Make the calls to wasm's memory.grow and memory.size indirect (#194)

* Make the calls to wasm's memory.grow and memory.size indirect

wasmtime-environ/src/vmoffsets.rs

@@ -2,6 +2,7 @@
 //! module.
 
 use crate::module::Module;
+use crate::BuiltinFunctionIndex;
 use core::convert::TryFrom;
 use cranelift_codegen::ir;
 use cranelift_wasm::{
@@ -332,9 +333,8 @@ impl VMOffsets {
             .unwrap()
     }
 
-    /// Return the size of the `VMContext` allocation.
-    #[allow(dead_code)]
-    pub fn size_of_vmctx(&self) -> u32 {
+    /// The offset of the builtin functions array.
+    pub fn vmctx_builtin_functions_begin(&self) -> u32 {
         self.vmctx_globals_begin()
             .checked_add(
                 self.num_defined_globals
@@ -344,6 +344,17 @@ impl VMOffsets {
             .unwrap()
     }
 
+    /// Return the size of the `VMContext` allocation.
+    pub fn size_of_vmctx(&self) -> u32 {
+        self.vmctx_builtin_functions_begin()
+            .checked_add(
+                BuiltinFunctionIndex::builtin_functions_total_number()
+                    .checked_mul(u32::from(self.pointer_size))
+                    .unwrap(),
+            )
+            .unwrap()
+    }
+
     /// Return the offset to `VMSharedSignatureId` index `index`.
     pub fn vmctx_vmshared_signature_id(&self, index: SignatureIndex) -> u32 {
         assert!(index.as_u32() < self.num_signature_ids);
@@ -517,6 +528,18 @@ impl VMOffsets {
             .checked_add(u32::from(self.vmglobal_import_from()))
             .unwrap()
     }
+
+    /// Return the offset to builtin function in `VMBuiltinFunctionsArray` index `index`.
+    pub fn vmctx_builtin_function(&self, index: BuiltinFunctionIndex) -> u32 {
+        self.vmctx_builtin_functions_begin()
+            .checked_add(
+                index
+                    .index()
+                    .checked_mul(u32::from(self.pointer_size))
+                    .unwrap(),
+            )
+            .unwrap()
+    }
 }
 
 /// Target specific type for shared signature index.