Break down emitStoresForConstant into optimized operations

[bcardosolopes]

The function in emitStoresForConstant in clang/lib/CIR/CodeGen/CIRGenDecl.cpp contains lots of unimplemented heuristics, namely:

  assert(!UnimplementedFeature::addAutoInitAnnotation());
  assert(!UnimplementedFeature::vectorConstants());
  assert(!UnimplementedFeature::shouldUseBZeroPlusStoresToInitialize());
  assert(!UnimplementedFeature::shouldUseMemSetToInitialize());
  assert(!UnimplementedFeature::shouldSplitConstantStore());
  assert(!UnimplementedFeature::shouldCreateMemCpyFromGlobal());

Those should be implemented in LoweringPrepare. Some context from a previous PR follows:

The original codegen doesn't have this problem at all, there is just a memcpy there, I provide LLVM IR just for reference:

Right, this one sounds like UnimplementedFeature::shouldCreateMemCpyFromGlobal(), which can now be implemented in LoweringPrepare (we didn't had it before). Out of curiosity, what LLVM lowering do we get for this right now?

Originally posted by @bcardosolopes in #463 (comment)

[Ris-Bali]

I would like to work on this. I found the implementation of some of these functions in clang/CodeGen/CGDecl.cpp for reference ; although I couldn't find most of the functions there. Is there any other place I can look for the implementation ? Or are these functions named differently in clang codegen.

[bcardosolopes]

Hi! These functions don't exist, their names refer to snippets of code in traditional CodeGen that needs to be implemented in ClangIR as part of LoweringPrepare.

[Ris-Bali]

Hi! These functions don't exist, their names refer to snippets of code in traditional CodeGen that needs to be implemented in ClangIR as part of LoweringPrepare.

Thanks for replying, there has been a misunderstanding I meant I am referring to these functions in the traditional clang codegen for implementing these operations in clangIR; although I couldn't find some of the above snippets there eg : shouldCreateMemCpyFromGlobal(maybe named differently ?). I'll start by working on the functions I found in clang's codegen.

[bcardosolopes]

Sure, starting by the easier ones is the way to go. Please work on them incrementally, one PR each!

above snippets there eg : shouldCreateMemCpyFromGlobal(maybe named differently ?)

That one doesn't have a snippet necessarily, it's just tracking lacking behavior, see #463 (comment) for more details