[Vm-dev] Performance of primitiveFailFor: and use of primFailCode

David T. Lewis lewis at mail.msen.com
Sun May 22 15:54:18 UTC 2011


I have been trying to gradually update trunk VMMaker to better align
with oscog VMMaker (an admittedly slow process, but hopefully still
worthwhile).  I have gotten the interpreter primitives moved into class
InterpreterPrimitives and verified no changes to generated code. This
greatly reduces the clutter in class Interpreter, so it's a nice change
I think.

My next step was to update all of the primitives to use the #primitiveFailFor:
idiom, in which the successFlag variable is replaced with primFailCode
(integer value, 0 for success, 1, 2, 3... for failure codes). This would
get us closer to the point where the standard interpreter and stack/cog
would use a common set of primitives. A lot of changes were required for
this, but the resulting VM works fine ... except for performance.

On a standard interpreter, use of primFailCode seems to result in a
nearly 12% reduction in bytecode performance as measured by tinyBenchmarks:

Standard interpreter (using successFlag):
  0 tinyBenchmarks. '439108061 bytecodes/sec; 15264622 sends/sec'
  0 tinyBenchmarks. '433164128 bytecodes/sec; 14740358 sends/sec'
  0 tinyBenchmarks. '445993031 bytecodes/sec; 15040691 sends/sec'
  0 tinyBenchmarks. '440999138 bytecodes/sec; 15052960 sends/sec'
  0 tinyBenchmarks. '445993031 bytecodes/sec; 14485815 sends/sec'

After updating the standard interpreter (using primFailCode):
  0 tinyBenchmarks. '393241167 bytecodes/sec; 14066256 sends/sec'
  0 tinyBenchmarks. '392036753 bytecodes/sec; 15040691 sends/sec'
  0 tinyBenchmarks. '393846153 bytecodes/sec; 14272953 sends/sec'
  0 tinyBenchmarks. '400625978 bytecodes/sec; 14991818 sends/sec'
  0 tinyBenchmarks. '393846153 bytecodes/sec; 15176750 sends/sec'

This is a much larger performance difference than I expected to see.
Actually I expected no measurable difference at all, and I was just
testing to verify this. But 12% is a lot, so I want to ask if I'm
missing something?

The changes to generated code generally take the form of:

Testing success status, original:
	if (successFlag) { ... }

Testing success status, new:
	if (foo->primFailCode == 0) { ... }

Setting failure status, original:
	successFlag = 0;

Setting failure status, new:
	if (foo->primFailCode == 0) {
		foo->primFailCode = 1;
	}

My approach to doing the updates was as follows:
- Replace all occurrences of "successFlag := true" with "self initPrimCall",
  which initialize primFailCode to 0.
- Replace all "successFlag := false" with "self primitiveFail".
- Replace all "successFlag ifTrue: [] ifFalse: []" with
  "self successful ifTrue: [] ifFalse: []".
- Update #primitiveFail, #failed and #success: to use primFailCode rather
  than successFlag.
- Remove successFlag variable.

Obviously I don't want to publish the code on SqS/VMMaker, but I can mail
an interp.c if anyone wants to see the gory details (It is too large to
post on this mailing list though).

Any advice appreciated. I suspect I'm missing something basic here.

Thanks,
Dave



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