LLVM Passes

Big Idea

LLVM passes are operations performed between the frontend and the backend, typically for optimization. The advantage of doing operations in this space is that because it lies between the frontend and backend, it can operate on a range of languages and compile to a range of architectures.

Types of Passes

The two main types of passes are analysis passes which only collect information and do not change the IR while transformation passes change the IR. There are also different levels of scope passes can be conducted at which are as follows: loop, function, and module.

Example Analysis Passes

Analysis	Description
Range Analysis	Will finds the range of values a variable may assume
Scalar Evolution	Can determine how a variable value evolves in a loop statically
Dominator Tree	Will make a data structure that will tell you what parts of code must run before other parts

Example Transformation Passes

Analysis	Description
Dead Code Elimination	Finds parts of code that cannot logically execute and removes them
Constant Propagation	Can evaluate expressions with constants

Sources

• Introduction to LLVM Passes by CompilersLab