On 15.08.2011 00:21, Levente Uzonyi wrote:
On Sun, 14 Aug 2011, Philippe Marschall wrote:
On 14.08.2011 22:00, Levente Uzonyi wrote:
On Sun, 14 Aug 2011, Philippe Marschall wrote:
Hi
In Seaside we get a lot of performance gains out of primitiveFindFirstInString. One thing that always annoyed me a bit is that it's not as optimized as it could be.
The inclusionMap you give it are 256 consecutive boolean values (0 or 1). There is no need for this to be a 256 element ByteArray when each element can only be 0 or 1. We could as well make it a 32 element ByteArray and each byte holding eight bit values. Instead of using the asciiValue to directly index into the inclusionMap we would use the top five bits to index into the inclusionMap and the bottom 3 bits to "index into the byte".
Did that make any sense?
Do you want to save space?
I'm trying to trade memory access (which is slow) for a bit shift and two bit ands (which is fast).
256 bytes + object header easily fit into the L1 cache of any recent x86 CPU, even the old P4's had 8kB of it.
Sure once it's in the cache then it's in the cache. But a cache line is only 64 bytes. So a 32 element array should fit and a 256 element doesn't. So a 32 element array should be there in one memory access while a 256 element array probably takes five.
Moving less bytes around may make some difference though, but if you create your ByteArrays right before you use them, then it's pretty likely that your object is already in the cache.
I don't of course.
Are you storing lots of inclusion maps (maybe CharacterSets)? If not, then IMHO it's not worth to adding this feature to this primitive, because runtime performance will be worse on both the image side and the VM side.
Why?
Filling the contents of a compact 32 sized ByteArray in the image takes more time than a 256 byte ByteArray. On the VM side memory access + shifts + bitands cost more than just memory access.
Uhm, it's a primitive. My understanding is this would result in "C shifts + bitands".
But (as usual) a benchmark can tell if it's really worth to do it or not.
This there somewhere some documentation how one adds a primitive and compiles a VM (Cog ideally)? Cheers Philippe