[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/+/3218159Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77345}

src/flags/flags.cc

@@ -796,23 +796,16 @@ 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)) {
+  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.
-      continue;
-    }
-    if (!current->IsDefault()) {
-      modified_args_as_string << i;
-      modified_args_as_string << *current;
-    }
+    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());
   flag_hash = static_cast<uint32_t>(
@@ -837,6 +830,7 @@ bool TriggerImplication(bool premise, const char* premise_name,
 
 // static
 void FlagList::EnforceFlagImplications() {
+  flag_hash = 0;
   bool changed;
   do {
     changed = false;
@@ -844,10 +838,14 @@ 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; }
+uint32_t FlagList::Hash() {
+  if (flag_hash == 0) {
+    ComputeFlagListHash();
+  }
+  return flag_hash;
+}
 
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS