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-runtime/src/vmcontext.rs

@@ -4,6 +4,7 @@
 use crate::instance::Instance;
 use core::any::Any;
 use core::{ptr, u32};
+use wasmtime_environ::BuiltinFunctionIndex;
 
 /// An imported function.
 #[derive(Debug, Copy, Clone)]
@@ -472,6 +473,33 @@ impl Default for VMCallerCheckedAnyfunc {
     }
 }
 
+/// An array that stores addresses of builtin functions. We translate code
+/// to use indirect calls. This way, we don't have to patch the code.
+#[repr(C)]
+pub struct VMBuiltinFunctionsArray {
+    ptrs: [usize; Self::len()],
+}
+
+impl VMBuiltinFunctionsArray {
+    pub const fn len() -> usize {
+        BuiltinFunctionIndex::builtin_functions_total_number() as usize
+    }
+
+    pub fn initialized() -> Self {
+        use crate::libcalls::*;
+        let mut ptrs = [0; Self::len()];
+        ptrs[BuiltinFunctionIndex::get_memory32_grow_index().index() as usize] =
+            wasmtime_memory32_grow as usize;
+        ptrs[BuiltinFunctionIndex::get_imported_memory32_grow_index().index() as usize] =
+            wasmtime_imported_memory32_grow as usize;
+        ptrs[BuiltinFunctionIndex::get_memory32_size_index().index() as usize] =
+            wasmtime_memory32_size as usize;
+        ptrs[BuiltinFunctionIndex::get_imported_memory32_size_index().index() as usize] =
+            wasmtime_imported_memory32_size as usize;
+        Self { ptrs }
+    }
+}
+
 /// The VM "context", which is pointed to by the `vmctx` arg in Cranelift.
 /// This has information about globals, memories, tables, and other runtime
 /// state associated with the current instance.