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Make local continuations direct style if possible (#777)
This PR tries to remove the overhead of CPS (and trampolining) for
direct style code.
Essentially, for the following code in CPS
```
let cont k(x, ks) = ...
if (cond) {
k(42, ks')
} else {
k(43, ks')
}
```
we are now generating the following JS:
```javascript
let x;
if (cond) {
x = 42
} else {
x = 43
};
...
```
This only works under the assumption that:
- `k` is not used somewhere in a more first-class manner (under a
different context, as part of a continuation passed to a function, etc.)
- `k` is always called with the currently in scope meta continuation
`ks'` (so that we can avoid assigning it -- this turns out to be
essential with the feature interaction of loops)
The interaction with loops was difficult to get right:
- turning recursive functions into direct style loops makes additional
continuations more local
- making a continuation more local turns more functions into loops
As a result, the function `parse_worker` (from the `parsing_dollars`
benchmark) is now a tight loop.
**Before the PR**
```javascript
function parse_worker_0(a_1, ks_7, k_4) {
const x_0 = i_0.value;
function k_3(c_0, ks_4) {
if (c_0 === (36)) {
return () => parse_worker_0((a_1 + (1)), ks_4, k_4);
} else if (c_0 === (10)) {
const x_1 = s_0.value;
s_0.value = (x_1 + a_1);
return () => parse_worker_0(0, ks_4, k_4);
} else {
return SHIFT(p_0, (k_5, ks_5, k_6) => k_6($effekt.unit, ks_5), ks_4, k_4);
}
}
if ((x_0 > n_0)) {
return SHIFT(p_0, (k_7, ks_6, k_8) => k_8($effekt.unit, ks_6), ks_7, (v_r_1, ks_8) =>
$effekt.emptyMatch());
} else {
const x_2 = j_0.value;
if (x_2 === (0)) {
const x_3 = i_0.value;
i_0.value = (x_3 + (1));
const x_4 = i_0.value;
j_0.value = x_4;
return () => k_3(10, ks_7);
} else {
const x_5 = j_0.value;
j_0.value = (x_5 - (1));
return () => k_3(36, ks_7);
}
}
}
```
**After the PR**
```javascript
function parse_worker_0(a_9, ks_293, k_236) {
parse_worker_0: while (true) {
const x_11 = i_13.value;
let c_0 = undefined;
if ((x_11 > n_17)) {
return SHIFT(p_32, (k_232, ks_292, k_233) =>
k_233($effekt.unit, ks_292), ks_293, (v_r_95, ks_294) =>
$effekt.emptyMatch());
} else {
const x_12 = j_0.value;
if (x_12 === (0)) {
const x_13 = i_13.value;
i_13.value = (x_13 + (1));
const x_14 = i_13.value;
j_0.value = x_14;
c_0 = 10;
} else {
const x_15 = j_0.value;
j_0.value = (x_15 - (1));
c_0 = 36;
}
}
if (c_0 === (36)) {
/* prepare tail call */
const a_8 = (a_9 + (1));
a_9 = a_8;
continue parse_worker_0;
} else if (c_0 === (10)) {
const x_16 = s_4.value;
s_4.value = (x_16 + a_9);
/* prepare tail call */
const a_10 = 0;
a_9 = a_10;
continue parse_worker_0;
} else {
return SHIFT(p_32, (k_234, ks_295, k_235) =>
k_235($effekt.unit, ks_295), ks_293, k_236);
}
}
}
```
Additionally, here are a few preliminary benchmark results:
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