Commit 8fb583c4 authored by George Spelvin's avatar George Spelvin Committed by Linus Torvalds
Browse files

lib/sort: avoid indirect calls to built-in swap 

Similar to what's being done in the net code, this takes advantage of
the fact that most invocations use only a few common swap functions, and
replaces indirect calls to them with (highly predictable) conditional
branches.  (The downside, of course, is that if you *do* use a custom
swap function, there are a few extra predicted branches on the code
path.)

This actually *shrinks* the x86-64 code, because it inlines the various
swap functions inside do_swap, eliding function prologues & epilogues.

x86-64 code size 767 -> 703 bytes (-64)

Link: http://lkml.kernel.org/r/d10c5d4b393a1847f32f5b26f4bbaa2857140e1e.1552704200.git.lkml@sdf.org


Signed-off-by: default avatarGeorge Spelvin <lkml@sdf.org>
Acked-by: default avatarAndrey Abramov <st5pub@yandex.ru>
Acked-by: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: default avatarAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Daniel Wagner <daniel.wagner@siemens.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Don Mullis <don.mullis@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 22a241cc

lib/sort.c

+36 −15
Original line number Diff line number Diff line
@@ -54,10 +54,8 @@ static bool is_aligned(const void *base, size_t size, unsigned char align) 
 * subtract (since the intervening mov instructions don't alter the flags).

 * Gcc 8.1.0 doesn't have that problem.

 */

static void swap_words_32(void *a, void *b, int size)

static void swap_words_32(void *a, void *b, size_t n)

{

	size_t n = (unsigned int)size;



	do {

		u32 t = *(u32 *)(a + (n -= 4));

		*(u32 *)(a + n) = *(u32 *)(b + n);
@@ -80,10 +78,8 @@ static void swap_words_32(void *a, void *b, int size) 
 * but it's possible to have 64-bit loads without 64-bit pointers (e.g.

 * x32 ABI).  Are there any cases the kernel needs to worry about?

 */

static void swap_words_64(void *a, void *b, int size)

static void swap_words_64(void *a, void *b, size_t n)

{

	size_t n = (unsigned int)size;



	do {

#ifdef CONFIG_64BIT

		u64 t = *(u64 *)(a + (n -= 8));
@@ -109,10 +105,8 @@ static void swap_words_64(void *a, void *b, int size) 
 *

 * This is the fallback if alignment doesn't allow using larger chunks.

 */

static void swap_bytes(void *a, void *b, int size)

static void swap_bytes(void *a, void *b, size_t n)

{

	size_t n = (unsigned int)size;



	do {

		char t = ((char *)a)[--n];

		((char *)a)[n] = ((char *)b)[n];
@@ -120,6 +114,33 @@ static void swap_bytes(void *a, void *b, int size) 
	} while (n);

}



typedef void (*swap_func_t)(void *a, void *b, int size);



/*

 * The values are arbitrary as long as they can't be confused with

 * a pointer, but small integers make for the smallest compare

 * instructions.

 */

#define SWAP_WORDS_64 (swap_func_t)0

#define SWAP_WORDS_32 (swap_func_t)1

#define SWAP_BYTES    (swap_func_t)2



/*

 * The function pointer is last to make tail calls most efficient if the

 * compiler decides not to inline this function.

 */

static void do_swap(void *a, void *b, size_t size, swap_func_t swap_func)

{

	if (swap_func == SWAP_WORDS_64)

		swap_words_64(a, b, size);

	else if (swap_func == SWAP_WORDS_32)

		swap_words_32(a, b, size);

	else if (swap_func == SWAP_BYTES)

		swap_bytes(a, b, size);

	else

		swap_func(a, b, (int)size);

}



/**

 * 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.
@@ -157,7 +178,7 @@ static size_t parent(size_t i, unsigned int lsbit, size_t size) 
 * 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.

 * avoids a slow retpoline and so is significantly faster.

 *

 * Sorting time is O(n log n) both on average and worst-case. While

 * quicksort is slightly faster on average, it suffers from exploitable
@@ -177,11 +198,11 @@ void sort(void *base, size_t num, size_t size, 


	if (!swap_func) {

		if (is_aligned(base, size, 8))

			swap_func = swap_words_64;

			swap_func = SWAP_WORDS_64;

		else if (is_aligned(base, size, 4))

			swap_func = swap_words_32;

			swap_func = SWAP_WORDS_32;

		else

			swap_func = swap_bytes;

			swap_func = SWAP_BYTES;

	}



	/*
@@ -197,7 +218,7 @@ void sort(void *base, size_t num, size_t size, 
		if (a)			/* Building heap: sift down --a */

			a -= size;

		else if (n -= size)	/* Sorting: Extract root to --n */

			swap_func(base, base + n, size);

			do_swap(base, base + n, size, swap_func);

		else			/* Sort complete */

			break;
@@ -224,7 +245,7 @@ void sort(void *base, size_t num, size_t 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);

			do_swap(base + b, base + c, size, swap_func);

		}

	}

}

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