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Stop emitting the `dso_local` LLVM attribute for external symbols under the static relocation model on macOS. This matches Clang's behavior:973cb2c326/clang/lib/CodeGen/CodeGenModule.cpp (L1038-L1040)Even if `dso_local` were properly supported in this way on macOS, it seems incorrect to add this annotation as liberally as we did. The `dso_local` annotation is for symbols that ultimately end up in the same linkage unit, but we were adding this annotation even for `static` values inside `extern` blocks marked with `#[link(type="framework")]`, which should be considered dynamically linked. Note that Clang likewise avoids emitting `dso_local` for `dllimport` symbols:973cb2c326/clang/lib/CodeGen/CodeGenModule.cpp (L1005-L1007)This issue caused breakage in the `ring` crate, which links to a symbol defined in `Security.framework` that ultimately resolves to address `0x0`:b94d61e044/src/rand.rs (L390)For this symbol, the use of `dso_local` causes LLVM to emit a relocation of type `X86_64_RELOC_SIGNED`, which is a 32-bit signed PC-relative offset. If the binary is large enough, `0x0` might be out of range, and the link will fail. Avoiding `dso_local` causes LLVM to use the GOT instead, emitting a relocation of type `X86_64_RELOC_GOT_LOAD`, which will properly handle the large offset and cause the link to succeed. As a side note, the static relocation model is effectively deprecated for security reasons on macOS, as it prohibits PIE. It's also completely unsupported on Apple Silicon, so I don't think it's worth going to the effort of properly supporting this model on that platform.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.