[PATCH 3/5] lib/sort: Avoid indirect calls to built-in swap

[George Spelvin]

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@xxxxxxx>
---
 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