Add a version tag for cached data.

BUG=399580,431699
LOG=N
R=dcarney@chromium.org, marja@chromium.org

Review URL: https://codereview.chromium.org/718043002

Cr-Commit-Position: refs/heads/master@{#25339}

include/v8.h

@@ -1238,6 +1238,26 @@ class V8_EXPORT ScriptCompiler {
   static Local<Script> Compile(Isolate* isolate, StreamedSource* source,
                                Handle<String> full_source_string,
                                const ScriptOrigin& origin);
+
+  /**
+   * Return a version tag for CachedData for the current V8 version & flags.
+   *
+   * This value is meant only for determining whether a previously generated
+   * CachedData instance is still valid; the tag has no other meaing.
+   *
+   * Background: The data carried by CachedData may depend on the exact
+   *   V8 version number or currently compiler flags. This means when
+   *   persisting CachedData, the embedder must take care to not pass in
+   *   data from another V8 version, or the same version with different
+   *   features enabled.
+   *
+   *   The easiest way to do so is to clear the embedder's cache on any
+   *   such change.
+   *
+   *   Alternatively, this tag can be stored alongside the cached data and
+   *   compared when it is being used.
+   */
+  static uint32_t CachedDataVersionTag();
 };
 
 

src/api.cc

@@ -14,6 +14,7 @@
 #include "include/v8-testing.h"
 #include "src/assert-scope.h"
 #include "src/background-parsing-task.h"
+#include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/base/platform/time.h"
 #include "src/base/utils/random-number-generator.h"
@@ -1846,6 +1847,13 @@ Local<Script> ScriptCompiler::Compile(Isolate* v8_isolate,
 }
 
 
+uint32_t ScriptCompiler::CachedDataVersionTag() {
+  return static_cast<uint32_t>(base::hash_combine(
+      internal::Version::Hash(), internal::FlagList::Hash(),
+      static_cast<uint32_t>(internal::CpuFeatures::SupportedFeatures())));
+}
+
+
 Local<Script> Script::Compile(v8::Handle<String> source,
                               v8::ScriptOrigin* origin) {
   i::Handle<i::String> str = Utils::OpenHandle(*source);

src/flags.cc

@@ -9,6 +9,7 @@
 #include "src/v8.h"
 
 #include "src/assembler.h"
+#include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/ostreams.h"
 
@@ -549,4 +550,17 @@ void FlagList::EnforceFlagImplications() {
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 }
 
+
+uint32_t FlagList::Hash() {
+  std::ostringstream modified_args_as_string;
+  for (size_t i = 0; i < num_flags; ++i) {
+    Flag* current = &flags[i];
+    if (!current->IsDefault()) {
+      modified_args_as_string << *current;
+    }
+  }
+  std::string args(modified_args_as_string.str());
+  return static_cast<uint32_t>(
+      base::hash_range(args.c_str(), args.c_str() + args.length()));
+}
 } }  // namespace v8::internal

src/flags.h

@@ -57,6 +57,9 @@ class FlagList {
 
   // Set flags as consequence of being implied by another flag.
   static void EnforceFlagImplications();
+
+  // Hash of current flags (to quickly determine flag changes).
+  static uint32_t Hash();
 };
 
 } }  // namespace v8::internal