Revert "Speed up tail processing when hashing aligned C strings, take two"

This reverts commit a365d9e2e8c1ead27203a4431211098292777d3b.

Older versions of Valgrind raise an error, so go back to the bytewise
loop for the final word in the input.

Reported-by: Anton A. Melnikov <a.melnikov@postgrespro.ru>
Discussion: https://postgr.es/m/a3a959f6-14b8-4819-ac04-eaf2aa2e868d@postgrespro.ru
Backpatch-through: 17

diff --git a/src/include/common/hashfn_unstable.h b/src/include/common/hashfn_unstable.h
index a09c0b7d5ce556e9332a1b28a973f5fed5e55b7e..e07c0226c1f05da1fdf0980e2c769a9faddf0ed6 100644 (file)
--- a/src/include/common/hashfn_unstable.h
+++ b/src/include/common/hashfn_unstable.h
@@ -219,13 +219,6 @@ fasthash_accum(fasthash_state *hs, const char *k, size_t len)
 #define haszero64(v) \
        (((v) - 0x0101010101010101) & ~(v) & 0x8080808080808080)
 
-/* get first byte in memory order */
-#ifdef WORDS_BIGENDIAN
-#define firstbyte64(v) ((v) >> 56)
-#else
-#define firstbyte64(v) ((v) & 0xFF)
-#endif
-
 /*
  * all-purpose workhorse for fasthash_accum_cstring
  */
@@ -262,7 +255,7 @@ static inline size_t
 fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
 {
        const char *const start = str;
-       uint64          chunk;
+       size_t          remainder;
        uint64          zero_byte_low;
 
        Assert(PointerIsAligned(start, uint64));
@@ -282,7 +275,7 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
         */
        for (;;)
        {
-               chunk = *(uint64 *) str;
+               uint64          chunk = *(uint64 *) str;
 
 #ifdef WORDS_BIGENDIAN
                zero_byte_low = haszero64(pg_bswap64(chunk));
@@ -297,33 +290,14 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
                str += FH_SIZEOF_ACCUM;
        }
 
-       if (firstbyte64(chunk) != 0)
-       {
-               size_t          remainder;
-               uint64          mask;
-
-               /*
-                * The byte corresponding to the NUL will be 0x80, so the rightmost
-                * bit position will be in the range 15, 23, ..., 63. Turn this into
-                * byte position by dividing by 8.
-                */
-               remainder = pg_rightmost_one_pos64(zero_byte_low) / BITS_PER_BYTE;
-
-               /*
-                * Create a mask for the remaining bytes so we can combine them into
-                * the hash. This must have the same result as mixing the remaining
-                * bytes with fasthash_accum().
-                */
-#ifdef WORDS_BIGENDIAN
-               mask = ~UINT64CONST(0) << BITS_PER_BYTE * (FH_SIZEOF_ACCUM - remainder);
-#else
-               mask = ~UINT64CONST(0) >> BITS_PER_BYTE * (FH_SIZEOF_ACCUM - remainder);
-#endif
-               hs->accum = chunk & mask;
-               fasthash_combine(hs);
-
-               str += remainder;
-       }
+       /*
+        * The byte corresponding to the NUL will be 0x80, so the rightmost bit
+        * position will be in the range 7, 15, ..., 63. Turn this into byte
+        * position by dividing by 8.
+        */
+       remainder = pg_rightmost_one_pos64(zero_byte_low) / BITS_PER_BYTE;
+       fasthash_accum(hs, str, remainder);
+       str += remainder;
 
        return str - start;
 }