[DNM/SILGen] Default argument generator visibility.

[johnno1962]

Hi Apple,

Thought I’d raise this PR as a possible change to make default argument generators non-hidden which would help me (in the long term) resolve an issue I’m seeing with “code Injection” for Swift. If a user tries to inject code which contains a call to a function using a default argument, InjectionIII fails as is shown in this issue. This is because while the function is public and accessible the default argument generators are "hidden" and the symbol is not available for dynamic linking which seems inconsistent.

Is there any chance it would be possible these functions be emitted public then this issue would not arise. @slavapestov, I see you made this commit for which I’m sure there are ABI “reasons” but could this be revisited specifically for default argument generators? At present I’m having to suggest users work around by running a program that patches object files to publish these symbols.

Cheers,

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)

[jckarter]

The PublicNonABI is intentional. Default argument generators are not part of the ABI, they have purely compile-time effect, and a new SDK is allowed to change the default, or add defaults to existing arguments, without affecting ABI.

[johnno1962]

Thanks for the response Joe. Would you entertain a patch that enabled this option with a command line switch or would that be too "niche"?

[jckarter]

What are you trying to do?

[johnno1962]

I have an open Source project https://github.com/johnno1962/InjectionIII which brings "Hot Reloading" to Swift apps by recompiling an individual source file into a .dylib, loading it and interposing new implementations on top of the old. One problem that crops up is that if you "inject" a file that uses a default argument, as their symbol has "hidden" visibility they are not visible to the dynamic linker and the load of the .dylib fails. I have a work-around of sorts but the real solution would be if the Swift compiler had an option such as "-export-default-arguments" people could use which changed the visibility as in this patch.