Question
Answer and Explanation
LLVM (Low Level Virtual Machine) is a powerful compiler infrastructure that uses a specific structure to store different kinds of data. Understanding this structure is crucial for anyone working with LLVM. What data resides where?
1. LLVM Modules:
- At the highest level, an LLVM Module contains all the information for a single compilation unit. This includes:
- Functions: Represents code, including instructions, basic blocks, and metadata.
- Global Variables: Global data accessible from anywhere in the module.
- Symbol Table: A mapping of names to LLVM values (functions, global variables, etc.).
- Metadata: Information about the code, such as debug information, source locations, and optimization hints.
2. Functions:
- Functions contain the executable code and associated information:
- Basic Blocks: A sequence of instructions that execute sequentially. Each function contains at least one basic block.
- Instructions: The fundamental operations that make up the code. These can include arithmetic operations, memory access, control flow, etc.
- Argument List: The list of arguments that the function accepts.
- Function Attributes: Metadata that describes the function's behavior, such as calling convention, optimization attributes, and other properties.
3. Basic Blocks:
- Basic blocks store a sequence of instructions:
- Instruction List: An ordered list of LLVM instructions.
- Terminator Instruction: The last instruction in a basic block, which determines control flow (e.g., branch, return).
4. Instructions:
- Instructions represent individual operations and hold:
- Opcode: Specifies the type of operation to be performed (e.g., addition, subtraction, load, store).
- Operands: The input values required by the instruction. These can be constants, registers, or other instructions.
- Type: The data type of the result of the instruction.
- Metadata: Debug information, optimization hints, and other annotations associated with the instruction.
5. Global Variables:
- Global variables represent data that can be accessed from anywhere in the module:
- Type: The data type of the variable.
- Initializer: The initial value of the variable.
- Linkage: Determines the visibility of the variable between modules (e.g., external, internal).
- Constant Flag: Indicates whether the variable is read-only.
6. Metadata:
- Metadata provides additional information about the code that doesn't directly affect its execution but is useful for debugging, optimization, and other analyses:
- Debug Information: Source location information, variable names, and other data used by debuggers.
- Optimization Hints: Hints to the optimizer about likely execution paths, branch probabilities, and other performance-related information.
- Custom Annotations: User-defined metadata that can be used for various purposes.
In Summary:
- Modules contain everything related to a compilation unit: functions, global variables, metadata.
- Functions encapsulate code as a collection of basic blocks and instructions, along with argument lists and attributes.
- Basic Blocks hold sequences of instructions, terminated by a control-flow instruction.
- Instructions define the individual operations performed by the code, including their operands and types.
- Global Variables store data that can be accessed throughout the module.
- Metadata provides additional information about the code for debugging, optimization, and other analyses.
Understanding where data is stored within the LLVM infrastructure allows developers to effectively work with and optimize their code using LLVM's powerful toolset.