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Subject[PATCH v2 3/5] 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)

Signed-off-by: George Spelvin <lkml@sdf.org>
Acked-by: Andrey Abramov <st5pub@yandex.ru>
---
lib/sort.c | 51 ++++++++++++++++++++++++++++++++++++---------------
1 file changed, 36 insertions(+), 15 deletions(-)

diff --git a/lib/sort.c b/lib/sort.c
index 0d24d0c5c0fc..50855ea8c262 100644
--- a/lib/sort.c
+++ b/lib/sort.c
@@ -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);
}
}
}
--
2.20.1
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