Refactoring plan of SILVisitor for LTO

[kateinoigakukun (Yuta Saito)]

Hi, everyone. Since last week, I had been prototyping @Arnold and @Michael_Gottesman 's architecture.

Here is my rough implementation of the architecture:

[Prototype] Thin LTO

Overview

Almost the same as LLVM's Thin LTO

1. Emit module summary
   • Add a new file type .swiftmodule.sumary.
   • It serializes a module's call graph, Witness Table, and VTable information.
   • The structure is similar to LLVM Thin LTO summary.
   • swift-frontend's option -emit-module-summary-path corresponds the emission.
   • This can be done in parallel.
2. Merge summaries
   • swift-frontend -cross-module-opt [module summary file...] links and merges multiple summaries.
   • And prepare for optimization at this phase (e.g. Marking dead functions)
   • This is sequential stage

• Performing Optimizations for each modules
  • Pass merged module summary to swift-frontend via -module-summary-path
  • My prototype implements only simple Dead Function Elimination.
  • This can be done in parallel

# 1. Emit module summary for 'module1' into './module1.swiftmodule.summary'
$ swift-frontend -emit-sib module1.swift \
                 -emit-module-summary-path module1.swiftmodule.summary \
                 -parse-as-library

$ swift-frontend -emit-module module1.swift -parse-as-library

# 2. Emit module summary for 'main' into './main.swiftmodule.summary'
$ swift-frontend -emit-sib main.swift \
                 -emit-module-summary-path main.swiftmodule.summary

# 3. Merge module summaries into one summary file, link them and mark dead functions
$ swift-frontend -cross-module-opt \
                 main.swiftmodule.summary module1.swiftmodule.summary \
                 -o merged-module.summary

# 4. Do Dead Function Elimination for 'module1' module using the combined module summary
$ sil-opt -emit-sil module1.sib \
          -module-summary-path merged-module.summary \
          --sil-cross-deadfuncelim

# 5. Do again for 'main' module.
$ sil-opt -emit-sil main.sib \
          -module-summary-path merged-module.summary \
          --sil-cross-deadfuncelim

Module Summary file format

The module summary file consists of call graph information and virtual method table information.

func myPrint(_ text: String) { ... }
public protocol Animal {
  func bark()
}

public struct Cat: Animal {
  public func bark() { myPrint("mew") }
}

public struct Dog: Animal {
  public func bark() { myPrint("bow")}
}

public func callBark<T: Animal>(_ animal: T) {
  animal.bark()
}

For example, this swift file would be summarized as: