AMD crammed 64-bit support into the Sempron without a redesign. This necessarily entailed a lot of compromises as a design optimized for 64-bit operation would have a lot of fundamental differences at a very low level.
As a result, 64-bit operations in 64-bit mode are about 35% slower on the Sempron than similar 32-bit operations in 32-bit mode. Of course if you would need two 32-bit operations instead of one 64-bit operation, that's still a win for 64-bit mode. But if you replace 64-bit operations with 32-bit operations, as the kernel does for pointers, you take that performance penalty.
There are still a lot of benefits to 64-bit operation. 64-bit operations are significantly faster for those times when you need them. And you have twice as many general purpose registers.
Unfortunately, 64-bit programs tend to require more memory bandwidth than 32-bit programs, and the Sempron has a memory controller with just a single channel. It relies on caches to hide this cost, but 64-bit programs move more data and so tend to have higher cache miss rates for the same cache size.
So your results aren't really all that surprising.
For which host machine? – 174140 – 2013-08-30T06:27:06.147
@uprego 64bit laptop, I think that you can't install 64bit on 32bit processor machine. – jcubic – 2013-08-30T06:28:20.793
how much memory do you have? – Keltari – 2013-08-30T06:29:12.363
For which x86_64 compatible host laptop? – 174140 – 2013-08-30T06:31:33.353
Updated info about my machine. – jcubic – 2013-08-30T06:44:54.190