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

[Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>]

On Fri, Oct 15, 2021 at 8:29 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> 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.

That's fair. Although I still think there are some improvements.

Looking at the assembly for all three in fact it seems GCC optimizes all of them
to larger register copies: https://godbolt.org/z/bd9nnnoEY

The biggest difference seems to be the setup / register spills for
the generic version so for the common case of a relatively small key
the special case for 4/8 makes sense.

Have you checked that GCC is able to use the conditions for selecting
the swap function to optimize the functions themselves? In the godbolt
link I got reasonable value out of adding the invariants to swap_64/swap_32.

It also may be worth it to write a custom memcpy implementation for
size = 0..SWAP_GENERIC_SIZE so it can be inlined (and probably
more optimized than what generic memcpy can get away with).