Optimize rotate

Previously, we used three rotations.  This was simple, but was aggressively
inlined by gcc at higher optimization levels, massively bloating the code.

Now, we take the front of the array, which is to be rotated to the end, and
using a series of swaps, move it progressively closer to the end.  When we
can no longer perform a swap of the required size because we're too close
to the end, we view the remaining operation as a rotate to the right with
a smaller shift size.  We repeat as necessary.

This generates somewhat smaller code than the three reverses, even at -Os.
It also shows up to a 25% overall performance improvement for grailsort on
some inputs.

1 files changed, 13 insertions, 15 deletions

diff --git a/common.h b/common.h
index c1bfb6c..087ee4f 100644
--- a/common.h
+++ b/common.h
@@ -28,22 +28,20 @@ static void swap(char *a, char *b, size_t width)
 	}
 }
 
-static void reverse(char *base, size_t nmel, size_t width)
-{
-	char *last = base + (nmel - 1) * width;
-	while (last > base) {
-		swap(base, last, width);
-		base += width;
-		last -= width;
-	}
-}
-
 /* rotates left */
-static void rotate(char *base, size_t nmel, size_t width, size_t shift)
+static void rotate(char *base, size_t size, size_t shift)
 {
-	reverse(base, shift, width);
-	reverse(base + shift * width, nmel - shift, width);
-	reverse(base, nmel, width);
+	int dir = 1;
+	while (shift) {
+		while (2*shift <= size) {
+			swap(base, base + dir*shift, shift);
+			size -= shift;
+			base += shift*dir;
+		}
+		shift = size - shift;
+		base  = dir > 0 ? base + size - shift : base - shift;
+		dir  *= -1;
+	}
 }
 
 static void distribute_buffer(char *base, size_t bufnmel, size_t sortnmel, size_t width, cmpfun cmp)
@@ -55,7 +53,7 @@ static void distribute_buffer(char *base, size_t bufnmel, size_t sortnmel, size_
 		for (; insertpos < sortnmel && cmp(base, sorted + insertpos * width) > 0; insertpos++);
 
 		if (insertpos > 0) {
-			rotate(base, bufnmel + insertpos, width, bufnmel);
+			rotate(base, (bufnmel + insertpos) * width, bufnmel * width);
 		}
 
 		base     += (insertpos + 1) * width;