I've been saying that we can convert this:
{
var x: int
break-if-=
...
}
..into this:
{
68/push 0/imm32
{
0f 84/jump-if-= break/disp32
...
}
81 0/subop/add %esp 4/imm32
}
All subsequent instructions go into a nested block, so that they can be
easily skipped without skipping the stack cleanup.
However, I've been growing aware that this is a special case. Most of the
time we can't use this trick:
for loops
for non-local breaks
for non-local loops
In most cases we need to figure out all the intervening variables on the
stack and emit code to clean them up.
And now it turns out even for local breaks like above, the trick doesn't
work. Consider what happens when there's a loop later in the block:
{
var x: int
break-if-=
...
}
If we emitted a nested block for the break, the local loop would become
non-local. So we replace one kind of state with another.
Easiest course of action is to just emit the exact same cleanup code for
all conditional branches.
Turns out we can't handle them like conditional loops.
This function to emit cleanup code for jumps is getting quite terrible.
I don't yet know what subsidiary abstractions it needs.
Start pushing dummy vars for labels on the stack as we encounter them.
This won't affect cleanup code, but will make it easy to ensure that jumps
are well-structured.
Clean up data structures and eliminate the notion of named blocks.
Named blocks still exist in the Mu language. But they get parsed into a
uniform block data structure, same as unamed blocks.
This will come in handy for the remaining cases where we need to clean
up locals on the stack:
loop after var
non-local break with vars in intervening blocks
non-local loop with vars in intervening blocks
It was possible for an instruction to write out of bounds of the memory
data structure. Most of the time this worked fine. However if the block
ever got resized and moved the out-of-bounds bytes no longer went along.
Before:
we detected labels using a '$' at the start of an arg, and turned them
into literals.
After:
we put labels on the var stack and let the regular lookup of the var
stack handle labels.
This adds complexity in one place and removes it from another. The crucial
benefit is that it allows us to store a block depth for each label. That
will come in handy later.
All this works only because of a salubrious coincidence: Mu labels are
always at the start of a block, and jumps always refer to the name at the
start of a block, even when the jump is in the forwards direction. So we
never see label uses before definitions.
Note on CI: this currently only works natively, not emulated.
Continuing to think about how to translate vars in a block when they're
followed by early exits. To clean up everything between a statement and
a target label, we need to know somehow the depth the target is defined
at.
So far we only handle unlabeled break instructions correctly. That part
is elegance itself. But the rest will need more work:
a) For labeled breaks we need to insert code to unwind all intervening
blocks.
b) For unlabeled loops we need to insert code to unwind the current block
and then loop.
c) For labeled loops we need to insert code to unwind all intervening blocks
and then loop.
Is this even worth doing? I think so. It's pretty common for a conditional
block inside a loop to 'continue'. That requires looping to somewhere non-local.
Kartik Agaram