diff --git a/js/src/jit/RegisterSets.h b/js/src/jit/RegisterSets.h
index 2a5e16b792e0..65ba34459363 100644
--- a/js/src/jit/RegisterSets.h
+++ b/js/src/jit/RegisterSets.h
@@ -449,11 +449,7 @@ class TypedRegisterSet
         return bits_;
     }
     uint32_t size() const {
-        uint32_t sum2  = (bits_ & 0x55555555) + ((bits_ & 0xaaaaaaaa) >> 1);
-        uint32_t sum4  = (sum2  & 0x33333333) + ((sum2  & 0xcccccccc) >> 2);
-        uint32_t sum8  = (sum4  & 0x0f0f0f0f) + ((sum4  & 0xf0f0f0f0) >> 4);
-        uint32_t sum16 = (sum8  & 0x00ff00ff) + ((sum8  & 0xff00ff00) >> 8);
-        return sum16;
+        return mozilla::CountPopulation32(bits_);
     }
     bool operator ==(const TypedRegisterSet<T> &other) const {
         return other.bits_ == bits_;
diff --git a/mfbt/MathAlgorithms.h b/mfbt/MathAlgorithms.h
index 6d58691e0612..941ac812718d 100644
--- a/mfbt/MathAlgorithms.h
+++ b/mfbt/MathAlgorithms.h
@@ -186,6 +186,16 @@ namespace detail {
     return uint_fast8_t(index);
   }
 
+  inline uint_fast8_t
+  CountPopulation32(uint32_t u)
+  {
+     uint32_t sum2  = (u     & 0x55555555) + ((u     & 0xaaaaaaaa) >> 1);
+     uint32_t sum4  = (sum2  & 0x33333333) + ((sum2  & 0xcccccccc) >> 2);
+     uint32_t sum8  = (sum4  & 0x0f0f0f0f) + ((sum4  & 0xf0f0f0f0) >> 4);
+     uint32_t sum16 = (sum8  & 0x00ff00ff) + ((sum8  & 0xff00ff00) >> 8);
+     return sum16;
+  }
+
   inline uint_fast8_t
   CountLeadingZeroes64(uint64_t u)
   {
@@ -242,6 +252,12 @@ namespace detail {
     return __builtin_ctz(u);
   }
 
+  inline uint_fast8_t
+  CountPopulation32(uint32_t u)
+  {
+    return __builtin_popcount(u);
+  }
+
   inline uint_fast8_t
   CountLeadingZeroes64(uint64_t u)
   {
@@ -258,6 +274,7 @@ namespace detail {
 #  error "Implement these!"
   inline uint_fast8_t CountLeadingZeroes32(uint32_t u) MOZ_DELETE;
   inline uint_fast8_t CountTrailingZeroes32(uint32_t u) MOZ_DELETE;
+  inline uint_fast8_t CountPopulation32(uint32_t u) MOZ_DELETE;
   inline uint_fast8_t CountLeadingZeroes64(uint64_t u) MOZ_DELETE;
   inline uint_fast8_t CountTrailingZeroes64(uint64_t u) MOZ_DELETE;
 #endif
@@ -300,6 +317,15 @@ CountTrailingZeroes32(uint32_t u)
   return detail::CountTrailingZeroes32(u);
 }
 
+/**
+ * Compute the number of one bits in the number |u|,
+ */
+inline uint_fast8_t
+CountPopulation32(uint32_t u)
+{
+  return detail::CountPopulation32(u);
+}
+
 /** Analogous to CountLeadingZeroes32, but for 64-bit numbers. */
 inline uint_fast8_t
 CountLeadingZeroes64(uint64_t u)
diff --git a/mfbt/tests/TestCountPopulation.cpp b/mfbt/tests/TestCountPopulation.cpp
new file mode 100644
index 000000000000..e02cb1384538
--- /dev/null
+++ b/mfbt/tests/TestCountPopulation.cpp
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/MathAlgorithms.h"
+
+using mozilla::CountPopulation32;
+
+static void
+TestCountPopulation32()
+{
+  MOZ_ASSERT(CountPopulation32(0xFFFFFFFF) == 32);
+  MOZ_ASSERT(CountPopulation32(0xF0FF1000) == 13);
+  MOZ_ASSERT(CountPopulation32(0x7F8F0001) == 13);
+  MOZ_ASSERT(CountPopulation32(0x3FFF0100) == 15);
+  MOZ_ASSERT(CountPopulation32(0x1FF50010) == 12);
+  MOZ_ASSERT(CountPopulation32(0x00800000) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00400000) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00008000) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00004000) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00000080) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00000040) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00000001) == 1);
+  MOZ_ASSERT(CountPopulation32(0x00000000) == 0);
+}
+
+int main()
+{
+  TestCountPopulation32();
+  return 0;
+}