# SSA form

## Basic idea

In LLVM IR every register is assigned exactly once. To represent values that depend on which control-flow predecessor was taken, LLVM uses `phi` instructions at the start of basic blocks.

## Key formulas

* $\phi$-function: `%x = phi i32 [%a, %bb1], [%b, %bb2]`
* Dominance frontier $DF(B)$: where $\phi$-nodes are inserted.

See [SSA](/isithas-wiki/computer_science/programming_language_theory/codegen/single_static_assignment_form.md) LLVM does not allow multiple assignments to registers

LLVM offers the phi function to choose between values.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://isubasinghe.gitbook.io/isithas-wiki/software_engineering/frameworks/llvm/ssa.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.