tinysun212
commented
8 years ago Welcome to my git page.
For convenience, I'll suppose you are in Cygwin environment.
The swift compiler generates the object file (*.o) from the source file (*.swift). To build an executable, the system linker combines the generated object file, swift libraries (cygswiftCore.dll, cygswiftGlibc.dll, cygswiftSwiftOnoneSupport.dll, libFoundation.dll, etc) and other system libraries.
For example, if you compile this one line file with none optimization, the linker combines Hello.o, cygswiftCore.dll, cygswiftSwiftOnoneSupport.dll.
$ cat Hello.swift
print("Hello")
$ swiftc Hello.swift -Onone, and if you compile this Foundation sample file, the linker combines TestFoundation.o, cygswiftCore.dll, libFoundation.dll, cygswiftGlibc.dll, etc.
$ cat TestFoundation.swift
import Foundation
// Make an URLComponents instance
let swifty = NSURLComponents(string: "https://swift.org")!
// Print something useful about the URL
print("\(swifty.host!)")
$ swiftc TestFoundation.swift -OThe list of libraries to combine varies as the modules you imported in the source and varies as the compiler optimization option you used.
swift-autolink-extract extracts the list of libraries to combine from the generated object files, and pass the list to the linker through clang.
For this, the section name .swift1_autolink_entries for object file is promised between the swift compiler and swift-autolink-extract. The compiler saves the list of the libraries to the section, and swift-autolink-extract reads the section.
swift-autolink-extract function can be tested simply.
$ swiftc -c Hello.swift -Onone
$ swift-autolink-extract Hello.o
-lswiftCore
-lswiftSwiftOnoneSupport
$ swiftc -c TestFoundation.swift -O
$ swift-autolink-extract TestFoundation.o
-lswiftCore
-lFoundation
-lswiftGlibc
-lpthread
-lutil
-ldlYou can use the utility objdump to view the internal of the object files generated by the compiler.
$ objdump -s -j .swift1_autolink_entries Hello.o
$ objdump -s -j .swift1_autolink_entries TestFoundation.oIf you have any questions feel free to ask me.
mominul
Hi! I know that it isn't the right place to ask, but because you have recently work on autolink-extract driver. I have some curiosity about how swift compiler links compiled source code. As far as I figured When swiftc compiles files then it initiates swift-autolink-extract, after building *.autollink file then it initiates clang to communicate with linker.
But what does the swift-autolink-extract do and why swiftc initiates clang to pass arguments(?) to linker?
Thanks for helping!