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[ares] simplify unifying equality, remove redundant checks, and better document

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This commit is contained in:
Edward Amsden 2023-02-02 13:35:06 -06:00
parent e04329f2b6
commit 06fe914c34
1 changed files with 91 additions and 91 deletions
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182
rust/ares/src/mem.rs
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@ -622,122 +622,122 @@ impl NockStack {
}
}
/** Unifying equality compares nouns for equality in the obvious way, and replaces a noun pointing
* to a more junior allocation with a noun pointing to a more senior allocation if the two are
* equal.
/** Unifying equality compares nouns for equality structurally. It *unifies* noun representations
* by rewriting a reference to the more junior (resident in a more recently allocated stack frame)
* noun to a reference to the more senior noun, but does so partially and recursively, so as
* subnouns are discovered to be equal, they too are unified. Thus even if the overall result is
* disequality, some equal subnouns may have been unified.
*
* This function is unsafe because it demands that all atoms be normalized: direct and indirect atoms
* will be considered non-equal without comparing their values, and indirects of different sizes
* will be considered non-equal.
*
* TODO: we really should try to tie lifetimes into the stack and use mut references instead of raw
* pointers wherever we can, but this is hard and delaying progress right now
*/
pub unsafe fn unifying_equality(stack: &mut NockStack, a: *mut Noun, b: *mut Noun) -> bool {
/* This version of unifying equality is not like that of vere.
* Vere does a tree comparison (accelerated by pointer equality and short-circuited by mug
* equality) and then unifies the nouns at the top level if they are equal.
*
* Here we recursively attempt to unify nouns. Pointer-equal nouns are already unified.
* Disequal mugs again short-circuit the unification and equality check.
*
* Since we expect atoms to be normalized, direct and indirect atoms do not unify with each
* other. For direct atoms, no unification is possible as there is no pointer involved in their
* representation. Equality is simply direct equality on the word representation. Indirect
* atoms require equality first of the size and then of the memory buffers' contents.
*
* Cell equality is tested (after mug and pointer equality) by attempting to unify the heads and tails,
* respectively, of cells, and then re-testing. If unification succeeds then the heads and
* tails will be pointer-wise equal and the cell itself can be unified. A failed unification of
* the head or the tail will already short-circuit the unification/equality test, so we will
* not return to re-test the pointer equality.
*
* When actually mutating references for unification, we must be careful to respect seniority.
* A reference to a more junior noun should always be replaced with a reference to a more
* senior noun, *never vice versa*, to avoid introducing references from more senior frames
* into more junior frames, which would result in incorrect operation of the copier.
*/
stack.push(1);
stack.save_prev_stack_pointer_to_local(0);
*(stack.alloc_in_previous_frame()) = (a, b);
loop {
if stack.prev_stack_pointer_equals_local(0) {
break;
} else {
let (x, y): (*mut Noun, *mut Noun) = *(stack.top_in_previous_frame());
if (*x).raw_equals(*y) {
break;
};
let (x, y): (*mut Noun, *mut Noun) = *(stack.top_in_previous_frame());
if (*x).raw_equals(*y) {
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
};
if let (Ok(x_alloc), Ok(y_alloc)) = (
// equal direct atoms return true for raw_equals()
(*x).as_allocated(),
(*y).as_allocated(),
) {
if let (Some(x_mug), Some(y_mug)) = (x_alloc.get_cached_mug(), y_alloc.get_cached_mug())
{
if x_mug != y_mug {
break; // short-circuit, the mugs differ therefore the nouns must differ
}
};
match (
(*x).as_either_direct_allocated(),
(*y).as_either_direct_allocated(),
) {
(Left(x_direct), Left(y_direct)) => {
if x_direct.data() == y_direct.data() {
match (x_alloc.as_either(), y_alloc.as_either()) {
(Left(x_indirect), Left(y_indirect)) => {
let x_as_ptr = x_indirect.to_raw_pointer();
let y_as_ptr = y_indirect.to_raw_pointer();
if x_indirect.size() == y_indirect.size()
&& memcmp(
x_indirect.data_pointer() as *const c_void,
y_indirect.data_pointer() as *const c_void,
indirect_raw_size(x_indirect),
) == 0
{
let (_senior, junior) = senior_pointer_first(stack, x_as_ptr, y_as_ptr);
// unify
if x_as_ptr == junior {
*x = *y;
} else {
*y = *x;
}
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else {
break;
}
}
(Right(x_alloc), Right(y_alloc)) => {
match (x_alloc.get_cached_mug(), y_alloc.get_cached_mug()) {
(Some(x_mug), Some(y_mug)) => {
if x_mug != y_mug {
break; // short-circuit, the mugs differ therefore the nouns must differ
}
}
_ => {}
};
match (x_alloc.as_either(), y_alloc.as_either()) {
(Left(x_indirect), Left(y_indirect)) => {
let x_as_ptr = x_indirect.to_raw_pointer();
let y_as_ptr = y_indirect.to_raw_pointer();
if x_as_ptr == y_as_ptr {
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else if x_indirect.size() == y_indirect.size()
&& memcmp(
x_indirect.data_pointer() as *const c_void,
y_indirect.data_pointer() as *const c_void,
indirect_raw_size(x_indirect),
) == 0
{
let (_senior, junior) =
senior_pointer_first(stack, x_as_ptr, y_as_ptr);
// unify
if x_as_ptr == junior {
*x = *y;
} else {
*y = *x;
}
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else {
break;
}
}
(Right(x_cell), Right(y_cell)) => {
let x_as_ptr = x_cell.to_raw_pointer();
let y_as_ptr = y_cell.to_raw_pointer();
if x_as_ptr == y_as_ptr {
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else {
if x_cell.head().raw_equals(y_cell.head())
&& x_cell.tail().raw_equals(y_cell.tail())
{
let (_senior, junior) =
senior_pointer_first(stack, x_as_ptr, y_as_ptr);
if x_as_ptr == junior {
*x = *y;
} else {
*y = *x;
}
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else {
/* THIS ISN'T AN INFINITE LOOP
* If we discover a disequality in either side, we will
* short-circuit the entire loop and reset the work stack.
*
* If both sides are equal, then we will discover pointer
* equality when we return and unify the cell.
*/
*(stack.alloc_in_previous_frame()) =
(x_cell.tail_as_mut(), y_cell.tail_as_mut());
*(stack.alloc_in_previous_frame()) =
(x_cell.head_as_mut(), y_cell.head_as_mut());
continue;
}
}
}
(_, _) => {
break;
(Right(x_cell), Right(y_cell)) => {
let x_as_ptr = x_cell.to_raw_pointer();
let y_as_ptr = y_cell.to_raw_pointer();
if x_cell.head().raw_equals(y_cell.head())
&& x_cell.tail().raw_equals(y_cell.tail())
{
let (_senior, junior) = senior_pointer_first(stack, x_as_ptr, y_as_ptr);
if x_as_ptr == junior {
*x = *y;
} else {
*y = *x;
}
stack.reclaim_in_previous_frame::<(*mut Noun, *mut Noun)>();
continue;
} else {
/* THIS ISN'T AN INFINITE LOOP
* If we discover a disequality in either side, we will
* short-circuit the entire loop and reset the work stack.
*
* If both sides are equal, then we will discover pointer
* equality when we return and unify the cell.
*/
*(stack.alloc_in_previous_frame()) =
(x_cell.tail_as_mut(), y_cell.tail_as_mut());
*(stack.alloc_in_previous_frame()) =
(x_cell.head_as_mut(), y_cell.head_as_mut());
continue;
}
}
(_, _) => {
break;
break; // cells don't unify with atoms
}
}
} else {
break; // direct atom not raw equal, so short circuit
}
}
stack.restore_prev_stack_pointer_from_local(0);