How do I hold an array of pointers in LLVM?

Question

For some background, I am implementing a compiler using the llvmpy library which is a wrapper around the LLVM IR generation.

I have created a character type which represents one or more UTF-8 code points. These code points are stored in an array so a character can be one of the following arrays:

[1 x i32], [2 x i32], ..., [6 x i32]

Now, I would like to implement a string type. This would be an array of pointers to arrays:

[n x [1-6 x i32]*] where n is the string length

However, (as far as I know) it seems that LLVM requires me to declare the length of the inner array. So, while I can store this:

[[1 x i32], [1 x i32], [1 x i32]]

I cannot store this:

[[1 x i32], [2 x i32]]

Is there a way to store an array of array pointers if the array pointers lead to arrays of different length?

Answer 1

Much like in C, LLVM IR requires all the elements of an array to be of the same type.

I guess the simplest way to work around this is to just store some arbitrary pointer type (e.g. i32*), and perform bitcasts whenever you want to access the array - though that of course assumes that you know in advance the size of the internal array at each index.

If it's only known at run-time, you can make each array element point to some { i32, i32* } struct which holds the size of the internal array as well as a pointer to it, and then switch on that size and bitcast accordingly in each branch target - or just calculate the size at run-time from the i32* pointer, which is easy as this is UTF-8.