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Now that we require at least LLVM 13, that codegen backend is always using its intrinsic `fptosi.sat` and `fptoui.sat` conversions, so it doesn't need the manual implementation. However, the GCC backend still needs it, so we can move all of that code down there.
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.