From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: libc-alpha@sourceware.org, Paul Eggert <eggert@cs.ucla.edu>,
Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v8 2/7] stdlib: Optimization qsort{_r} swap implementation
Date: Mon, 30 Oct 2023 14:02:57 -0500 [thread overview]
Message-ID: <CAFUsyfKDo1UofOxv1+q-WCgbOQn-qxBZsNUN1K3JhDuwmWj=aQ@mail.gmail.com> (raw)
In-Reply-To: <20231003122251.3325435-3-adhemerval.zanella@linaro.org>
On Tue, Oct 3, 2023 at 7:23 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> The optimization takes 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 what msort does.
>
> For large buffer the swap operation uses memcpy/mempcpy with a
> small fixed size buffer (so compiler might inline the operations).
>
> Checked on x86_64-linux-gnu.
> ---
> stdlib/qsort.c | 95 ++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 77 insertions(+), 18 deletions(-)
>
> diff --git a/stdlib/qsort.c b/stdlib/qsort.c
> index 728a0ed370..072ccdfb95 100644
> --- a/stdlib/qsort.c
> +++ b/stdlib/qsort.c
> @@ -21,22 +21,73 @@
>
> #include <alloca.h>
> #include <limits.h>
> +#include <memswap.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. */
> +
> +enum swap_type_t
> + {
> + SWAP_WORDS_64,
> + SWAP_WORDS_32,
> + SWAP_BYTES
> + };
> +
> +/* If this function returns true, elements can be safely copied using word
> + loads and stores. Otherwise, it might not be safe. BASE (as an integer)
> + must be a multiple of the word alignment. SIZE must be a multiple of
> + WORDSIZE. Since WORDSIZE must be a multiple of the word alignment, and
> + WORDSIZE is a power of two on all supported platforms, this function for
> + speed merely checks that BASE and SIZE are both multiples of the word
> + size. */
> +static inline bool
> +is_aligned (const void *base, size_t size, size_t wordsize)
> +{
> + return (((uintptr_t) base | size) & (wordsize - 1)) == 0;
> +}
> +
> +static inline void
> +swap_words_64 (void * restrict a, void * restrict b, size_t n)
> +{
> + typedef uint64_t __attribute__ ((__may_alias__)) u64_alias_t;
> + do
> + {
> + n -= 8;
> + u64_alias_t t = *(u64_alias_t *)(a + n);
> + *(u64_alias_t *)(a + n) = *(u64_alias_t *)(b + n);
> + *(u64_alias_t *)(b + n) = t;
> + } while (n);
> +}
> +
> +static inline void
> +swap_words_32 (void * restrict a, void * restrict b, size_t n)
> +{
> + typedef uint32_t __attribute__ ((__may_alias__)) u32_alias_t;
> + do
> + {
> + n -= 4;
> + u32_alias_t t = *(u32_alias_t *)(a + n);
> + *(u32_alias_t *)(a + n) = *(u32_alias_t *)(b + n);
> + *(u32_alias_t *)(b + n) = t;
> + } while (n);
> +}
> +
> +/* 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,
> + enum swap_type_t swap_type)
> +{
> + if (swap_type == SWAP_WORDS_64)
> + swap_words_64 (a, b, size);
> + else if (swap_type == SWAP_WORDS_32)
> + swap_words_32 (a, b, size);
> + else
> + __memswap (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 +147,14 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
> /* Avoid lossage with unsigned arithmetic below. */
> return;
>
> + enum swap_type_t swap_type;
> + if (is_aligned (pbase, size, 8))
> + swap_type = SWAP_WORDS_64;
> + else if (is_aligned (pbase, size, 4))
> + swap_type = SWAP_WORDS_32;
> + else
> + swap_type = SWAP_BYTES;
> +
> if (total_elems > MAX_THRESH)
> {
> char *lo = base_ptr;
> @@ -119,13 +178,13 @@ _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_type);
> if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
> - SWAP (mid, hi, size);
> + do_swap (mid, hi, size, swap_type);
> else
> goto jump_over;
> if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
> - SWAP (mid, lo, size);
> + do_swap (mid, lo, size, swap_type);
> jump_over:;
>
> left_ptr = lo + size;
> @@ -144,7 +203,7 @@ _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_type);
> if (mid == left_ptr)
> mid = right_ptr;
> else if (mid == right_ptr)
> @@ -216,7 +275,7 @@ _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_type);
>
> /* Insertion sort, running from left-hand-side up to right-hand-side. */
>
> --
> 2.34.1
>
LGTM
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
next prev parent reply other threads:[~2023-10-30 19:03 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-03 12:22 [PATCH v8 0/7] Use introsort for qsort Adhemerval Zanella
2023-10-03 12:22 ` [PATCH v8 1/7] string: Add internal memswap implementation Adhemerval Zanella
2023-10-03 17:42 ` Noah Goldstein
2023-10-27 20:23 ` Adhemerval Zanella Netto
2023-10-27 22:47 ` Gabriel Ravier
2023-10-28 1:30 ` Noah Goldstein
2023-10-30 19:02 ` Noah Goldstein
2023-10-03 12:22 ` [PATCH v8 2/7] stdlib: Optimization qsort{_r} swap implementation Adhemerval Zanella
2023-10-30 19:02 ` Noah Goldstein [this message]
2023-10-03 12:22 ` [PATCH v8 3/7] stdlib: Move insertion sort out qsort Adhemerval Zanella
2023-10-30 19:03 ` Noah Goldstein
2023-10-03 12:22 ` [PATCH v8 4/7] stdlib: qsort: Move some macros to inline function Adhemerval Zanella
2023-10-30 19:03 ` Noah Goldstein
2023-10-03 12:22 ` [PATCH v8 5/7] stdlib: Implement introsort for qsort (BZ 19305) Adhemerval Zanella
2023-10-30 19:03 ` Noah Goldstein
2023-10-03 12:22 ` [PATCH v8 6/7] stdlib: Remove use of mergesort on qsort (BZ 21719) Adhemerval Zanella
2023-10-30 19:04 ` Noah Goldstein
2023-10-03 12:22 ` [PATCH v8 7/7] stdlib: Add more qsort{_r} coverage Adhemerval Zanella
2023-10-30 19:04 ` Noah Goldstein
2023-10-03 18:13 ` [PATCH v8 0/7] Use introsort for qsort Noah Goldstein
2023-10-27 20:24 ` Adhemerval Zanella Netto
2023-10-28 1:31 ` Noah Goldstein
-- strict thread matches above, loose matches on Subject: below --
2023-10-02 19:33 Adhemerval Zanella
2023-10-02 19:33 ` [PATCH v8 2/7] stdlib: Optimization qsort{_r} swap implementation Adhemerval Zanella
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