Bug 978397 - Fix the implementation of CountPopulation_32 on Windows. r=nbp

mfbt/MathAlgorithms.h

@@ -189,11 +189,9 @@ namespace detail {
   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;
+    uint32_t x = u - ((u >> 1) & 0x55555555);
+    x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
+    return (((x + (x >> 4)) & 0xf0f0f0f) * 0x1010101) >> 24;
   }
 
   inline uint_fast8_t

mfbt/tests/moz.build

@@ -10,6 +10,7 @@ CPP_UNIT_TESTS += [
     'TestCasting.cpp',
     'TestCeilingFloor.cpp',
     'TestCheckedInt.cpp',
+    'TestCountPopulation.cpp',
     'TestCountZeroes.cpp',
     'TestEndian.cpp',
     'TestEnumSet.cpp',