On Saturday, 2 February 2013 at 15:47:37 UTC, FG wrote:
Yeah but let us move reallocation out of the equation. Reserving space limits the amount of RAM used and can avoid reallocations all together but in a little test it came out that still appender is 2.5-4 times faster than tab ~= str, where tab is char[] (same when it is ubyte[]). Why is that?
Reserving doesn't mean it has to allocate any memory at all; That's up to the implementation of the runtime and OS, it may just say 'any allocations not related to this object start after address xxx', then as the reserved space needs more it requests the larger space from the OS. The limitations of how much you can use memory-wise doesn't change as that's determined by the infrastructure. (VM and harddrive space doesn't count).
Why cannot tab ~= fill achieve similar performance as this code? Am I missing something important?
As for how many checks and overhead the runtime has I'm not sure. Try compiling the code again with full optimizations on and see if it makes a difference. Concatenation may perform better like that (as long as relocation/copying isn't done) or maybe not...
