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/api/src/trampoline/func.rs

@@ -64,7 +64,12 @@ impl Drop for TrampolineState {
     }
 }
 
-unsafe extern "C" fn stub_fn(vmctx: *mut VMContext, call_id: u32, values_vec: *mut i128) -> u32 {
+unsafe extern "C" fn stub_fn(
+    vmctx: *mut VMContext,
+    _caller_vmctx: *mut VMContext,
+    call_id: u32,
+    values_vec: *mut i128,
+) -> u32 {
     let mut instance = InstanceHandle::from_vmctx(vmctx);
 
     let (args, returns_len) = {
@@ -72,9 +77,9 @@ unsafe extern "C" fn stub_fn(vmctx: *mut VMContext, call_id: u32, values_vec: *m
         let signature = &module.signatures[module.functions[FuncIndex::new(call_id as usize)]];
 
         let mut args = Vec::new();
-        for i in 1..signature.params.len() {
+        for i in 2..signature.params.len() {
             args.push(Val::read_value_from(
-                values_vec.offset(i as isize - 1),
+                values_vec.offset(i as isize - 2),
                 signature.params[i].value_type,
             ))
         }
@@ -116,12 +121,15 @@ fn make_trampoline(
     let pointer_type = isa.pointer_type();
     let mut stub_sig = ir::Signature::new(isa.frontend_config().default_call_conv);
 
-    // Add the `vmctx` parameter.
+    // Add the caller/callee `vmctx` parameters.
     stub_sig.params.push(ir::AbiParam::special(
         pointer_type,
         ir::ArgumentPurpose::VMContext,
     ));
 
+    // Add the caller `vmctx` parameter.
+    stub_sig.params.push(ir::AbiParam::new(pointer_type));
+
     // Add the `call_id` parameter.
     stub_sig.params.push(ir::AbiParam::new(types::I32));
 
@@ -131,9 +139,10 @@ fn make_trampoline(
     // Add error/trap return.
     stub_sig.returns.push(ir::AbiParam::new(types::I32));
 
+    // Compute the size of the values vector. The vmctx and caller vmctx are passed separately.
     let value_size = 16;
     let values_vec_len = ((value_size as usize)
-        * cmp::max(signature.params.len() - 1, signature.returns.len()))
+        * cmp::max(signature.params.len() - 2, signature.returns.len()))
         as u32;
 
     let mut context = Context::new();
@@ -155,7 +164,7 @@ fn make_trampoline(
 
         let values_vec_ptr_val = builder.ins().stack_addr(pointer_type, ss, 0);
         let mflags = MemFlags::trusted();
-        for i in 1..signature.params.len() {
+        for i in 2..signature.params.len() {
             if i == 0 {
                 continue;
             }
@@ -165,14 +174,21 @@ fn make_trampoline(
                 mflags,
                 val,
                 values_vec_ptr_val,
-                ((i - 1) * value_size) as i32,
+                ((i - 2) * value_size) as i32,
             );
         }
 
-        let vmctx_ptr_val = builder.func.dfg.ebb_params(block0)[0];
+        let ebb_params = builder.func.dfg.ebb_params(block0);
+        let vmctx_ptr_val = ebb_params[0];
+        let caller_vmctx_ptr_val = ebb_params[1];
         let call_id_val = builder.ins().iconst(types::I32, call_id as i64);
 
-        let callee_args = vec![vmctx_ptr_val, call_id_val, values_vec_ptr_val];
+        let callee_args = vec![
+            vmctx_ptr_val,
+            caller_vmctx_ptr_val,
+            call_id_val,
+            values_vec_ptr_val,
+        ];
 
         let new_sig = builder.import_signature(stub_sig);
 
@@ -236,17 +252,18 @@ pub fn create_handle_with_function(
     func: &Rc<dyn Callable + 'static>,
     store: &Store,
 ) -> Result<InstanceHandle> {
-    let sig = match ft.get_wasmtime_signature() {
-        Some(sig) => sig.clone(),
-        None => bail!("not a supported core wasm signature {:?}", ft),
-    };
-
     let isa = {
         let isa_builder = native::builder();
         let flag_builder = settings::builder();
         isa_builder.finish(settings::Flags::new(flag_builder))
     };
 
+    let pointer_type = isa.pointer_type();
+    let sig = match ft.get_wasmtime_signature(pointer_type) {
+        Some(sig) => sig.clone(),
+        None => bail!("not a supported core wasm signature {:?}", ft),
+    };
+
     let mut fn_builder_ctx = FunctionBuilderContext::new();
     let mut module = Module::new();
     let mut finished_functions: PrimaryMap<DefinedFuncIndex, *const VMFunctionBody> =