[llvm] [refactor] LLVMProgramImpl code clean up: part-2

[jim19930609]

Related issue = #4800, #5114

Removed Program::get_llvm_program_impl() interface, so we dont have to expose backend-specific LlvmProgramImpl along with Program.

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[ailzhang]

Hmmm I vaguely remember this function - the main motivation of get_llvm_program_implwas a "temp" hack and we wanted to remove it in the future (as opposed to exposing a more backend agnostic version). Please correct me if I'm wrong! @k-ye

[jim19930609]

Hmmm I vaguely remember this function - the main motivation of get_llvm_program_implwas a "temp" hack and we wanted to remove it in the future (as opposed to exposing a more backend agnostic version). Please correct me if I'm wrong! @k-ye

I guess we'll have to get rid of get_llvm_program_impl() since it exposes LlvmProgramImpl in the header file, which couples the entire LLVM backend with the higher-level Base classes (any file that include "program.h").

IMO, removing the backend-agnostic interface get_program_impl() could be a second step, which helps keep the higher-level interface clean. This is difficult because we put too many members and implementations in the "Program" object:

Ideally, we should move most of these members to "ProgramImpl", and make "Program" an interface class. In that way, lower-level objects such as CodeGenLLVM can keep "LlvmProgramImpl" instead of "Program".

[k-ye]

I agree that we should remove get_llvm_program_impl() first. One nit: inside codegen, instead of writing static_cast<LlvmProgramImpl*>(program->get_program_impl()), we could write a small helper here?

[ailzhang]

LGTM!