[SR-3635] Swift stdlib gets wrong EH Personality on FreeBSD

[swift-ci]

Previous ID	SR-3635
Radar	None
Original Reporter	nether_cat (JIRA User)
Type	Bug

Environment

FreeBSD 11.0-RELEASE

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: 3a144b0e6f779d3d5ab04e4b3b28d9df

Issue Description:

Currently building Swift on FreeBSD leads to libswiftCore.so having an undefined symbol '__gnustep_objcxx_personality_v0' which is the Exception Handling Personality function from the GNUstep libobjc. This results in undefined symbol errors when trying to run the Swift REPL and when executing binaries build with swiftc as long as you don't link them to GNUstep's Objective-C library. As @belkadan stated in #4804 we shouldn't be linking it. I couldn't find the source for this problem yet. But I found https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/CGException.cpp to be an interesting place to start.

I also like to mention, that my builds on Ubuntu 16.04 seem not to use any EH personality functions. I tried grep and nm, giving results in Swift binary/stdlib on FreeBSD only for __gnustep_objcxx_personality_v0 and on macOS for __objc_personality_v0, __objcxx_personality_v0, __gcc_personality_v0 and __gxx_personality_v0, but no matches on Linux. I believe the usage on macOS is because of the Objective-C/C++ Bridging support, which is not present on Linux and FreeBSD. Please fix 🙂

[belkadan]

Maybe we're compiling a .mm file on non-Apple platforms that should just be a .cpp.

[dcci]

This is responsible for a bunch of failures on FreeBSD. More generally, it seems to make swift useless as everything built without passing `-lojbc2` will fail.
In theory, we could just modify the Driver to always pass `-lobjc2`, but that feels very very wrong (and it's a sledgehammer).
https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20170417/005192.html seems to mention a knob that should be disabled on non-MachO, but I can't navigate the build easily to find out where that's set.