Reland "[flags] Skip --random-seed in FlagList::Hash"

This is a reland of 9fe53c4f

- Fix data-race by using an atomic for flag_hash;
- Make sure flag_hash != 0
- Initialize flag_hash in V8::InitializeOncePerProcessImpl
- Clear flag_hash in more cases

Original change's description:
> [flags] Skip --random-seed in FlagList::Hash
>
> Node and friends use --random-seed to temporary reset the seed for
> predictable code-cache creation. To allow custom random seeds at runtime
> the flag is reset for encoding the FlagList::Hash in the snapshots.
>
> We will soon disallow changing flags via the API after V8 has been
> initialized. In order to make node work we will exclude --random-seed
> from the FlagList::Hash calculation.
>
> Drive-by-fix:
> * Lazily initialize flag_hash instead of calculating it after every call
>   to SetFlagsFromString / EnforceFlagImplications.
> * Simplify hash string source creation since out << flag now includes
>   the full flag information
>
> Bug: v8:12309
> Change-Id: I1a168f4702d8c4d160ff12fdbea881731e4ea8b6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218159
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#77345}

Bug: v8:12309
Change-Id: I12cd2931d81dc74e07a4da3564e4bf8dd151300a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218981
Commit-Queue: Marja Hölttä <marja@chromium.org>
Auto-Submit: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77373}

src/flags/flags.cc

@@ -493,6 +493,34 @@ std::ostream& operator<<(std::ostream& os, const Flag& flag) {
   return os;
 }
 
+namespace {
+
+static std::atomic<uint32_t> flag_hash(0);
+
+void ComputeFlagListHash() {
+  std::ostringstream modified_args_as_string;
+  if (COMPRESS_POINTERS_BOOL) modified_args_as_string << "ptr-compr";
+  if (DEBUG_BOOL) modified_args_as_string << "debug";
+  for (const Flag& flag : flags) {
+    if (flag.IsDefault()) continue;
+    // We want to be able to flip --profile-deserialization without
+    // causing the code cache to get invalidated by this hash.
+    if (flag.PointsTo(&FLAG_profile_deserialization)) continue;
+    // Skip FLAG_random_seed to allow predictable code caching.
+    if (flag.PointsTo(&FLAG_random_seed)) continue;
+    modified_args_as_string << flag;
+  }
+  std::string args(modified_args_as_string.str());
+  // Generate a hash that is not 0.
+  uint32_t hash = static_cast<uint32_t>(base::hash_range(
+                      args.c_str(), args.c_str() + args.length())) |
+                  1;
+  DCHECK_NE(hash, 0);
+  flag_hash.store(hash, std::memory_order_relaxed);
+}
+
+}  // namespace
+
 // Helper function to parse flags: Takes an argument arg and splits it into
 // a flag name and flag value (or nullptr if they are missing). negated is set
 // if the arg started with "-no" or "--no". The buffer may be used to NUL-
@@ -754,6 +782,7 @@ int FlagList::SetFlagsFromString(const char* str, size_t len) {
 
 // static
 void FlagList::ResetAllFlags() {
+  flag_hash = 0;
   for (size_t i = 0; i < num_flags; ++i) {
     flags[i].Reset();
   }
@@ -792,33 +821,6 @@ void FlagList::PrintValues() {
 
 namespace {
 
-static uint32_t flag_hash = 0;
-
-void ComputeFlagListHash() {
-  std::ostringstream modified_args_as_string;
-  if (COMPRESS_POINTERS_BOOL) {
-    modified_args_as_string << "ptr-compr";
-  }
-  if (DEBUG_BOOL) {
-    modified_args_as_string << "debug";
-  }
-  for (size_t i = 0; i < num_flags; ++i) {
-    Flag* current = &flags[i];
-    if (current->PointsTo(&FLAG_profile_deserialization)) {
-      // We want to be able to flip --profile-deserialization without
-      // causing the code cache to get invalidated by this hash.
-      continue;
-    }
-    if (!current->IsDefault()) {
-      modified_args_as_string << i;
-      modified_args_as_string << *current;
-    }
-  }
-  std::string args(modified_args_as_string.str());
-  flag_hash = static_cast<uint32_t>(
-      base::hash_range(args.c_str(), args.c_str() + args.length()));
-}
-
 template <class A, class B>
 bool TriggerImplication(bool premise, const char* premise_name,
                         A* conclusion_pointer, B value, bool weak_implication) {
@@ -837,6 +839,7 @@ bool TriggerImplication(bool premise, const char* premise_name,
 
 // static
 void FlagList::EnforceFlagImplications() {
+  flag_hash = 0;
   bool changed;
   do {
     changed = false;
@@ -844,10 +847,13 @@ void FlagList::EnforceFlagImplications() {
 #include "src/flags/flag-definitions.h"  // NOLINT(build/include)
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
   } while (changed);
-  ComputeFlagListHash();
 }
 
-uint32_t FlagList::Hash() { return flag_hash; }
+// static
+uint32_t FlagList::Hash() {
+  if (flag_hash.load() == 0) ComputeFlagListHash();
+  return flag_hash.load();
+}
 
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS

src/init/v8.cc

@@ -183,6 +183,9 @@ void V8::InitializeOncePerProcessImpl() {
 
   if (FLAG_print_flag_values) FlagList::PrintValues();
 
+  // Initialize the default FlagList::Hash
+  FlagList::Hash();
+
 #if defined(V8_USE_PERFETTO)
   if (perfetto::Tracing::IsInitialized()) TrackEvent::Register();
 #endif