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n_popped, n_pushed is the wrong abstraction for stack effect #105034
Description
The code generator produces metadata for use by the compiler that tells it how many stack entries were pushed and popped by an opcode. These two quantities are subtracted to compute the net stack effect. (The metadata is in the form of functions _PyOpcode_num_popped and _PyOpcode_num_pushed in opcode_metadata.h.)
But what we really need to know, if we want accurate information about what an opcode (or micro-op) does to the stack, is three numbers: how low it goes, how high it goes, and where it ends. For example, if we have a micro-op that pops two entries and then pushes one, and another that pushes two and then pops one, and we combine them into a macro, the macro's net effect is zero, but the lowest point is two below the initial stack level, and the highest point is one above it.
There are various ways to express this, but there is no way to express all this information with just two numbers.
Note that the relevant calculation is already present in the generator, as part of analyze_macro().
Activity
gvanrossum commentedon May 30, 2023
I might be wrong here. I'm ending up doing the equivalent of
So two numbers (
lowannet) would seem to be enough...markshannon commentedon May 30, 2023
Abstractly, each instruction, or op, pops some number of values, then either fails or pushes some other number of values.
Two numbers are sufficient.
For example, instruction
Iis composed of opAand opB. OpApops 2 values, and pushes 1. OpBpops 2 values and pushes 2, the net effect is thatIpops 3 values and pushes 2.You need to model the stack, rather than just adding up values.
gvanrossum commentedon May 30, 2023
The tricky bits happen when the stack effect depends on
oparg(e.g.BUILD_SET). The stack effect must then be represented symbolically. I don't know if there are any of those that need to be split up though -- will do an audit later.gvanrossum commentedon May 30, 2023
Fortunately there are (as yet) no ops with variable-size stack effects. So in #104909 we can use Mark's algorithm.
There is a hypothetical case where the tuple
(popped, pushed)isn't enough for the computation of maximum stack level needed: considerIcomposed ofA+B+C, whereApushes 1,Bpops 2, andCpushes 1. The net effect is 0, but we need to know that this requires at least one item on the stack at the beginning, and we also need to know that it requires room for an extra item. However, this case doesn't occur in practice.carljm commentedon May 30, 2023
Does it? Doesn't the cases generator convert temporaries pushed to the stack by one op and popped by the next into temporary variables, meaning the
A+B+Cexample would not actually use any additional stack (though it would temporarily consume one item from the stack.)gvanrossum commentedon Jun 13, 2023
Let's close this since we don't need it.