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[spec] Add note about control stack invariant to algorithm #1498

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Jun 27, 2022
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[spec] Add note about control stack invariant to algorithm #1498

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2 changes: 2 additions & 0 deletions document/core/appendix/algorithm.rst
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Expand Up @@ -140,6 +140,8 @@ The type of the :ref:`label <syntax-label>` associated with a control frame is e

Finally, the current frame can be marked as unreachable.
In that case, all existing operand types are purged from the value stack, in order to allow for the :ref:`stack-polymorphism <polymorphism>` logic in :code:`pop_val` to take effect.
Because every function has an implicit outermost label that corresponds to an implicit block frame,
it is an invariant of the validation algorithm that there always is at least one frame on the control stack when validating an instruction, and hence, `ctrls[0]` is always defined.

.. note::
Even with the unreachable flag set, consecutive operands are still pushed to and popped from the operand stack.
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