[PATCH] D25932: Unconditionally pass `-lto_library` to the linker on Darwin

[Mehdi AMINI via cfe-commits]

mehdi_amini added a comment.

In https://reviews.llvm.org/D25932#601842, @rnk wrote:

> In https://reviews.llvm.org/D25932#601820, @mehdi_amini wrote:
>
> > We ship `clang + libLTO + ld64` bundled in the toolchain, so even if you don't package libLTO yourself, it is already accessible from the linker: it will use the one in the toolchain when needed.
> >
> > I don't have an immediate idea to prevent this and have the linker issue an error (other than removing manually libLTO from the Xcode installation).
>
>
> So, even if clang doesn't pass -lto_library to ld64, ld64 will auto-discover the bundled libLTO that happens to be next to it? That could go badly.


Right: until LLVM 3.8, clang was *never* passing the `-lto_library` option. The only way to have your own libLTO used by ld64 instead of the one in the Xcode toolchain was setting the environment variable "DYLD_LIBRARY_PATH".
Of course was has many issues, and that's what lead us to have clang passing this option to ld64. Initially only when the driver was invoked with -flto, but recently I had issues with clients that didn't use LTO themselves but were having static archives they depend on that were containing bitcode.

> 
> 
>> This is the motivation behind the warning I believe: we're trying to prevent this situation where a user would have clang but not his own libLTO and may encounter an unexpected issue. Even when the user does not opt-in to build his own project with LTO enabled, there can be static archive dependencies that contain bitcode.
> 
> I guess ld64 doesn't expose a flag like `-fno-lto` to disable this completely? Maybe we could pass `-lto_library /dev/null` or something?

No there is no such flag, what you suggest seems to work:

  $ clang -flto main.c -Wl,-lto_library,/dev/null
  ld: could not process llvm bitcode object file, because /dev/null could not be loaded file '/var/folders/4z/k9mg8rls7wsfm6t6hbm0_bxw0000gn/T/main-8e28a4.o' for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Also with an non-existing path:

  $ clang -flto main.c -Wl,-lto_library,imaginary_path
  ld: could not process llvm bitcode object file, because imaginary_path could not be loaded file '/var/folders/4z/k9mg8rls7wsfm6t6hbm0_bxw0000gn/T/main-319618.o' for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)


Repository:
  rL LLVM

https://reviews.llvm.org/D25932