When doing linker-plugin based LTO, write LLVM bitcode obj-files instead of embedding the bitcode into the regular object file. #52109
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instead of embedding the bitcode into the regular object file.
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@bors: r+ Nice! I like the idea of one day doing this by default for |
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Jul 7, 2018
When doing linker-plugin based LTO, write LLVM bitcode obj-files instead of embedding the bitcode into the regular object file. This PR makes the compiler emit LLVM bitcode object files instead of regular object files with the IR embed when compiling for linker-plugin-based LTO. The reasoning for switching the strategy is this: - Embedding bitcode in a section of the object file actually makes us save bitcode twice in rlibs and Rust dylibs, once for linker-based LTO and once for rustc-based LTO. That's a waste of space. - When compiling for plugin-based LTO, one usually has no use for the machine code also present in the object file. Generating it is a waste of time. - When compiling for plugin-based LTO, `rustc` will skip running ThinLTO because the linker will do that anyway. This has the side effect of then generating poorly optimized machine code, which makes it even less useful (and may lead to users not knowing why their code is slow instead of getting an error). - Not having machine code available makes it impossible for the linker to silently fall back to not inlining stuff across language boundaries. - This is what Clang does and according to [the documentation](https://llvm.org/docs/BitCodeFormat.html#native-object-file-wrapper-format) is the better supported option. - The current behavior (minus the runtime performance problems) is still available via `-Z embed-bitcode` (we might want to do this for `libstd` at some point). r? @alexcrichton
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☀️ Test successful - status-appveyor, status-travis |
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This PR makes the compiler emit LLVM bitcode object files instead of regular object files with the IR embed when compiling for linker-plugin-based LTO. The reasoning for switching the strategy is this:
rustcwill skip running ThinLTO because the linker will do that anyway. This has the side effect of then generating poorly optimized machine code, which makes it even less useful (and may lead to users not knowing why their code is slow instead of getting an error).-Z embed-bitcode(we might want to do this forlibstdat some point).r? @alexcrichton