Re: [PATCH v3 3/7] stdlib: Optimization qsort{_r} swap implementation (BZ #19305)

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 13/10/2021 00:39, Noah Goldstein wrote:
> 
> 
> On Tue, Oct 12, 2021 at 11:29 PM Noah Goldstein <goldstein.w.n@gmail.com <mailto:goldstein.w.n@gmail.com>> wrote:
> 
> 
> 
>     On Fri, Sep 3, 2021 at 1:14 PM Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org <mailto:libc-alpha@sourceware.org>> wrote:
> 
>         It optimizes take in consideration both the most common elements are
>         either 32 or 64 bit in size [1] 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 either if architecture defines
>         _STRING_ARCH_unaligned or 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.
> 
>         [1] https://sourceware.org/pipermail/libc-alpha/2018-August/096984.html <https://sourceware.org/pipermail/libc-alpha/2018-August/096984.html>
>         ---
>          stdlib/qsort.c | 109 +++++++++++++++++++++++++++++++++++++++++--------
>          1 file changed, 91 insertions(+), 18 deletions(-)
> 
>         diff --git a/stdlib/qsort.c b/stdlib/qsort.c
>         index 23f2d28314..59458d151b 100644
>         --- a/stdlib/qsort.c
>         +++ b/stdlib/qsort.c
>         @@ -24,20 +24,85 @@
>          #include <limits.h>
>          #include <stdlib.h>
>          #include <string.h>
>         +#include <stdbool.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);
>         +
>         +/* Return trues is elements can be copied used word load and sortes.
>         +   The size must be a multiple of the alignment, and the base address.  */
>         +static inline bool
>         +is_aligned_to_copy (const void *base, size_t size, size_t align)
>         +{
>         +  unsigned char lsbits = size;
>         +#if !_STRING_ARCH_unaligned
>         +  lsbits |= (unsigned char)(uintptr_t) base;
>         +#endif
>         +  return (lsbits & (align - 1)) == 0;
>         +}
>         +
>         +#define SWAP_WORDS_64 (swap_func_t)0
>         +#define SWAP_WORDS_32 (swap_func_t)1
>         +#define SWAP_BYTES    (swap_func_t)2
>         +
>         +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);
>         +}
> 
> 
> I'm not certain swap_words_32 / swap_words_8 will be optimal for larger
> key sizes. Looking at GCC's implementation of swap_generic on modern x86_64: 
> https://godbolt.org/z/638h3Y9va <https://godbolt.org/z/638h3Y9va>
> It's able to optimize the temporary buffer out of the loop and use xmm registers which
> will likely win out for larger sizes. 

It is probably not the most optimized code compiler can generated since
I tried to go for a more generic code that should results in a somewhat
better code in a architecture agnostic code.  I trying to mimic some
of optimization Linux did on 37d0ec34d111acfdb, and the swap_bytes I
used the same strategy used on d3496c9f4f27d (Linux did not optimize
anything for the byte version).

I think it would be possible to tune it for an specific architecture
and/or compiler, but I would prefer to use a good enough algorithm that
work reasonable on multiple architectures.