From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: libc-alpha@sourceware.org,
Noah Goldstein <goldstein.w.n@gmail.com>,
Paul Eggert <eggert@cs.ucla.edu>,
Florian Weimer <fweimer@redhat.com>
Subject: [PATCH v8 5/7] stdlib: Implement introsort for qsort (BZ 19305)
Date: Mon, 2 Oct 2023 16:33:09 -0300 [thread overview]
Message-ID: <20231002193311.3985890-6-adhemerval.zanella@linaro.org> (raw)
In-Reply-To: <20231002193311.3985890-1-adhemerval.zanella@linaro.org>
This patch makes the quicksort implementation to acts as introsort, to
avoid worse-case performance (and thus making it O(nlog n)). It switch
to heapsort when the depth level reaches 2*log2(total elements). The
heapsort is a textbook implementation.
Checked on x86_64-linux-gnu.
---
stdlib/qsort.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 80 insertions(+), 7 deletions(-)
diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index 4082f5f9c1..e160932657 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -97,6 +97,7 @@ typedef struct
{
char *lo;
char *hi;
+ size_t depth;
} stack_node;
/* The stack needs log (total_elements) entries (we could even subtract
@@ -106,22 +107,83 @@ typedef struct
enum { STACK_SIZE = CHAR_BIT * sizeof (size_t) };
static inline stack_node *
-push (stack_node *top, char *lo, char *hi)
+push (stack_node *top, char *lo, char *hi, size_t depth)
{
top->lo = lo;
top->hi = hi;
+ top->depth = depth;
return ++top;
}
static inline stack_node *
-pop (stack_node *top, char **lo, char **hi)
+pop (stack_node *top, char **lo, char **hi, size_t *depth)
{
--top;
*lo = top->lo;
*hi = top->hi;
+ *depth = top->depth;
return top;
}
+/* A fast, small, non-recursive O(nlog n) heapsort, adapted from Linux
+ lib/sort.c. Used on introsort implementation as a fallback routine with
+ worst-case performance of O(nlog n) and worst-case space complexity of
+ O(1). */
+
+static inline void
+siftdown (void *base, size_t size, size_t k, size_t n,
+ enum swap_type_t swap_type, __compar_d_fn_t cmp, void *arg)
+{
+ while (k <= n / 2)
+ {
+ size_t j = 2 * k;
+ if (j < n && cmp (base + (j * size), base + ((j + 1) * size), arg) < 0)
+ j++;
+
+ if (cmp (base + (k * size), base + (j * size), arg) >= 0)
+ break;
+
+ do_swap (base + (size * j), base + (k * size), size, swap_type);
+ k = j;
+ }
+}
+
+static inline void
+heapify (void *base, size_t size, size_t n, enum swap_type_t swap_type,
+ __compar_d_fn_t cmp, void *arg)
+{
+ size_t k = n / 2;
+ while (1)
+ {
+ siftdown (base, size, k, n, swap_type, cmp, arg);
+ if (k-- == 0)
+ break;
+ }
+}
+
+static void
+heapsort_r (void *base, void *end, size_t size, enum swap_type_t swap_type,
+ __compar_d_fn_t cmp, void *arg)
+{
+ const size_t count = ((uintptr_t) end - (uintptr_t) base) / size;
+
+ if (count < 2)
+ return;
+
+ size_t n = count - 1;
+
+ /* Build the binary heap, largest value at the base[0]. */
+ heapify (base, size, n, swap_type, cmp, arg);
+
+ /* On each iteration base[0:n] is the binary heap, while base[n:count]
+ is sorted. */
+ while (n > 0)
+ {
+ do_swap (base, base + (n * size), size, swap_type);
+ n--;
+ siftdown (base, size, 0, n, swap_type, cmp, arg);
+ }
+}
static inline void
insertion_sort_qsort_partitions (void *const pbase, size_t total_elems,
@@ -207,7 +269,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
const size_t max_thresh = MAX_THRESH * size;
- if (total_elems == 0)
+ if (total_elems <= 1)
/* Avoid lossage with unsigned arithmetic below. */
return;
@@ -219,15 +281,26 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
else
swap_type = SWAP_BYTES;
+ /* Maximum depth before quicksort switches to heapsort. */
+ size_t depth = 2 * (sizeof (size_t) * CHAR_BIT - 1
+ - __builtin_clzl (total_elems));
+
if (total_elems > MAX_THRESH)
{
char *lo = base_ptr;
char *hi = &lo[size * (total_elems - 1)];
stack_node stack[STACK_SIZE];
- stack_node *top = stack + 1;
+ stack_node *top = push (stack, NULL, NULL, depth);
while (stack < top)
{
+ if (depth == 0)
+ {
+ heapsort_r (lo, hi, size, swap_type, cmp, arg);
+ top = pop (top, &lo, &hi, &depth);
+ continue;
+ }
+
char *left_ptr;
char *right_ptr;
@@ -291,7 +364,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
{
if ((size_t) (hi - left_ptr) <= max_thresh)
/* Ignore both small partitions. */
- top = pop (top, &lo, &hi);
+ top = pop (top, &lo, &hi, &depth);
else
/* Ignore small left partition. */
lo = left_ptr;
@@ -302,13 +375,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
else if ((right_ptr - lo) > (hi - left_ptr))
{
/* Push larger left partition indices. */
- top = push (top, lo, right_ptr);
+ top = push (top, lo, right_ptr, depth - 1);
lo = left_ptr;
}
else
{
/* Push larger right partition indices. */
- top = push (top, left_ptr, hi);
+ top = push (top, left_ptr, hi, depth - 1);
hi = right_ptr;
}
}
--
2.34.1
next prev parent reply other threads:[~2023-10-02 19:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-02 19:33 [PATCH v8 0/7] Use introsort for qsort Adhemerval Zanella
2023-10-02 19:33 ` [PATCH v8 1/7] string: Add internal memswap implementation Adhemerval Zanella
2023-10-03 5:43 ` Noah Goldstein
2023-10-03 11:00 ` Adhemerval Zanella Netto
2023-10-02 19:33 ` [PATCH v8 2/7] stdlib: Optimization qsort{_r} swap implementation Adhemerval Zanella
2023-10-02 19:33 ` [PATCH v8 3/7] stdlib: Move insertion sort out qsort Adhemerval Zanella
2023-10-02 19:33 ` [PATCH v8 4/7] stdlib: qsort: Move some macros to inline function Adhemerval Zanella
2023-10-02 19:33 ` Adhemerval Zanella [this message]
2023-10-02 19:33 ` [PATCH v8 6/7] stdlib: Remove use of mergesort on qsort (BZ 21719) Adhemerval Zanella
2023-10-02 19:33 ` [PATCH v8 7/7] stdlib: Add more qsort{_r} coverage Adhemerval Zanella
2023-10-03 12:22 [PATCH v8 0/7] Use introsort for qsort Adhemerval Zanella
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
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