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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]:
b58b2a34d5/llvm/cmake/modules/HandleLLVMOptions.cmake (L1278-L1310)