solidity/libyul/optimiser

Note that the Yul optimiser is still in research phase. Because of that, the following description might not fully reflect the current or even planned state of the optimiser.

Yul Optimiser

The Yul optimiser consists of several stages and components that all transform the AST in a semantically equivalent way. The goal is to end up either with code that is shorter or at least only marginally longer but will allow further optimisation steps.

The optimiser currently follows a purely greedy strategy and does not do any backtracking.

Disambiguator

The disambiguator takes an AST and returns a fresh copy where all identifiers have names unique to the input AST. This is a prerequisite for all other optimiser stages. One of the benefits is that identifier lookup does not need to take scopes into account and we can basically ignore the result of the analysis phase.

All subsequent stages have the property that all names stay unique. This means if a new identifier needs to be introduced, a new unique name is generated.

Function Hoister

The function hoister moves all function definitions to the end of the topmost block. This is a semantically equivalent transformation as long as it is performed after the disambiguation stage. The reason is that moving a definition to a higher-level block cannot decrease its visibility and it is impossible to reference variables defined in a different function.

The benefit of this stage is that function definitions can be looked up more easily.

Function Grouper

The function grouper has to be applied after the disambiguator and the function hoister. Its effect is that all topmost elements that are not function definitions are moved into a single block which is the first statement of the root block.

After this step, a program has the following normal form:

{ I F... }

Where I is a block that does not contain any function definitions (not even recursively) and F is a list of function definitions such that no function contains a function definition.

Functional Inliner

The functional inliner depends on the disambiguator, the function hoister and function grouper. It performs function inlining such that the result of the inlining is an expression. This can only be done if the body of the function to be inlined has the form { r := E } where r is the single return value of the function, E is an expression and all arguments in the function call are so-called movable expressions. A movable expression is either a literal, a variable or a function call (or EVM opcode) which does not have side-effects and also does not depend on any side-effects.

As an example, neither mload nor mstore would be allowed.

Expression Splitter

The expression splitter turns expressions like add(mload(x), mul(mload(y), 0x20)) into a sequence of declarations of unique variables that are assigned sub-expressions of that expression so that each function call has only variables or literals as arguments.

The above would be transformed into

{
    let _1 := mload(y)
    let _2 := mul(_1, 0x20)
    let _3 := mload(x)
    let z := add(_3, _2)
}

Note that this transformation does not change the order of opcodes or function calls.

It is not applied to loop conditions, because the loop control flow does not allow this "outlining" of the inner expressions in all cases.

The final program should be in a form such that with the exception of loop conditions, function calls can only appear in the right-hand side of a variable declaration, assignments or expression statements and all arguments have to be constants or variables.

The benefits of this form are that it is much easier to re-order the sequence of opcodes and it is also easier to perform function call inlining. The drawback is that such code is much harder to read for humans.

Expression Joiner

This is the opposite operation of the expression splitter. It turns a sequence of variable declarations that have exactly one reference into a complex expression. This stage again fully preserves the order of function calls and opcode executions. It does not make use of any information concerning the commutability of opcodes; if moving the value of a variable to its place of use would change the order of any function call or opcode execution, the transformation is not performed.

Note that the component will not move the assigned value of a variable assignment or a variable that is referenced more than once.

Common Subexpression Eliminator

This step replaces a subexpression by the value of a pre-existing variable that currently has the same value (only if the value is movable), based on a syntactic comparison.

This can be used to compute a local value numbering, especially if the expression splitter is used before.

The expression simplifier will be able to perform better replacements if the common subexpression eliminator was run right before it.

Prerequisites: Disambiguator

Full Function Inliner

Rematerialisation

The rematerialisation stage tries to replace variable references by the expression that was last assigned to the variable. This is of course only beneficial if this expression is comparatively cheap to evaluate. Furthermore, it is only semantically equivalent if the value of the expression did not change between the point of assignment and the point of use. The main benefit of this stage is that it can save stack slots if it leads to a variable being eliminated completely (see below), but it can also save a DUP opcode on the EVM if the expression is very cheap.

The algorithm only allows movable expressions (see above for a definition) in this case. Expressions that contain other variables are also disallowed if one of those variables have been assigned to in the meantime. This is also not applied to variables where assignment and use span across loops and conditionals.

Unused Definition Pruner

If a variable or function is not referenced, it is removed from the code. If there are two assignments to a variable where the first one is a movable expression and the variable is not used between the two assignments (and the second is not inside a loop or conditional, the first one is not inside), the first assignment is removed.

This step also removes movable expression statements.

Function Unifier

Expression Simplifier

This step can only be applied for the EVM-flavoured dialect of Yul. It applies simple rules like x + 0 == x to simplify expressions.

Ineffective Statement Remover

This step removes statements that have no side-effects.

WebAssembly specific

Main Function

Changes the topmost block to be a function with a specific name ("main") which has no inputs nor outputs.

Depends on the Function Grouper.