> For the complete documentation index, see [llms.txt](https://isubasinghe.gitbook.io/isithas-wiki/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://isubasinghe.gitbook.io/isithas-wiki/software_engineering/frameworks/llvm/ssa.md).

# 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.