[clang] [Clang][CodeGen] Lookup standard library declarations for builtins to… (PR #204792)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem commented:

Added modules and codegen experts.

Looking at `CodeGenModule::getBuiltinLibFunction` it's not clear to me what difference it makes whether the function (e.g. `fabsf`) exists in the AST or not.

The code just seems to figure out the name, and then call `GetOrCreateLLVMFunction` on it?

I also wonder how this should work in the case when the math function is not in the AST before -- neither by including math.h or through a module. Shouldn't the builtin work anyway?

Clang (clang-cl) knows whether it's compiling with `/MD` or `/MT` etc. Maybe it should just add the `dllimport` as appropriate for these functions, rather than relying on the math header/module being included?

https://github.com/llvm/llvm-project/pull/204792