lkml.org 
[lkml]   [2019]   [Mar]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Date
Subject[PATCH 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 additional (highly predictable) conditional
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 770 -> 709 bytes (-61)

Signed-off-by: George Spelvin <lkml@sdf.org>
---
lib/sort.c | 45 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/lib/sort.c b/lib/sort.c
index 2aef4631e7d3..226a8c7e4b9a 100644
--- a/lib/sort.c
+++ b/lib/sort.c
@@ -117,6 +117,33 @@ static void generic_swap(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 U64_SWAP (swap_func_t)0
+#define U32_SWAP (swap_func_t)1
+#define GENERIC_SWAP (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, int size, swap_func_t swap_func)
+{
+ if (swap_func == U64_SWAP)
+ u64_swap(a, b, size);
+ else if (swap_func == U32_SWAP)
+ u32_swap(a, b, size);
+ else if (swap_func == GENERIC_SWAP)
+ generic_swap(a, b, size);
+ else
+ swap_func(a, b, 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.
@@ -151,10 +178,10 @@ static size_t parent(size_t i, unsigned int lsbit, size_t 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 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.
+ * 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
+ * avoids a slow retpoline and so is significantly faster.
*
* 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
@@ -174,11 +201,11 @@ void sort(void *base, size_t num, size_t size,

if (!swap_func) {
if (alignment_ok(base, size, 8))
- swap_func = u64_swap;
+ swap_func = U64_SWAP;
else if (alignment_ok(base, size, 4))
- swap_func = u32_swap;
+ swap_func = U32_SWAP;
else
- swap_func = generic_swap;
+ swap_func = GENERIC_SWAP;
}

/*
@@ -194,7 +221,7 @@ 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;

@@ -221,7 +248,7 @@ 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
\
 
 \ /
  Last update: 2019-03-09 04:20    [W:0.214 / U:8.156 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site