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Merge pull request #13501 from ethereum/document-unused-store-eliminator

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Document UnusedStoreEliminator
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Nikola Matić 2022-10-21 07:06:20 -05:00 committed by GitHub
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49
docs/internals/optimizer.rst
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@ -322,6 +322,7 @@ Abbreviation Full name
``a`` :ref:`SSA-transform`
``t`` :ref:`structural-simplifier`
``p`` :ref:`unused-function-parameter-pruner`
``S`` :ref:`unused-store-eliminator`
``u`` :ref:`unused-pruner`
``d`` :ref:`var-decl-initializer`
============ ===============================
@ -958,7 +959,7 @@ DeadCodeEliminator
This optimization stage removes unreachable code.
Unreachable code is any code within a block which is preceded by a
leave, return, invalid, break, continue, selfdestruct or revert.
leave, return, invalid, break, continue, selfdestruct, revert or by a call to a user-defined function that recurses infinitely.
Function definitions are retained as they might be called by earlier
code and thus are considered reachable.
@ -1121,6 +1122,52 @@ The step LiteralRematerialiser is not required for correctness. It helps deal wi
``function f(x) -> y { revert(y, y} }`` where the literal ``y`` will be replaced by its value ``0``,
allowing us to rewrite the function.
.. index:: ! unused store eliminator
.. _unused-store-eliminator:
UnusedStoreEliminator
^^^^^^^^^^^^^^^^^^^^^
Optimizer component that removes redundant ``sstore`` and memory store statements.
In case of an ``sstore``, if all outgoing code paths revert (due to an explicit ``revert()``, ``invalid()``, or infinite recursion) or
lead to another ``sstore`` for which the optimizer can tell that it will overwrite the first store, the statement will be removed.
However, if there is a read operation between the initial ``sstore`` and the revert, or the overwriting ``sstore``, the statement
will not be removed.
Such read operations include: external calls, user-defined functions with any storage access, and ``sload`` of a slot that cannot be
proven to differ from the slot written by the initial ``sstore``.
For example, the following code
.. code-block:: yul
{
let c := calldataload(0)
sstore(c, 1)
if c {
sstore(c, 2)
}
sstore(c, 3)
}
will be transformed into the code below after the Unused Store Eliminator step is run
.. code-block:: yul
{
let c := calldataload(0)
if c { }
sstore(c, 3)
}
For memory store operations, things are generally simpler, at least in the outermost yul block as all such
statements will be removed if they are never read from in any code path.
At function analysis level, however, the approach is similar to ``sstore``, as we do not know whether the memory location will
be read once we leave the function's scope, so the statement will be removed only if all code code paths lead to a memory overwrite.
Best run in SSA form.
Prerequisites: Disambiguator, ForLoopInitRewriter.
.. _equivalent-function-combiner:
EquivalentFunctionCombiner

11
libyul/optimiser/UnusedStoreEliminator.h
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@ -39,8 +39,15 @@ struct Dialect;
struct AssignedValue;
/**
* Optimizer component that removes sstore statements if they
* are overwritten in all code paths or never read from.
* Optimizer component that removes sstore and memory store statements if conditions are met for their removal.
* In case of an sstore, if all outgoing code paths revert (due to an explicit revert(), invalid(),
* or infinite recursion) or lead to another ``sstore`` for which the optimizer can tell that it will overwrite the first store,
* the statement will be removed.
*
* For memory store operations, things are generally simpler, at least in the outermost yul block as all such statements
* will be removed if they are never read from in any code path. At function analysis level however, the approach is similar
* to sstore, as we don't know whether the memory location will be read once we leave the function's scope,
* so the statement will be removed only if all code code paths lead to a memory overwrite.
*
* The m_store member of UnusedStoreBase is only used with the empty yul string
* as key in the first dimension.