No way to make forward references with mr::Builder

[rrika]

When building an if-else–construct using branch_conditional(cond, true_label, false_label, …) I need the OpLabel-ids before they are generated.

If branch_conditional returned &mr::Instruction and forward references were Cell<Word> instead of Word I could patch it later. Though this would allow anyone with a non-mut reference to mess things up.

[rrika]

Never mind, I should’ve read the documentation. Though even if I use builder.id() how do I make a later basic block use exactly that id instead of the next auto-generated one? The builder gives no mutable references to any of the objects it creates, as far as I can tell.

[antiagainst]

Yep, confirmed. This is a flaw in the current builder API. I knew forward references would bite me so I left some comments in the doc about it. However, the solution covers some case but not all. I'll fix the rest soon. :)

Returning an instruction to allow fixing up things later was truly the way I'd like to take previously. But after a second thought, wrapping all instructions with Cell introduces too much overheads into the current data representation. So I think I'll instead change the builder methods to take an additional Option<spirv::Word> for result ids. It brings in some boilerplate code when using the API, but it keeps the current simple data representation.

Thanks for reporting! :)

[antiagainst]

Fixed. I'll push a new release after adding some more tests. :)