This adds a feature which allows one to look up an export by name
without knowing what module it's in -- `lookup_global_export` on an
`InstanceContents`.
The main expected use for this is to support APIs where module A
imports a function from module B, and module B needs to access module
A's memory. B can't import it from A in the normal way, because that
would create a dependency cycle. So for now, allow B to look up A's
exported memory dynamically with `lookup_global_export`.
In the future, with reference types and possibly host bindings, we'll be
able to pass references to memory as arguments, which will obviate the
need for this mechanism.
Currently we don't actually sandbox the memory at all, so you can do evil things
like read and write the host's memory. We also don't support growing memory or
cranelift-compatible ABI that passes the memory offset as an argument.
We also always immediately allocate the buffer when encountering a memory section,
there is preliminary support for translating a buffer which can then have the real
offset replaced using relocations (and returning a different type when doing so)
but I haven't written the code that actually does relocation so it doesn't work yet.
wasmtime-execute is now wasmtime-jit. Move `JITCode` and the TargetIsa
into a new `Compiler` type. `InstancePlus` is no more, with trampoline
functionality now handled by `Compiler`.
Dan Gohman
Jef
Sergey Pepyakin