lkml.org 
[lkml]   [2019]   [Mar]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 2/5] lib/sort: Use more efficient bottom-up heapsort variant
From
Date
On 21/02/2019 09.21, George Spelvin wrote:
>
> +/**
> + * parent - given the offset of the child, find the offset of the parent.
> + * @i: the offset of the heap element whose parent is sought. Non-zero.
> + * @lsbit: a precomputed 1-bit mask, equal to "size & -size"
> + * @size: size of each element
> + *
> + * In terms of array indexes, the parent of element j = i/size is simply
> + * (j-1)/2. But when working in byte offsets, we can't use implicit
> + * truncation of integer divides.
> + *
> + * Fortunately, we only need one bit of the quotient, not the full divide.
> + * size has a least significant bit. That bit will be clear if i is
> + * an even multiple of size, and set if it's an odd multiple.
> + *
> + * Logically, we're doing "if (i & lsbit) i -= size;", but since the
> + * branch is unpredictable, it's done with a bit of clever branch-free
> + * code instead.
> + */
> +__attribute_const__ __always_inline
> +static size_t parent(size_t i, unsigned int lsbit, size_t size)
> +{
> + i -= size;
> + i -= size & -(i & lsbit);
> + return i / 2;
> +}
> +

Really nice :) I had to work through this by hand, but it's solid.

> /**
> * sort - sort an array of elements
> * @base: pointer to data to sort
> @@ -125,21 +151,26 @@ static void generic_swap(void *a, void *b, int size)
> * @cmp_func: pointer to comparison function
> * @swap_func: pointer to swap function or NULL
> *
> - * This function does a heapsort on the given array. You may provide a
> - * swap_func function optimized to your element type.
> + * This function does a heapsort on the given array. You may provide a
> + * swap_func function if you need to do something more than a memory copy
> + * (e.g. fix up pointers or auxiliary data), but the built-in swap isn't
> + * usually a bottleneck.
> *
> * Sorting time is O(n log n) both on average and worst-case. While
> * qsort is about 20% faster on average, it suffers from exploitable
> * O(n*n) worst-case behavior and extra memory requirements that make
> * it less suitable for kernel use.
> */
> -
> void sort(void *base, size_t num, size_t size,
> int (*cmp_func)(const void *, const void *),
> void (*swap_func)(void *, void *, int size))
> {
> /* pre-scale counters for performance */
> - int i = (num/2 - 1) * size, n = num * size, c, r;
> + size_t n = num * size, a = (num/2) * size;
> + unsigned const lsbit = size & -size; /* Used to find parent */
> +

Nit: qualifier before type, "const unsigned". And this sets ZF, so a
paranoid check for zero size (cf. the other mail) by doing "if (!lsbit)
return;" is practically free. Though it's probably a bit obscure doing
it that way...

> + if (!n)
> + return;

I'd make that n <= 1. Shouldn't be much more costly.

> - }
> - }
> + /*
> + * Loop invariants:
> + * 1. elements [a,n) satisfy the heap property (compare greater than
> + * all of their children),
> + * 2. elements [n,num*size) are sorted, and
> + * 3. a <= b <= c <= d <= n (whenever they are valid).
> + */
> + for (;;) {
> + size_t b, c, d;
>

> + if (a) /* Building heap: sift down --a */
> + a -= size;
> + else if (n -= size) /* Sorting: Extract root to --n */
> + swap_func(base, base + n, size);
> + else /* Sort complete */
> + break;
> +
> + /*
> + * Sift element at "a" down into heap. This is the
> + * "bottom-up" variant, which significantly reduces
> + * calls to cmp_func(): we find the sift-down path all
> + * the way to the leaves (one compare per level), then
> + * backtrack to find where to insert the target element.
> + *
> + * Because elements tend to sift down close to the leaves,
> + * this uses fewer compares than doing two per level
> + * on the way down. (A bit more than half as many on
> + * average, 3/4 worst-case.)
> + */
> + for (b = a; c = 2*b + size, (d = c + size) < n;)
> + b = cmp_func(base + c, base + d) >= 0 ? c : d;
> + if (d == n) /* Special case last leaf with no sibling */
> + b = c;
> +
> + /* Now backtrack from "b" to the correct location for "a" */
> + while (b != a && cmp_func(base + a, base + b) >= 0)
> + b = parent(b, lsbit, size);
> + c = b; /* Where "a" belongs */
> + while (b != a) { /* Shift it into place */
> + b = parent(b, lsbit, size);
> + swap_func(base + b, base + c, size);
> }
> }
> }
>

Nice!

Rasmus

\
 
 \ /
  Last update: 2019-03-13 23:30    [W:0.301 / U:15.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site