From: Lucas De Marchi <lucas.demarchi@intel.com>
Date: Thu, 22 Aug 2013 04:10:13 +0000 (-0300)
Subject: util: Add ALIGN_POWER2
X-Git-Tag: v16~19
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=3ba7f59e84857eb4dbe56a68fc7a3ffe8a650393;p=thirdparty%2Fkmod.git

util: Add ALIGN_POWER2

Add static inline function to align a value to it's next power of 2.
This is commonly done by a SWAR like the one in:

http://aggregate.org/MAGIC/#Next Largest Power of 2

However a microbench shows that the implementation herer is a faster.
It doesn't really impact the possible user of this function, but it's
interesting nonetheless.

Using a x86_64 i7 Ivy Bridge it shows a ~4% advantage by using clz
instead instead of the OR and SHL chain. And this is by using a BSR
since Ivy Bridge doesn't have LZCNT. New Haswell processors have the
LZCNT instruction which can make this even better. ARM also has a CLZ
instruction so it should be better, too.

Code used to test:

	...
	v = val[i];
	t1 = get_cycles(0);
	a = ALIGN_POWER2(v);
	t1 = get_cycles(t1);

	t2 = get_cycles(0);
	v = nlpo2(v);
	t2 = get_cycles(t2);

	printf("%u\t%llu\t%llu\t%d\n", v, t1, t2, v == a);
	...

In which val is an array of 20 random unsigned int, nlop2 is the SWAR
implementation and get_cycles uses RDTSC to measure the performance.

Averages:
	ALIGN_POWER2: 	30 cycles
	nlop2:		31.4 cycles
---

diff --git a/libkmod/libkmod-util.h b/libkmod/libkmod-util.h
index f7f3e90d..8a70aebf 100644
--- a/libkmod/libkmod-util.h
+++ b/libkmod/libkmod-util.h
@@ -51,3 +51,8 @@ do {						\
 	} *__p = (typeof(__p)) (ptr);		\
 	__p->__v = (val);			\
 } while(0)
+
+static _always_inline_ unsigned int ALIGN_POWER2(unsigned int u)
+{
+	return 1 << ((sizeof(u) * 8) - __builtin_clz(u - 1));
+}