Blocks
Another central concept when developing extensions is the concept of a block. In source code, a
block contains both a set of local variables, and a sequence of expressions. In the abstract syntax
tree, these two concepts are separate. The class lang:bs:Block is an expression that only contains
a set of variables that are visible inside the block. This class therefore focuses on managing the
variables and their lifetimes. Below is a summary of the most relevant members in the Block class:
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Constructor
Since blocks deal with name lookup, they need to know their parent scope. This is achieved by either supplying a
Scopethat represents the parent scope, or a reference to another block. The former is typically used to create the root block in a function, and the latter is typically used inside functions to create nested blocks. -
lookupThis variable contains a
BlockLookupthat inherits fromcore:lang:NameLookup. This class thus represents a node linked to the name tree, so that it is possible to create aScopeinstance that represents the variables visible inside a block in Basic Storm code. Note that it is only possible to traverse from theBlockLookupand upwards in the name tree: there are no pointers from the function entity itself into all blocks. The blocks only exist briefly, during compilation of a function. -
scopeContains a
Scopethat refers to the current block. It is possible to create aScopefrom thelookupmember, but it is provided as a convenience. -
blockCodeWhen overriding a
Block, theblockCodemember should be overridden instead of thecodemember. TheBlockitself overridescodeto generate code for initializing variables, manage blocks in the IR, and so on. Thus, overridingcodewould cause theBlockto generate incorrect code.There are two overloads of
blockCode. One that takes acore:asm:Blockas a parameter, and one that does not. The former is for cases when the current block wishes to have control of when the block begins (e.g. if it wishes to place some code before the block in the IR begins), and the latter is the normal case where theBlockhandles all initialization. -
addAdds a variable to the block. This is typically done automatically when creating instances of the
lang:bs:Varclass.
Many of the control structures inherit from Block directly, since they need to scope variables
created in the condition of an if statement or a while loop for example. Basic Storm also
provides a class that simply contains other statement. This class is called lang:bs:ExprBlock, and
it is the class used for ordinary blocks that appear in the Basic Storm source code. This class
simply extends the Block with an array that stores the expressions contained in the block. When
asked to generate code, it simply generates code for each of the expressions in turn, and any code
required to pass the result of the last expression back to the parent expression. The ExprBlock
also keeps track of unreachable code in the block and avoids generating dead code (e.g. after a
return statement).
The ExprBlock class has the following members that are relevant when developing extensions:
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addAdd an expression at the end of the block.
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insertInsert an expression at a particular position. Note: this is a linear operation.