I wrote the algorithm that is there to sort the variables based on usage since I couldn't come up with a nicer metric, because as you thought cache line placement could be important. 8 years back (or so) it wasn't important, now it is, we noticed a 2% gain in a VM build by adding a single instance variable, which changed placement of other variables on a cache line.
Hand sorting couldn't hurt, didn't want to do that at the time.
Isn't everything Intel now anyway? Alas...
The other magic that is lurking btw is if you shared a global between different routines and this variable(s) become part of a sole routine after the merging of methods and inlining is done then the variable is moved out of the foo structure and made a local. There is a method where you can deny that behavior for a variable/routine if I remember.
The *most* important place this was effective in was the GC logic. The GC logic is split between various smalltalk methods, which all get inlined together. So on the powerpc those globals sharing state between the 4-5 routines would all become local register variables made quite a different in performance.
Andreas made a change a few years back to change scoping of local variables to the block they are used in, versus defining the locals at the top level of the routine, that help register allocation.
Lastly I've a change set, well somewhere, that would extrude slang and say void foobar{} if in fact method foobar didn't actually return anything. This was never accepted because we found certain methods in the VMMachine defs would return values, but in fact actually didn't return values. however one can never tell what the compiler will do given int foobar{} versus void foobar{}
On Feb 28, 2008, at 8:34 PM, Igor Stasenko wrote:
I just wondering, what if i place variables in interpreter struct in specific order, not in order, which code generator produces.
Since speed impact expected to be very small (if any), i used following code to measure difference.
[ 5 timesRepeat: [ 1 tinyBenchmarks] ] timeToRun
This is HydraVM without attempts to arrange interpreters ivars:
28921 28863 28863
This build shows a small slowdown (i placed all big-sized arrays at tail, and #stackTop #successFlag at head)
28938 28940
With this build i placed methodCache ivar to come first.
28279 28264
so, difference is small but noticeable. Placing methodCache first gives roughly 1-2% speedup.
I don't really sure if this worth experimenting at all. And i'm lacking of knowledge of different CPU/compiler details to predict that these changes will take effect on different CPUs (mine is AMD Athlon series).
If you having any ideas, concerning given changes, or any other optimizations which can probably improve speed, please feel free to uncover details/guidelines.
-- Best regards, Igor Stasenko AKA sig.
-- = = = ======================================================================== John M. McIntosh johnmci@smalltalkconsulting.com Corporate Smalltalk Consulting Ltd. http://www.smalltalkconsulting.com = = = ========================================================================