Re: [PATCH v3 04/18] Add string vectorized find and detection functions

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 01/10/2018 04:47 AM, Adhemerval Zanella wrote:
> +  op_t lsb = (op_t)-1 / 0xff;
> +  op_t msb = lsb << (CHAR_BIT - 1);
This would be simpler and clearer if it were rewritten as:

    opt_t lsb = repeat_bytes (0x01);
    opt_t msb = repeat_bytes (0x80);

There are several other opportunities for this kind of simplification.

> +static inline op_t
> +find_zero_eq_low (op_t x1, op_t x2)
> +{
> +  op_t lsb = (op_t)-1 / 0xff;
> +  op_t msb = lsb << (CHAR_BIT - 1);
> +  op_t eq = x1 ^ x2;
> +  return (((x1 - lsb) & ~x1) | ((eq - lsb) & ~eq)) & msb;
> +}

How about the following simpler implementation instead? I expect it's 
just as fast:

   return find_zero_low (x1) | find_zero_low (x1 ^ x2);

Similarly for find_zero_eq_all, find_zero_ne_low, find_zero_ne_all.

> +static inline unsigned
> +__clz (op_t x)
> +{
> +#if !HAVE_BUILTIN_CLZ
> +  unsigned r;
> +  op_t i;
> +
> +  x |= x >> 1;
> +  x |= x >> 2;
> +  x |= x >> 4;
> +  x |= x >> 8;
> +  x |= x >> 16;
> +# if __WORDSIZE == 64
> +  x |= x >> 32;
> +  i = x * 0x03F79D71B4CB0A89ull >> 58;
> +# else
> +  i = x * 0x07C4ACDDU >> 27;
> +# endif
> +  r = index_access (i);
> +  return r ^ (sizeof (op_t) * CHAR_BIT - 1);

The Gnulib integer_length module has a faster implementation, at least 
for 32-bit platforms. Do we still care about 32-bit platforms? If so, 
you might want to take a look at it.