BasicBlock

public struct BasicBlock : IRValue

extension BasicBlock: Equatable

A BasicBlock represents a basic block in an LLVM IR program. A basic block contains a sequence of instructions, a pointer to its parent block and its follower block, and an optional label that gives the basic block an entry in the symbol table. Because of this label, the type of every basic block is LabelType.

A basic block can be thought of as a sequence of instructions, and indeed its member instructions may be iterated over with a for-in loop. A well- formed basic block has as its last instruction a “terminator” that produces a transfer of control flow and possibly yields a value. All other instructions in the middle of the basic block may not be “terminator” instructions. Basic blocks are not required to be well-formed until code generation is complete.

Creating a Basic Block

By default, the initializer for a basic block merely creates the block but does not associate it with a function.

let module = Module(name: "Example")
let fun = builder.addFunction("example",
                              type: FunctionType([], VoidType()))

// This basic block is "floating" outside of a function.
let floatingBB = BasicBlock(name: "floating")
// Until we associate it with a function by calling `Function.append(_:)`.
fun.append(floatingBB)

A basic block may be created and automatically inserted at the end of a function by calling Function.appendBasicBlock(named:in:).

let module = Module(name: "Example")
let fun = builder.addFunction("example",
                              type: FunctionType([], VoidType()))

// This basic block is "attached" to the example function.
let attachedBB = fun.appendBasicBlock(named: "attached")

The Address of a Basic Block

Basic blocks (except the entry block) may have their labels appear in the symbol table. Naturally, these labels are associated with address values in the final object file. The value of that address may be accessed for the purpose of an indirect call or a direct comparisson by calling Function.address(of:) and providing one of the function’s child blocks as an argument. Providing any other basic block outside of the function as an argument value is undefined.

The Entry Block

The first basic block (the entry block) in a Function is special:

• The entry block is immediately executed when the flow of control enters its parent function.
• The entry block is not allowed to have predecessor basic blocks (i.e. there cannot be any branches to the entry block of a function).
• The address of the entry block is not a well-defined value.
• The entry block cannot have PHI nodes. This is enforced structurally, as the entry block can have no predecessor blocks to serve as operands to the PHI node.
• Static alloca instructions situated in the entry block are treated specially by most LLVM backends. For example, FastISel keeps track of static alloca values in the entry block to more efficiently reference them from the base pointer of the stack frame.