[wasm] Refactor code specialization / patching

src/wasm/wasm-code-specialization.h

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_WASM_CODE_SPECIALIZATION_H_
+#define V8_WASM_CODE_SPECIALIZATION_H_
+
+#include <unordered_map>
+
+#include "src/assembler.h"
+#include "src/wasm/wasm-objects.h"
+
+namespace v8 {
+namespace internal {
+namespace wasm {
+
+// Helper class to specialize wasm code for a specific instance, or to update
+// code when memory / globals / tables change.
+// This class in unhandlified, and contains a DisallowHeapAllocation field to
+// ensure that no allocations happen while it is alive.
+//
+// Set up all relocations / patching that should be performed by the Relocate* /
+// Patch* methods, then apply all changes in one step using the Apply* methods.
+class CodeSpecialization {
+ public:
+  // Update memory references.
+  void RelocateMemoryReferences(Address old_start, uint32_t old_size,
+                                Address new_start, uint32_t new_size);
+  // Update references to global variables.
+  void RelocateGlobals(Address old_start, Address new_start);
+  // Update function table size.
+  // TODO(wasm): Prepare this for more than one indirect function table.
+  void PatchTableSize(uint32_t old_size, uint32_t new_size);
+  // Update all direct call sites based on the code table in the given instance.
+  void RelocateDirectCalls(WasmInstanceObject* instance);
+  // Relocate an arbitrary object (e.g. function table).
+  void RelocateObject(Object* old_obj, Object* new_obj);
+
+  // Apply all relocations and patching to all code in the instance (wasm code
+  // and exported functions).
+  bool ApplyToWholeInstance(WasmInstanceObject* instance,
+                            ICacheFlushMode = FLUSH_ICACHE_IF_NEEDED);
+  // Apply all relocations and patching to one wasm code object.
+  bool ApplyToWasmCode(Code* code, ICacheFlushMode = FLUSH_ICACHE_IF_NEEDED);
+
+ private:
+  DisallowHeapAllocation no_gc_during_specialization;
+
+  Address old_mem_start = 0;
+  uint32_t old_mem_size = 0;
+  Address new_mem_start = 0;
+  uint32_t new_mem_size = 0;
+
+  Address old_globals_start = 0;
+  Address new_globals_start = 0;
+
+  uint32_t old_function_table_size = 0;
+  uint32_t new_function_table_size = 0;
+
+  WasmInstanceObject* relocate_direct_calls_instance = nullptr;
+
+  std::unordered_map<Object*, Object*> objects_to_relocate;

[titzer, 2017/02/17 13:38:16]

I think we can replace this with an IdentityMap, which is stable over GC (and
potentially more efficient because it is a hashmap underneath).

[Clemens Hammacher, 2017/02/20 11:05:57]

Also note that I am already using a hash map here.
identity-map.h sais: "The value type {V} must not be a heap type."
Hence we would have to map to Handle<Object>, which would work, but introduces
unnecessary overhead. What do you think?

+};
+
+}  // namespace wasm
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_WASM_CODE_SPECIALIZATION_H_