Add Attributes Metadata

Skill: Add Attributes and Metadata to LLVM 22 IR

Use this skill to annotate your generated IR so the optimizer can make better decisions — inlining, alias analysis, vectorization, branch layout, and more.

---

Step 0 — Why this matters

Correct attributes are one of the highest-ROI improvements you can make to a frontend:

• nounwind eliminates EH cleanup code on every call.
• nocapture on parameters enables escape analysis and alias analysis.
• noundef enables constant folding and value-range propagation.
• Loop metadata controls auto-vectorization and unrolling.

---

Step 1 — Function attributes

Set once per function, before or after emitting its body:

// Never throws — most important; enables call-site EH cleanup elision
F->addFnAttr(llvm::Attribute::NoUnwind);

// Reads no memory (pure function)
F->addFnAttr(llvm::Attribute::ReadNone);

// Reads memory but never writes
F->addFnAttr(llvm::Attribute::ReadOnly);

// Only accesses memory via its own arguments (no globals)
F->addFnAttr(llvm::Attribute::ArgMemOnly);

// Does not recurse (direct or indirect)
F->addFnAttr(llvm::Attribute::NoRecurse);

// Always returns (no infinite loops, no exit() calls)
F->addFnAttr(llvm::Attribute::WillReturn);

// Never returns (e.g., panic(), abort()) — always emit unreachable after call
F->addFnAttr(llvm::Attribute::NoReturn);

// Rarely called — deprioritize in layout
F->addFnAttr(llvm::Attribute::Cold);

// On a call site (not function definition):
CI->addFnAttr(llvm::Attribute::NoUnwind);