Merge pull request #5220 from ethereum/libjulia-to-libyul

Renames `libjulia` directory to `libyul` & namespace `dev::julia` to `dev::yul`

1 files changed, 157 insertions, 0 deletions

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+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.
+
+
+## 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.