I need to agree with Toke!
"having Bird behave differently on different archs is bound to be incredibly confusing..."
And, on the other hand, what would be the downside of having everything in 64?
Using a little more memory than might be the minimum usable?
This kind of care I see being taken for applications that run platforms with serious memory limitations.
I don't see that this is BIRD's niche.
In a real scenario of high demand using BIRD, how much memory would be "wasted"?
In these real high demand scenarios, who uses 32bit these days?
I would worry more about CPU cycles.