[CMake] Explicitly disable LTO if no LTO option is provided #85621
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When compiling Swift as an LLVM external project (not the non-unified workflow from the build-script), if one tries to enable LTO for LLVM projects it will end up affecting Swift and the Swift standard library, even if the options `SWIFT_TOOLS_ENABLE_LTO` and `SWIFT_STDLIB_ENABLE_LTO` are disabled or using their default values, because [HandleLLVMOptions] modifies the `CMAKE_C/CXX_FLAGS` globally. By setting `-fno-lto` explicitly when those options are disabled or using their default values, the options from `CMAKE_C/CXX_FLAGS` are overriden, and the `SWIFT_*_ENABLE_LTO` controls the usage or not of LTO optimizations and the presence of bitcode in the object files. [HandleLLVMOptions]: https://github.com/swiftlang/llvm-project/blob/b58b2a34d5094928c4ee1b94a7d5412b14540c01/llvm/cmake/modules/HandleLLVMOptions.cmake#L1278-L1310
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@swift-ci please smoke test |
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@swift-ci please smoke test Windows platform |
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@swift-ci please smoke test Windows platform |
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@swift-ci please test Windows platform |
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When compiling Swift as an LLVM external project (not the non-unified workflow from the build-script), if one tries to enable LTO for LLVM projects it will end up affecting Swift and the Swift standard library, even if the options
SWIFT_TOOLS_ENABLE_LTOandSWIFT_STDLIB_ENABLE_LTOare disabled or using their default values, because HandleLLVMOptions modifies theCMAKE_C/CXX_FLAGSglobally.By setting
-fno-ltoexplicitly when those options are disabled or using their default values, the options fromCMAKE_C/CXX_FLAGSare overriden, and theSWIFT_*_ENABLE_LTOcontrols the usage or not of LTO optimizations and the presence of bitcode in the object files.