How do I teach codegen to handle i8 arrays as i32s?

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#1

Hello.

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?

Thanks in advance.

#2

Yes, you can accomplish this with custom lowering for the store

-Matt

#3

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