[Python-Dev] Bytecode idea

[Christian Tismer]

Christian Tismer wrote:
[much about bytecode, folding, ...]

Well, to stop myself blathering about bytecode and
how to make it different, which I'm expected to do
other things or at least sleep, I can't resist
to throw this rigorous idea into py-dev, which
struck me at 4:30 in the morning:

WHat would happen if we re-structured the ceval
loop this way:
We look at every similar pattern and group together
all the opcodes which have a similar pattern.
By "pattern" I mean the surrounding calls of simple
things like stack operations before and after, and
error handling.

So what I would like to try is to merge all opcodes
which have exactly the same pattern into one case,
which then does the common preparation stuff and
postprocessing including error handling, but internally
dispatches on the function to be called.

No idea if it may be an improvement, but just let me throw
it in.
Example:

		case UNARY_POSITIVE:
			v = POP();
			x = PyNumber_Positive(v);
			Py_DECREF(v);
			PUSH(x);
			if (x != NULL) continue;
			break;

		case UNARY_NEGATIVE:
			v = POP();
			x = PyNumber_Negative(v);
			Py_DECREF(v);
			PUSH(x);
			if (x != NULL) continue;
			break;
...
		case UNARY_CONVERT:
			v = POP();
			x = PyObject_Repr(v);
			Py_DECREF(v);
			PUSH(x);
			if (x != NULL) continue;
			break;

		case UNARY_INVERT:
			v = POP();
			x = PyNumber_Invert(v);
			Py_DECREF(v);
			PUSH(x);
			if (x != NULL) continue;
			break;

is converted into

		case UNARY_POSITIVE:
		case UNARY_NEGATIVE:
		case UNARY_CONVERT:
		case UNARY_INVERT:
			v = POP();
			switch (opcode) {
				case UNARY_POSITIVE:
					x = PyNumber_Positive(v); break;
				case UNARY_NEGATIVE:
					x = PyNumber_Negative(v); break;
				case UNARY_CONVERT:
					x = PyObject_Repr(v); break;
				case UNARY_INVERT:
					x = PyNumber_Invert(v);
			}
			Py_DECREF(v);
			PUSH(x);
			if (x != NULL) continue;
			break;

Maybe it also makes sense to use indexing into a static
array, instead of the case construct. Note that there
can be one single such table for all opcodes and all cases,
since opcodes are still disjoint. It depends where this
table is stored and if this can get in the cache.

While I don't know if this really makes the interpreter
more efficient, at least it makes it shorter to read
and maybe easier to maintain.

Ok ok, I will sleep now :-)

cheers - chris

-- 
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