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We have a for loop now.

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Chris Pressey 2018-03-14 16:45:35 +00:00
parent 96fbe03ce7
commit ed917566ea
2 changed files with 3 additions and 31 deletions
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HISTORY.md
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@ -4,6 +4,8 @@ History of SixtyPical
0.14
----
* Added the so-called "open-faced `for` loop", which spans a loop
variable over a range, the end of which is fixed.
* `--origin` and `--output-format` options added to reference compiler.
* "Tail position" is now more correctly determined for the purposes of
insisting that `goto` only appears in it.

32
README.md
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@ -67,37 +67,6 @@ Documentation
TODO
----
### `for`-like loop
We have range-checking in the abstract analysis now, but we lack practical ways
to use it.
We can `and` a value to ensure it is within a certain range. However, in the 6502
ISA the only register you can `and` is `A`, while loops are done with `X` or `Y`.
Insisting this as the way to do it would result in a lot of `TXA`s and `TAX`s.
What would be better is a dedicated `for` loop, like
for x in 0 to 15 {
// in here, we know the range of x is exactly 0-15 inclusive
// also in here: we are disallowed from changing x
}
However, this is slightly restrictive, and hides a lot.
However however, options which do not hide a lot, require a lot of looking at
(to ensure: did you increment the loop variable? only once? etc.)
The leading compromise so far is an "open-faced for loop", like
ld x, 15
for x downto 0 {
// same as above
}
This makes it a little more explicit, at least, even though the loop
decrementation is still hidden.
### Save registers on stack
This preserves them, so that, semantically, they can be used later even though they
@ -123,5 +92,6 @@ is probably NP-complete. But doing it adequately is probably not that hard.
* Automatic tail-call optimization (could be tricky, w/constraints?)
* Possibly `ld x, [ptr] + y`, possibly `st x, [ptr] + y`.
* Maybe even `copy [ptra] + y, [ptrb] + y`, which can be compiled to indirect LDA then indirect STA!
* Optimize `ld a, z` and `st a, z` to zero-page operations if address of z < 256.
[VICE]: http://vice-emu.sourceforge.net/