Replace the global-exports mechanism with a caller-vmctx mechanism. (#789)

* Replace the global-exports mechanism with a caller-vmctx mechanism.

This eliminates the global exports mechanism, and instead adds a
caller-vmctx argument to wasm functions so that WASI can obtain the
memory and other things from the caller rather than looking them up in a
global registry.

This replaces #390.

* Fixup some merge conflicts

* Rustfmt

* Ensure VMContext is aligned to 16 bytes

With the removal of `global_exports` it "just so happens" that this
isn't happening naturally any more.

* Fixup some bugs with double vmctx in wasmtime crate

* Trampoline stub needed adjusting
* Use pointer type instead of always using I64 for caller vmctx
* Don't store `ir::Signature` in `Func` since we don't know the pointer
  size at creation time.
* Skip the first 2 arguments in IR signatures since that's the two vmctx
  parameters.

* Update cranelift to 0.56.0

* Handle more merge conflicts

* Rustfmt

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/runtime/src/instance.rs

@@ -18,9 +18,8 @@ use crate::vmcontext::{
 use memoffset::offset_of;
 use more_asserts::assert_lt;
 use std::any::Any;
-use std::borrow::Borrow;
 use std::cell::RefCell;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::convert::TryFrom;
 use std::ptr::NonNull;
 use std::rc::Rc;
@@ -294,11 +293,6 @@ pub(crate) struct Instance {
     /// Offsets in the `vmctx` region.
     offsets: VMOffsets,
 
-    /// A global namespace of exports. This is a temporary mechanism to avoid
-    /// cyclic dependencies when one module wants to import from another and
-    /// make its memory available too, that will be obviated by host-bindings.
-    global_exports: Rc<RefCell<HashMap<String, Option<Export>>>>,
-
     /// WebAssembly linear memory data.
     memories: BoxedSlice<DefinedMemoryIndex, LinearMemory>,
 
@@ -567,7 +561,7 @@ impl Instance {
         };
 
         // Make the call.
-        unsafe { wasmtime_call(callee_vmctx, callee_address) }
+        unsafe { wasmtime_call(callee_vmctx, self.vmctx_mut_ptr(), callee_address) }
             .map_err(InstantiationError::StartTrap)
     }
 
@@ -689,15 +683,6 @@ impl Instance {
         foreign_instance.memory_size(foreign_index)
     }
 
-    pub(crate) fn lookup_global_export(&self, field: &str) -> Option<Export> {
-        let cell: &RefCell<HashMap<String, Option<Export>>> = self.global_exports.borrow();
-        let map: &mut HashMap<String, Option<Export>> = &mut cell.borrow_mut();
-        if let Some(Some(export)) = map.get(field) {
-            return Some(export.clone());
-        }
-        None
-    }
-
     /// Grow table by the specified amount of elements.
     ///
     /// Returns `None` if table can't be grown by the specified amount
@@ -772,7 +757,6 @@ impl InstanceHandle {
     /// Create a new `InstanceHandle` pointing at a new `Instance`.
     pub fn new(
         module: Rc<Module>,
-        global_exports: Rc<RefCell<HashMap<String, Option<Export>>>>,
         finished_functions: BoxedSlice<DefinedFuncIndex, *const VMFunctionBody>,
         imports: Imports,
         data_initializers: &[DataInitializer<'_>],
@@ -814,7 +798,6 @@ impl InstanceHandle {
                 dependencies: imports.dependencies,
                 mmap: instance_mmap,
                 module,
-                global_exports,
                 offsets,
                 memories,
                 tables,
@@ -886,25 +869,6 @@ impl InstanceHandle {
         initialize_memories(instance, data_initializers)?;
         initialize_globals(instance);
 
-        // Collect the exports for the global export map.
-        for (field, decl) in &instance.module.exports {
-            use std::collections::hash_map::Entry::*;
-            let cell: &RefCell<HashMap<String, Option<Export>>> = instance.global_exports.borrow();
-            let map: &mut HashMap<String, Option<Export>> = &mut cell.borrow_mut();
-            match map.entry(field.to_string()) {
-                Vacant(entry) => {
-                    entry.insert(Some(lookup_by_declaration(
-                        &instance.module,
-                        &mut instance.vmctx,
-                        &instance.offsets,
-                        &instance.finished_functions,
-                        &decl,
-                    )));
-                }
-                Occupied(ref mut entry) => *entry.get_mut() = None,
-            }
-        }
-
         // Ensure that our signal handlers are ready for action.
         // TODO: Move these calls out of `InstanceHandle`.
         wasmtime_init_eager();