Clean up how we reclaim local scopes.
It used to work like this (commit 3216):
1. Update refcounts of products after every instruction, EXCEPT:
a) when instruction is a non-primitive and the callee starts with
'local-scope' (because it's already not decremented in 'return')
OR:
b) when instruction is primitive 'next-ingredient' or
'next-ingredient-without-typechecking', and its result is saved to a
variable in the default space (because it's already incremented at
the time of the call)
2. If a function starts with 'local-scope', force it to be reclaimed
before each return. However, since locals may be returned, *very
carefully* don't reclaim those. (See the logic in the old `escaping`
and `should_update_refcount` functions.)
However, this approach had issues. We needed two separate commands for
'local-scope' (reclaim locals on exit) and 'new-default-space'
(programmer takes charge of reclaiming locals). The hard-coded
reclamation duplicated refcounting logic. In addition to adding
complexity, this implementation failed to work if a function overwrites
default-space after setting up a local-scope (the old default-space is
leaked). It also fails in the presence of continuations. Calling a
continuation more than once was guaranteed to corrupt memory (commit
3986).
After this commit, reclaiming local scopes now works like this:
Update refcounts of products for every PRIMITIVE instruction.
For non-primitive instructions, all the work happens in the `return`
instruction:
increment refcount of ingredients to `return`
(unless -- one last bit of ugliness -- they aren't saved in the
caller)
decrement the refcount of the default-space
use existing infrastructure for reclaiming as necessary
if reclaiming default-space, first decrement refcount of each
local
again, use existing infrastructure for reclaiming as necessary
This commit (finally!) completes the bulk[1] of step 2 of the plan in
commit 3991. It was very hard until I gave up trying to tweak the
existing implementation and just test-drove layer 43 from scratch.
[1] There's still potential for memory corruption if we abuse
`default-space`. I should probably try to add warnings about that at
some point (todo in layer 45).
I'm dropping all mention of 'recipe' terminology from the Readme. That
way I hope to avoid further bike-shedding discussions while I very
slowly decide on the right terminology with my students.
I could be smarter in my error messages and use 'recipe' when code uses
it and 'function' otherwise. But what about other words like ingredient?
It would all add complexity that I'm not yet sure is worthwhile. But I
do want separate experiences for veteran programmers reading about Mu on
github and for people learning programming using Mu.
This is the one major refinement on the C programming model I'm planning
to introduce in mu. Instead of Rust's menagerie of pointer types and
static checking, I want to introduce just one new type, and use it to
perform ref-counting at runtime.
So far all we're doing is updating new's interface. The actual
ref-counting implementation is next.
One implication: I might sometimes need duplicate implementations for a
recipe with allocated vs vanilla addresses of the same type. So far it
seems I can get away with just always passing in allocated addresses;
the situations when you want to pass an unallocated address to a recipe
should be few and far between.
All primitives now always write to all their products. If a product is
not used that's fine, but if an instruction seems to expect too many
products mu will complain.
In the process, many primitives can operate on more than two ingredients
where it seems intuitive. You can add or divide more than two numbers
together, copy or negate multiple corresponding locations, etc.
There's one remaining bit of ugliness. Some instructions like
get/get-address, index/index-address, wait-for-location, these can
unnecessarily load values from memory when they don't need to.
Useful vim commands:
%s/ingredients\[\([^\]]*\)\]/ingredients.at(\1)/gc
%s/products\[\([^\]]*\)\]/products.at(\1)/gc
.,$s/\[\(.\)]/.at(\1)/gc
Swap printing generalized objects using arc's infrastructure to be the
$-prefixed debug helper, while the erstwhile $print-key-to-host becomes
the primitive print-character to host.
This will let me swap in a fake in tests.
Still hacky, though. I'm sure I'm not managing the parameter right in
the chessboard app.
And then there's the question of whether it should also appear as an
output operand.
But it's a start. And using nil to mean 'real' is a reasonable
convention.
If I ever need to handle multiple screens perhaps we'll have to switch
to 1:literal/terminal and 2:literal/terminal, etc. But those are equally
easy to guard on.
'default-scope' is now 'default-space'
'closure-generator' is now 'next-space-generator'
The connection to high-level syntax for closures is now tenuous, so
we'll call the 'outer scope' the 'next space'.
So, let's try to create a few sentences with all these related ideas:
Names map to addresses offset from a default-space when it's provided.
Spaces can be strung together. The zeroth variable points to the next
space, the one that is accessed when a variable has /space:1.
To map a name to an address in the next space, you need to know what
function generated that space. A corollary is that the space passed in
to a function should always be generated by a single function.
Spaces can be used to construct lexical scopes and objects.
Kartik K. Agaram