Function alignment does not work on wasm

[RalfJung]

For #[align], see the error here and the discussion here. I assume this also affects -Zmin-function-align.

In wasm, functions don't have an "address", it's not a von-Neumann architecture. Instead, there is a dedicated table and one can pick an integer index into the table when doing a dynamic call. So the actual address of a function cannot be observed and "aligning" it has no meaning. However, the fn ptr value can be observed by being cast to an integer, and then one will see the index into that table. One could attempt to align that but it seems not very useful...

Apart from the usefulness, I guess the question is what can we do?

• Error when the attribute is used on wasm, since relying on fn ptr being divisible by N will just not work
• Try to convince LLVM to implement this (is there an LLVM issue?)

[RalfJung]

@folkertdev

So, should we just apply a target-specific minimum of #[align(1)] for wasm? (much like on windows targets the limit will be 8192)

I assume you mean "target-specific maxmimum"?

[folkertdev]

Right so an additional option is to allow only #[align(1)] and error on any higher alignment for the wasm targets. It is just the highest alignment we can guarantee. If LLVM ever moves on this, we can bump that limit.

[RalfJung]

Right, that's morally equivalent to just erroring (since why would you ever use align(1) but fits better into the existing framework.

Either way it makes lots of code with reasonable small alignments in the range of 2..=64 non-portable, which is a bummer... but seems hard to avoid.