Re: [PATCH v4 1/6] stdlib: Optimization qsort{_r} swap implementation (BZ 19305)

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Tue, Jul 11, 2023 at 2:08 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> It optimizes take in consideration both the most common elements are
> either 32 or 64 bit in size and inputs are aligned to the word
> boundary.  This is similar to the optimization done on lib/sort.c
> from Linux.
>
> This patchs adds an optimized swap operation on qsort based in previous
> msort one.  Instead of byte operation, three variants are provided:
>
>   1. Using uint32_t loads and stores.
>   2. Using uint64_t loads and stores.
>   3. Generic one with a temporary buffer and memcpy/mempcpy.
>
> The 1. and 2. options are selected only if base pointer is aligned
> to required type.
>
> It also fixes BZ#19305 by checking input size against number of
> elements 1 besides 0.
>
> Checked on x86_64-linux-gnu.
> ---
>  stdlib/qsort.c | 113 +++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 95 insertions(+), 18 deletions(-)
>
> diff --git a/stdlib/qsort.c b/stdlib/qsort.c
> index 728a0ed370..8a3331fdb4 100644
> --- a/stdlib/qsort.c
> +++ b/stdlib/qsort.c
> @@ -23,20 +23,89 @@
>  #include <limits.h>
>  #include <stdlib.h>
>  #include <string.h>
> +#include <stdbool.h>
> +#include <libc-pointer-arith.h>
>
> -/* Byte-wise swap two items of size SIZE. */
> -#define SWAP(a, b, size)                                                     \
> -  do                                                                         \
> -    {                                                                        \
> -      size_t __size = (size);                                                \
> -      char *__a = (a), *__b = (b);                                           \
> -      do                                                                     \
> -       {                                                                     \
> -         char __tmp = *__a;                                                  \
> -         *__a++ = *__b;                                                      \
> -         *__b++ = __tmp;                                                     \
> -       } while (--__size > 0);                                               \
> -    } while (0)
> +/* Swap SIZE bytes between addresses A and B.  These helpers are provided
> +   along the generic one as an optimization.  */
> +
> +typedef void (*swap_func_t)(void * restrict, void * restrict, size_t);
> +
> +#define SWAP_WORDS_64 (swap_func_t)0
> +#define SWAP_WORDS_32 (swap_func_t)1
> +#define SWAP_BYTES    (swap_func_t)2
> +

Why both making this a function pointer? Why not just make it an enum?
> +/* Returns true if elements can be copied using word loads and stores.
> +   The SIZE and BASE must be a multiple of the ALIGN.  */
> +__attribute_const__ __always_inline
> +static bool
> +is_aligned (const void *base, size_t size, unsigned char align)
> +{
> +  unsigned char lsbits = (unsigned char) size;
> +
> +  lsbits |= (unsigned char)(uintptr_t) base;
> +  return (lsbits & (align - 1)) == 0;
> +}
> +
> +static void
> +swap_words_64 (void * restrict a, void * restrict b, size_t n)
> +{
> +  do
> +   {
> +     n -= 8;
> +     uint64_t t = *(uint64_t *)(a + n);
> +     *(uint64_t *)(a + n) = *(uint64_t *)(b + n);
> +     *(uint64_t *)(b + n) = t;
> +   } while (n);
> +}
> +
> +static void
> +swap_words_32 (void * restrict a, void * restrict b, size_t n)
> +{
> +  do
> +   {
> +     n -= 4;
> +     uint32_t t = *(uint32_t *)(a + n);
> +     *(uint32_t *)(a + n) = *(uint32_t *)(b + n);
> +     *(uint32_t *)(b + n) = t;
> +   } while (n);
> +}
> +
> +static void
> +swap_bytes (void * restrict a, void * restrict b, size_t n)
> +{
> +  /* Use multiple small memcpys with constant size to enable inlining
> +     on most targets.  */
> +  enum { SWAP_GENERIC_SIZE = 32 };
> +  unsigned char tmp[SWAP_GENERIC_SIZE];
> +  while (n > SWAP_GENERIC_SIZE)
> +    {
> +      __builtin_memcpy (tmp, a, SWAP_GENERIC_SIZE);
> +      a = __mempcpy (a, b, SWAP_GENERIC_SIZE);
> +      b = __mempcpy (b, tmp, SWAP_GENERIC_SIZE);
> +      n -= SWAP_GENERIC_SIZE;
> +    }
> +  while (n > 0)
> +    {
> +      unsigned char t = ((unsigned char *)a)[--n];
> +      ((unsigned char *)a)[n] = ((unsigned char *)b)[n];
> +      ((unsigned char *)b)[n] = t;
> +    }
> +}
> +
> +/* Replace the indirect call with a serie of if statements.  It should help
> +   the branch predictor.  */
> +static void
> +do_swap (void * restrict a, void * restrict b, size_t size,
> +        swap_func_t swap_func)
> +{
> +  if (swap_func == SWAP_WORDS_64)
> +    swap_words_64 (a, b, size);
> +  else if (swap_func == SWAP_WORDS_32)
> +    swap_words_32 (a, b, size);
> +  else
> +    swap_bytes (a, b, size);
> +}
>
>  /* Discontinue quicksort algorithm when partition gets below this size.
>     This particular magic number was chosen to work best on a Sun 4/260. */
> @@ -96,6 +165,14 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
>      /* Avoid lossage with unsigned arithmetic below.  */
>      return;
>
> +  swap_func_t swap_func;
> +  if (is_aligned (pbase, size, sizeof (uint64_t)))
> +    swap_func = SWAP_WORDS_64;
> +  else if (is_aligned (pbase, size, sizeof (uint32_t)))
> +    swap_func = SWAP_WORDS_32;
> +  else
> +    swap_func = SWAP_BYTES;
> +
>    if (total_elems > MAX_THRESH)
>      {
>        char *lo = base_ptr;
> @@ -119,13 +196,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
>           char *mid = lo + size * ((hi - lo) / size >> 1);
>
>           if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
> -           SWAP (mid, lo, size);
> +           do_swap (mid, lo, size, swap_func);
>           if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
> -           SWAP (mid, hi, size);
> +           do_swap (mid, hi, size, swap_func);
>           else
>             goto jump_over;
>           if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
> -           SWAP (mid, lo, size);
> +           do_swap (mid, lo, size, swap_func);
>         jump_over:;
>
>           left_ptr  = lo + size;
> @@ -144,7 +221,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
>
>               if (left_ptr < right_ptr)
>                 {
> -                 SWAP (left_ptr, right_ptr, size);
> +                 do_swap (left_ptr, right_ptr, size, swap_func);
>                   if (mid == left_ptr)
>                     mid = right_ptr;
>                   else if (mid == right_ptr)
> @@ -216,7 +293,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
>          tmp_ptr = run_ptr;
>
>      if (tmp_ptr != base_ptr)
> -      SWAP (tmp_ptr, base_ptr, size);
> +      do_swap (tmp_ptr, base_ptr, size, swap_func);
>
>      /* Insertion sort, running from left-hand-side up to right-hand-side.  */
>
> --
> 2.34.1
>