[PATCH] Optimize hweight32 for x86

Akinobu Mita
2009-11-19T04:13:09Z

Optimize hweight32 by using the same technique in hweight64.
The proof of this technique can be found in the commit log for
f9b4192923fa6e38331e88214b1fe5fc21583fcc

The userspece benchmark on x86_32 showed 20% speedup with
bitmap_weight() which uses hweight32 to count bits for each
unsigned long on 32bit architectures.

int main(void)
{
#define SZ (1024 * 1024 * 512)

static DECLARE_BITMAP(bitmap, SZ) = {
[0 ... 100] = 1,
};

return bitmap_weight(bitmap, SZ);
}

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
lib/hweight.c | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/lib/hweight.c b/lib/hweight.c
index 389424e..63ee4eb 100644
--- a/lib/hweight.c
+++ b/lib/hweight.c
@@ -11,11 +11,18 @@

unsigned int hweight32(unsigned int w)
{
+#ifdef ARCH_HAS_FAST_MULTIPLIER
+ w -= (w >> 1) & 0x55555555;
+ w = (w & 0x33333333) + ((w >> 2) & 0x33333333);
+ w = (w + (w >> 4)) & 0x0f0f0f0f;
+ return (w * 0x01010101) >> 24;
+#else
unsigned int res = w - ((w >> 1) & 0x55555555);
res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
res = (res + (res >> 4)) & 0x0F0F0F0F;
res = res + (res >> 8);
return (res + (res >> 16)) & 0x000000FF;
+#endif
}
EXPORT_SYMBOL(hweight32);

--
1.6.5.1