How to optimize store of constant arrays

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1

Hi all, I have this problem:

I’m using LLVM’s C++ API, mostly IRBuilder<> to generate code. Some of the generated code is just storing constant data to a location provided as a function argument. Something like ir_builder.CreateStore(get_default_data(), ptrValue) where get_default_data() may return a ConstantArray of i8 and ptrValue is a function argument. Looking at the generated assembly, it seems inefficient, it generates a separate instruction for storing each byte relative to the pointer. Each instruction takes several bytes, and I am trying to optimize for code size.

The Q: What should I do to make the generated code more size-efficient (think -Os)?

Current ideas, which may not include the best answer:

  • Store the data as-is in the IR, and use the MemCpyInst instead of the store when get_default_data() returns something big?
  • Add a custom – or any available built-in – pass to the output to transform the store instns to memcpy when appropriate?

Thanks for any help,
Boldizsar

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2

You could create a statically initialized global variable containing the constant data, and then insert a call to memcpy to copy it over to your destination.

-Krzysztof

3

Regarding the memcpy intrinsic---it will be expanded into individual loads/stores when the amount of data is small, depending on your target settings.
In any case, these two approaches you have below are pretty much equivalent. Use existing intrinsics whenever you can, or otherwise you'd need to add code to handle them in the codegen.

-Krzysztof