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Make continuations consistently "shallow" (#874)

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Prior to this change, `__await` would make a deep copy of the continuation stack, but shallow-copy the individual stack frames within it. As a result, continuations appeared to have shallow semantics so long as the continuation stack had only a single frame.

This change also removes an obsolete test from the brief period when we intended continuations to have deep-copy semantics, which has been passing basically by accident.
This commit is contained in:
Geoff Romer
2021-10-11 14:57:09 -07:00
committed by GitHub
parent 9211c98951
commit d177a08e01
5 changed files with 34 additions and 52 deletions
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executable_semantics/interpreter/interpreter.cpp
+12 -9
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@@ -1002,11 +1002,12 @@ auto Interpreter::StepStmt() -> Transition {
todo.Push(arena->New<StatementAction>(
arena->New<Return>(arena, stmt->source_loc())));
todo.Push(arena->New<StatementAction>(cast<Continuation>(*stmt).Body()));
auto continuation_stack = arena->New<std::vector<Nonnull<Frame*>>>();
auto continuation_frame =
arena->New<Frame>("__continuation", scopes, todo);
continuation_stack->push_back(continuation_frame);
Address continuation_address =
heap.AllocateValue(arena->New<ContinuationValue>(
std::vector<Nonnull<Frame*>>({continuation_frame})));
heap.AllocateValue(arena->New<ContinuationValue>(continuation_stack));
// Store the continuation's address in the frame.
continuation_frame->continuation = continuation_address;
// Bind the continuation object to the continuation variable
@@ -1031,11 +1032,11 @@ auto Interpreter::StepStmt() -> Transition {
arena->New<TupleLiteral>(stmt->source_loc())));
frame->todo.Push(ignore_result);
// Push the continuation onto the current stack.
const std::vector<Nonnull<Frame*>>& continuation_vector =
cast<ContinuationValue>(*act->results()[0]).Stack();
for (auto frame_iter = continuation_vector.rbegin();
frame_iter != continuation_vector.rend(); ++frame_iter) {
stack.Push(*frame_iter);
std::vector<Nonnull<Frame*>>& continuation_vector =
*cast<ContinuationValue>(*act->results()[0]).Stack();
while (!continuation_vector.empty()) {
stack.Push(continuation_vector.back());
continuation_vector.pop_back();
}
return ManualTransition{};
}
@@ -1048,8 +1049,10 @@ auto Interpreter::StepStmt() -> Transition {
paused.push_back(stack.Pop());
} while (paused.back()->continuation == std::nullopt);
// Update the continuation with the paused stack.
heap.Write(*paused.back()->continuation,
arena->New<ContinuationValue>(paused), stmt->source_loc());
const auto& continuation = cast<ContinuationValue>(
*heap.Read(*paused.back()->continuation, stmt->source_loc()));
CHECK(continuation.Stack()->empty());
*continuation.Stack() = std::move(paused);
return ManualTransition{};
}
}