From: Andre Noll <maan@tuebingen.mpg.de>
Date: Mon, 24 Jul 2017 20:10:42 +0000 (+0200)
Subject: mood: Speed up int_sqrt().
X-Git-Tag: v0.6.1~8^2~2
X-Git-Url: http://git.tue.mpg.de/?a=commitdiff_plain;h=e79198e9851faddfd64e47654b5bc66fbc574255;p=paraslash.git

mood: Speed up int_sqrt().

Following a recent discussion on lkml, the choice of the initial
value for the square root is sub-optimal. The change introduced
in this commit was proposed by Peter Zijlstra who also measured a
significant speed improvement for both the hot and the cold cache case.

The speed improvements for the hot-cache case were confirmed on a
32 bit system by running a simple test program which calculates the
square root of 10000000 random numbers. With the new initial value,
the running time went down by 23%. This matters because when a new
mood is loaded, int_sqrt() is called four times per admissible file.

The new initial value is computed in terms of the position of the
most significant bit set in the given argument to int_sqrt(). While
ffs(3) (find first set bit) is in POSIX.1‐2008, there is no fls(3)
(find last set bit), so we have to introduce our own implementation.
We chose an open-coded version because this turned out to be faster
than reversing the bits and calling ffs(3).
---

diff --git a/mood.c b/mood.c
index 54d7b85f..c24b3d8b 100644
--- a/mood.c
+++ b/mood.c
@@ -84,6 +84,42 @@ struct mood {
  */
 static struct mood *current_mood;
 
+/*
+ * Find the position of the most-significant set bit.
+ *
+ * Copied and slightly adapted from the linux source tree, version 4.9.39
+ * (2017-07).
+ */
+__a_const static uint32_t fls64(uint64_t v)
+{
+	int n = 63;
+	const uint64_t ones = ~(uint64_t)0U;
+
+	if ((v & (ones << 32)) == 0) {
+		n -= 32;
+		v <<= 32;
+	}
+	if ((v & (ones << (64 - 16))) == 0) {
+		n -= 16;
+		v <<= 16;
+	}
+	if ((v & (ones << (64 - 8))) == 0) {
+		n -= 8;
+		v <<= 8;
+	}
+	if ((v & (ones << (64 - 4))) == 0) {
+		n -= 4;
+		v <<= 4;
+	}
+	if ((v & (ones << (64 - 2))) == 0) {
+		n -= 2;
+		v <<= 2;
+	}
+	if ((v & (ones << (64 - 1))) == 0)
+		n -= 1;
+	return n;
+}
+
 /*
  * Rough approximation to sqrt.
  *
@@ -96,10 +132,7 @@ __a_const static uint64_t int_sqrt(uint64_t x)
 	op = x;
 	res = 0;
 
-	one = one << 62;
-	while (one > op)
-		one >>= 2;
-
+	one = one << (fls64(x) & ~one);
 	while (one != 0) {
 		if (op >= res + one) {
 			op = op - (res + one);