[llvm-dev] How to optimize store of constant arrays

[Krzysztof Parzyszek via llvm-dev]

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

On 3/13/2018 10:06 AM, via llvm-dev wrote:
> 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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