[Feature Request] Link-time optimization support

[Absolucy]

It would be nice, especially for larger projects, if there was support for Clang's LTO.

It would likely require changes to the compilation process, mainly because LTO consists of emitting LLVM bitcode (.bc) files rather than object fiiles (.o), and then linking them together with LLD

This would also allow for cross-language (ObjC / C / C++ / Swift) optimization, because well, it's all bitcode in the end

    .m --clang--> .bc --LLVM--> .bc (opt) ---------+
                                                   |
    .c --clang--> .bc --LLVM--> .bc (opt) ---------+
                                                   |
                                                   +-ld+LLVM--> bin/dylib
    .swift --+                                     |
             |                                     |
    .swift --+--swiftc--> .bc --LLVM--> .bc (opt) -+
             |
    .swift --+

[kabiroberai]

The structure of the compilation process seems roughly the same so I don't think it'd require many changes. Have you tried enabling LTO yourself by adding -flto to your CFLAGS and LDFLAGS? (Swift support for LTO is still WIP afaik.)

If it works for you we could consider making it a default.

[Absolucy]

Oh hm, that works. I forgot why it failed for me earlier but it does now?

Also, Swift seems to have -emit-bc, and there's also this

[kabiroberai]

Taking a look at the corresponding Swift Driver changes, it seems that enabling LTO in swiftc is as easy as -lto (EDIT: you might also need -Xfrontend -emit-bc). In conjunction with supplying -flto in the C and LDFLAGS, this should enable LTO across Theos. I'm not sure what the impact of making this the default setting would be but we could possibly add a flag to enable it; something like instance_LTO = 1.

[kirb]

Going to reopen as this is still of interest to the Theos team to implement.