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[spec] Additional possible implementation limits #2207

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11 changes: 8 additions & 3 deletions document/core/appendix/implementation.rst
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Expand Up @@ -56,14 +56,16 @@ An implementation may impose restrictions on the following dimensions of a modul
* the nesting depth of :ref:`structured control instructions <syntax-instr-control>`
* the number of :ref:`label indices <syntax-labelidx>` in a |BRTABLE| instruction
* the number of instructions in a :ref:`constant <valid-constant>` :ref:`expression <syntax-expr>`
* the number of instructions between a :ref:`branch instruction <syntax-br>` and its target
* the nesting depth of blocks between a :ref:`branch instruction <syntax-br>` and its target
* the length of the array in a |ARRAYNEWFIXED| instruction
* the length of an :ref:`element segment <syntax-elem>`
* the length of a :ref:`data segment <syntax-data>`
* the length of a :ref:`name <syntax-name>`
* the range of :ref:`characters <syntax-char>` in a :ref:`name <syntax-name>`

If the limits of an implementation are exceeded for a given module,
then the implementation may reject the :ref:`validation <valid>`, compilation, or :ref:`instantiation <exec-instantiation>` of that module with an embedder-specific error.
then the implementation may reject the :ref:`validation <valid>`, compilation, or :ref:`instantiation <exec-instantiation>` of that module with an implementation-specific error.

.. note::
The last item allows :ref:`embedders <embedder>` that operate in limited environments without support for
Expand Down Expand Up @@ -142,11 +144,14 @@ Restrictions on the following dimensions may be imposed during :ref:`execution <
* the number of :ref:`frames <syntax-frame>` on the :ref:`stack <stack>`
* the number of :ref:`labels <syntax-label>` on the :ref:`stack <stack>`
* the number of :ref:`values <syntax-val>` on the :ref:`stack <stack>`
* the number of :ref:`handlers <syntax-handler>` on the :ref:`stack <stack>`
* the number of :ref:`labels <syntax-val>` dropped from the :ref:`stack <stack>` by a :ref:`branch instruction <syntax-br>`
* the number of :ref:`values <syntax-val>` dropped from the :ref:`stack <stack>` by a :ref:`branch instruction <syntax-br>`

If the runtime limits of an implementation are exceeded during execution of a computation,
then it may terminate that computation and report an embedder-specific error to the invoking code.
then it may terminate that computation and report an implementation-specific error to the invoking code.

Some of the above limits may already be verified during instantiation, in which case an implementation may report exceedance in the same manner as for :ref:`syntactic limits <impl-syntax>`.
Some of the above limits may already be verified during :ref:`validation <valid>` or :ref:`instantiation <exec-instantiation>`, in which case an implementation may report exceedance in the same manner as for :ref:`syntactic limits <impl-syntax>`.

.. note::
Concrete limits are usually not fixed but may be dependent on specifics, interdependent, vary over time, or depend on other implementation- or embedder-specific situations or events.
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