📖2015 LLVM Developers’ Meeting: Joseph Groff & Chris Lattner “Swift's High-Level IR: A Case Study..."

even though C/C++ are pretty low-level languages, the gap between C/C++ and LLVM IR is still great

LLVM IR is too low-level for language analysis (uninitialized, unreachable, etc), so clang developed a side pipeline

design of SIL

• very similar to LLVM IR

• high-level type system (including generics)

• $Swift.Int is just a structure struct Int { var value: Builtin.Int64 }

• constants are instructions

  • attached source location

  • no value/constant divide

• phi nodes → basic block arguments (jump with arguments)

  • more uniform IR

  • more natural

  • (maps nicely to CPS’s continuation call)

• builtin can reference LLVM instructions directly

SIL passes

• raw SIL + canonical SIL

• there are some mandatory optimizations that always guaranteed run

Lowering

• SILGen handles operational lowering

• IRGen handles type lowering & concretization of the target

SIL is a big investment: lowering, pass manager, inliner, testing infrastructure, etc. (lots of duplication)