MINOR: tools: improve ptr hash distribution on 64 bits

When testing the pointer hash on 64-bit real pointers (map entries),
it appeared that the shift by 33 bits that hoped to compensate for the
3 nul LSB degrades the hash, and the centering is more optimal on
31-(bits+1)/2. This makes sense since the topmost bit of the
multiplicator is 31, so for an input of 1 bit and 1 bit of output we
would always get zero. With the formula adjusted this way, we can get
up to ~15% more unique entries at 10 bits and ~24% more at 11 bits.

diff --git a/include/haproxy/tools.h b/include/haproxy/tools.h
index cea68c868d3180de1c2ec07d7e80268a1bfc37fe..e0a91f0c4f6711545cfed50601dbf49d028aefeb 100644 (file)
--- a/include/haproxy/tools.h
+++ b/include/haproxy/tools.h
@@ -1085,12 +1085,13 @@ static inline uint statistical_prng_range(uint range)
  * to compute statistic buckets, in that it's fast and reasonably distributed
  * thanks to mixing the bits via a multiplication by a prime number and using
  * the middle bits on 64-bit platforms or remixing the topmost with lowest ones
- * on 32-bit. It provides ~2588 unique values (~1510 non-colliding) at 100%
- * fill ratio for 12 bits, ~1296 (~756 non-colliding) at 100% fill ratio for 11
- * bits, ~648 (~378 non-colliding) at 100% fill ratio for 10 bits, ~163 (95 non
- * colliding) at 100% fill ratio for 8 bits, hence 1-1/e and 1/e respectively.
- * It must be inlined so that <bits> is always a compile-time constant. It
- * supports output sizes from 0 to 32 bits.
+ * on 32-bit. It provides ~2491 unique values (~1354 non-colliding) for 2^12
+ * valid pointers at 12 bits, ~1454 (~941 non-colliding) for 2^11 valid ptrs
+ * at 11 bits, ~707 (~434 non-colliding) for 2^10 valid ptrs at 10 bits, ~347
+ * (212 non colliding) for 2^9 valid ptrs at 9 bits, and 165/99 for 2^8 ptrs
+ * at 8 bits, hence 1-1/e and 1/e respectively. It must be inlined so that
+ * <bits> is always a compile-time constant. It supports output sizes from 0
+ * to 32 bits.
  */
 static forceinline uint ptr_hash(const void *p, const int bits)
 {
@@ -1103,7 +1104,7 @@ static forceinline uint ptr_hash(const void *p, const int bits)
        if (sizeof(long) == 4)
                x ^= x >> 32;
        else
-               x >>= 33 - bits / 2;
+               x >>= 31 - (bits + 1) / 2;
        return x & (~0U >> (-bits & 31));
 }