Use bit trick for portable and arm popcount

1 files changed, 18 insertions, 5 deletions

diff --git a/src/arch/portable.h b/src/arch/portable.h
index 56d3074..bd67c18 100644
--- a/src/arch/portable.h
+++ b/src/arch/portable.h
@@ -9,16 +9,29 @@
 #define SOLVED_CUBE STATIC_CUBE( \
     0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
 
-/* TODO: optimize this (use bit tricks?) */
 STATIC_INLINE int
 popcount_u32(uint32_t x)
 {
-        int ret;
+        /*
+        Bit trick: accumulate in pairs of bits, quads of bits and so on,
+        until the final result is the sum of all bits.
 
-        for (ret = 0; x != 0; x >>= 1)
-                ret += x & 1;
+        x = (x & 0x55555555) + ((x >> 1) & 0x55555555);
+        x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
+        x = (x & 0x0F0F0F0F) + ((x >> 4) & 0x0F0F0F0F);
+        x = (x & 0x00FF00FF) + ((x >> 8) & 0x00FF00FF);
+        x = (x & 0x0000FFFF) + ((x >> 16) & 0x0000FFFF);
 
-        return ret;
+        The actual method we use is a small optimization of the one above.
+        */
+
+        x -= (x >> UINT32_C(1)) & UINT32_C(0x55555555);
+        x = (x & UINT32_C(0x33333333)) +
+            ((x >> UINT32_C(2)) & UINT32_C(0x33333333));
+        x = (x + (x >> UINT32_C(4))) & UINT32_C(0x0F0F0F0F);
+        x = (x * UINT32_C(0x01010101)) >> UINT32_C(24);
+
+        return (int)x;
 }
 
 STATIC void