[llvm-dev] How do I teach codegen to handle i8 arrays as i32s?

[Doerfert, Johannes via llvm-dev]

On 01/03, Gleb Popov via llvm-dev wrote:
> My backend supports only i32 stores. Given the following IR:
> 
>   %2 = alloca [2 x i8], align 1
> 
>   %4 = getelementptr inbounds [2 x i8], [2 x i8]* %2, i64 0, i64 0
>   store i8 1, i8* %4, align 1
>   %5 = getelementptr inbounds [2 x i8], [2 x i8]* %2, i64 0, i64 1
>   store i8 2, i8* %5, align 1
> 
> is it possible to convince LLVM codegen to first load 4 bytes, then blend
> it with the value being stored using "or" and then store 4 bytes back?
> 
> Or maybe it should be performed on the IR level?

In the example as shown that is on it's own not necessarily legal (on
the IR-level). If you expand the allocation to 4 x i8 it should be
allowed, or if you do it late enough (=backend) you can make it work
without introducing UB.

I personally would do it on IR, seems easy enough. I think we have
similar rules in InstCombine already.

Cheers,
  Johannes
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