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llvm-ir-and-passes

Use when reading LLVM IR, explaining SSA and PHI nodes, finding what -O2 changed, or running opt on a .ll or .bc file. Not for writing a pass plugin: use llvm-passes.

.devin/skills/llvm-ir-and-passes/SKILL.md

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# LLVM IR and passes

## Contract

| Field | Bound contract |
|---|---|
| Trigger | A user wants to know what `-O2` did to a function, which pass created an instruction or PHI, why a pass name is not found, why IR differs between two Clang versions, or how to read SSA, dominance, `undef`, and `poison` before writing a pass. |
| Authority | Reversible local: writes only `.ll` and `.bc` files under a scratch directory named in the report; rollback is deleting that directory. No remote mutation. |
| Side effect | Runs `clang`, `opt`, and `llvm-dis` on the supplied input and diffs the results. Project files are not modified. |
| Done | The IR question is answered with the IR itself as evidence: the instruction or block is quoted, the pass that produced or removed it is named from an `opt` run, and any version mismatch is stated. |

## Inputs

1. Source, `.ll`, or `.bc` input (required).