === "function")
+    reportCompare(true, true);
+}
+
+test_float32x4();
+test_int32x4();
diff --git a/mfbt/IntegerTypeTraits.h b/mfbt/IntegerTypeTraits.h
index 9414451f913..6144d2084c4 100644
--- a/mfbt/IntegerTypeTraits.h
+++ b/mfbt/IntegerTypeTraits.h
@@ -79,6 +79,11 @@ struct UnsignedStdintTypeForSize
   : detail::StdintTypeForSizeAndSignedness<Size, false>
 {};
 
+template<size_t Size>
+struct SignedStdintTypeForSize
+  : detail::StdintTypeForSizeAndSignedness<Size, true>
+{};
+
 template<typename IntegerType>
 struct PositionOfSignBit
 {