[flags] add --print-flag-values helper

It's not always easy to spot what exact configuration of V8 is run
within embedders. With --print-flag-values we can easily compare
different configurations.

Drive-by-fix:
- Use new FlagValue and FlagName helpers for printing
- Remove unused FlagList::argv helper

Change-Id: Ic8a25479d7b1e72f714b22ae7d2e56e06e810556
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3197713Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77189}

src/flags/flag-definitions.h

@@ -1816,6 +1816,8 @@ DEFINE_BOOL_READONLY(minor_mc, false,
 //
 
 DEFINE_BOOL(help, false, "Print usage message, including flags, on console")
+DEFINE_BOOL(print_flag_values, false, "Print all flag values of V8")
+
 DEFINE_BOOL(dump_counters, false, "Dump counters on exit")
 DEFINE_BOOL(slow_histograms, false,
             "Enable slow histograms with more overhead.")

src/flags/flags.cc

@@ -9,6 +9,7 @@
 #include <cinttypes>
 #include <cstdlib>
 #include <cstring>
+#include <iomanip>
 #include <sstream>
 
 #include "src/base/functional.h"
@@ -40,6 +41,8 @@ namespace internal {
 
 namespace {
 
+char NormalizeChar(char ch) { return ch == '_' ? '-' : ch; }
+
 struct Flag;
 Flag* FindFlagByPointer(const void* ptr);
 Flag* FindFlagByName(const char* name);
@@ -380,8 +383,6 @@ Flag flags[] = {
 
 const size_t num_flags = sizeof(flags) / sizeof(*flags);
 
-inline char NormalizeChar(char ch) { return ch == '_' ? '-' : ch; }
-
 bool EqualNames(const char* a, const char* b) {
   for (int i = 0; NormalizeChar(a[i]) == NormalizeChar(b[i]); i++) {
     if (a[i] == '\0') {
@@ -429,7 +430,27 @@ static const char* Type2String(Flag::FlagType type) {
   UNREACHABLE();
 }
 
-std::ostream& operator<<(std::ostream& os, const Flag& flag) {
+// Helper struct for printing normalize Flag names.
+struct FlagName {
+  explicit FlagName(const Flag& flag) : flag(flag) {}
+  const Flag& flag;
+};
+
+std::ostream& operator<<(std::ostream& os, const FlagName& flag_name) {
+  for (const char* c = flag_name.flag.name(); *c != '\0'; ++c) {
+    os << NormalizeChar(*c);
+  }
+  return os;
+}
+
+// Helper for printing flag values.
+struct FlagValue {
+  explicit FlagValue(const Flag& flag) : flag(flag) {}
+  const Flag& flag;
+};
+
+std::ostream& operator<<(std::ostream& os, const FlagValue& flag_value) {
+  const Flag& flag = flag_value.flag;
   switch (flag.type()) {
     case Flag::TYPE_BOOL:
       os << (flag.bool_variable() ? "true" : "false");
@@ -456,33 +477,20 @@ std::ostream& operator<<(std::ostream& os, const Flag& flag) {
       break;
     case Flag::TYPE_STRING: {
       const char* str = flag.string_value();
-      os << (str ? str : "nullptr");
+      os << std::quoted(str ? str : "");
       break;
     }
   }
   return os;
 }
 
-// static
-std::vector<const char*>* FlagList::argv() {
-  std::vector<const char*>* args = new std::vector<const char*>(8);
-  for (size_t i = 0; i < num_flags; ++i) {
-    Flag* f = &flags[i];
-    if (!f->IsDefault()) {
-      {
-        bool disabled = f->type() == Flag::TYPE_BOOL && !f->bool_variable();
-        std::ostringstream os;
-        os << (disabled ? "--no" : "--") << f->name();
-        args->push_back(StrDup(os.str().c_str()));
-      }
-      if (f->type() != Flag::TYPE_BOOL) {
-        std::ostringstream os;
-        os << *f;
-        args->push_back(StrDup(os.str().c_str()));
-      }
-    }
+std::ostream& operator<<(std::ostream& os, const Flag& flag) {
+  if (flag.type() == Flag::TYPE_BOOL) {
+    os << (flag.bool_variable() ? "--" : "--no") << FlagName(flag);
+  } else {
+    os << "--" << FlagName(flag) << "=" << FlagValue(flag);
   }
-  return args;
+  return os;
 }
 
 // Helper function to parse flags: Takes an argument arg and splits it into
@@ -768,16 +776,20 @@ void FlagList::PrintHelp() {
   os << "Options:\n";
 
   for (const Flag& f : flags) {
-    os << "  --";
-    for (const char* c = f.name(); *c != '\0'; ++c) {
-      os << NormalizeChar(*c);
-    }
-    os << " (" << f.comment() << ")\n"
+    os << "  --" << FlagName(f) << " (" << f.comment() << ")\n"
        << "        type: " << Type2String(f.type()) << "  default: " << f
        << "\n";
   }
 }
 
+// static
+void FlagList::PrintValues() {
+  StdoutStream os;
+  for (const Flag& f : flags) {
+    os << f << "\n";
+  }
+}
+
 namespace {
 
 static uint32_t flag_hash = 0;

src/flags/flags.h

@@ -19,15 +19,6 @@ namespace internal {
 // The global list of all flags.
 class V8_EXPORT_PRIVATE FlagList {
  public:
-  // The list of all flags with a value different from the default
-  // and their values. The format of the list is like the format of the
-  // argv array passed to the main function, e.g.
-  // ("--prof", "--log-file", "v8.prof", "--nolazy").
-  //
-  // The caller is responsible for disposing the list, as well
-  // as every element of it.
-  static std::vector<const char*>* argv();
-
   class HelpOptions {
    public:
     enum ExitBehavior : bool { kExit = true, kDontExit = false };
@@ -78,6 +69,8 @@ class V8_EXPORT_PRIVATE FlagList {
   // Print help to stdout with flags, types, and default values.
   static void PrintHelp();
 
+  static void PrintValues();
+
   // Set flags as consequence of being implied by another flag.
   static void EnforceFlagImplications();
 

src/init/v8.cc

@@ -181,6 +181,8 @@ void V8::InitializeOncePerProcessImpl() {
 
   if (FLAG_random_seed) SetRandomMmapSeed(FLAG_random_seed);
 
+  if (FLAG_print_flag_values) FlagList::PrintValues();
+
 #if defined(V8_USE_PERFETTO)
   if (perfetto::Tracing::IsInitialized()) TrackEvent::Register();
 #endif