[LLVMdev] Union type, is it really used or necessary?

[Dan Gohman]

On Sep 7, 2010, at 6:22 AM, Erik de Castro Lopo wrote:
> 
> I'm currently use llvm-2.7 and have been using unions, not being
> aware that they are going to be removed. The use case is for 
> forcing field alignments in a packed struct to be correct for
> 32 and 64 bits. In particular I have a struct with an i32 tag
> field followed by a pointer.
> 
> When generating 32 bit code the struct looks like:
> 
>     <{ i32, pointer }>
> 
> and for 64 bit code:
> 
>     <{ union { i32, i64 }, pointer }>
> 
> The nice thing about this is that in my LLVM code generator,
> I have a function offsetOf which can get me the byte offset of
> and element in a struct. In the case above,
> 
>    offsetOf (1)
> 
> returns 4 when generating 32 bit code and 8 when generating 64
> bit code.
> 
> If there's another of guaranteeing struct alignment as well as
> and easy way to get struct field offsets I'd like hear of it.

If you want to make sure the pointer field is properly aligned, why not
just use a non-packed struct: { i32, pointer } ?  Or if you really want
a packed struct, can you use <{ i32, i32, pointer }>, since you're
already emitting target-dependent IR anyway?

If you're computing the offset in order to use as a value
within the program, you can use ConstantExpr::getOffsetOf.
(If this isn't exposed in whatever bindings you're using,
that can be fixed.)

Dan