Adds a --help option and usage message listing all flags to V8.

src/flags.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 299 matching lines @@
       // lookup the flag
       Flag* flag = FindFlag(name);
       if (flag == NULL) {
         if (remove_flags) {
           // We don't recognize this flag but since we're removing
           // the flags we recognize we assume that the remaining flags
           // will be processed somewhere else so this flag might make
           // sense there.
           continue;
         } else {
-          fprintf(stderr, "Error: unrecognized flag %s\n", arg);
+          fprintf(stderr, "Error: unrecognized flag %s\n"
+                  "Try --help for options\n", arg);
           return j;
         }
       }
 
       // if we still need a flag value, use the next argument if available
       if (flag->type() != Flag::TYPE_BOOL && value == NULL) {
         if (i < *argc) {
           value = argv[i++];
         } else {
-          fprintf(stderr, "Error: missing value for flag %s of type %s\n",
+          fprintf(stderr, "Error: missing value for flag %s of type %s\n"
+                  "Try --help for options\n",
                   arg, Type2String(flag->type()));
           return j;
         }
       }
 
       // set the flag
       char* endp = const_cast<char*>("");  // *endp is only read
       switch (flag->type()) {
         case Flag::TYPE_BOOL:
           *flag->bool_variable() = !is_bool;
           break;
         case Flag::TYPE_INT:
           *flag->int_variable() = strtol(value, &endp, 10);  // NOLINT
           break;
         case Flag::TYPE_FLOAT:
           *flag->float_variable() = strtod(value, &endp);
           break;
         case Flag::TYPE_STRING:
           *flag->string_variable() = value;
           break;
       }
 
       // handle errors
       if ((flag->type() == Flag::TYPE_BOOL && value != NULL) ||
           (flag->type() != Flag::TYPE_BOOL && is_bool) ||
           *endp != '\0') {
-        fprintf(stderr, "Error: illegal value for flag %s of type %s\n",
+        fprintf(stderr, "Error: illegal value for flag %s of type %s\n"
+                "Try --help for options\n",
                 arg, Type2String(flag->type()));
         return j;
       }
 
       // remove the flag & value from the command
       if (remove_flags)
         while (j < i)
           argv[j++] = NULL;
     }
   }
 
   // shrink the argument list
   if (remove_flags) {
     int j = 1;
     for (int i = 1; i < *argc; i++) {
       if (argv[i] != NULL)
         argv[j++] = argv[i];
     }
     *argc = j;
   }
 
+  if (FLAG_help) {
+    PrintHelp();
+  }
   // parsed all flags successfully
   return 0;
 }
 
 
 static char* SkipWhiteSpace(char* p) {
   while (*p != '\0' && isspace(*p) != 0) p++;
   return p;
 }
 
@@ skipping 51 matching lines @@
 // static
 void FlagList::ResetAllFlags() {
   for (size_t i = 0; i < num_flags; ++i) {
     flags[i].Reset();
   }
 }
 
 
 // static
 void FlagList::PrintHelp() {
+  printf("Usage:\n");
+  printf("  shell [options] -e string\n");
+  printf("    execute string in V8\n");
+  printf("  shell [options] file1 file2 ... filek\n");
+  printf("    run JavaScript scripts in file1, file2, ..., filek\n");
+  printf("  shell [options]\n");
+  printf("  shell [options] --shell\n");
+  printf("    run an interactive JavaScript shell");
+  printf("  d8 [options] file\n");
+  printf("  d8 [options]\n");
+  printf("    run the new debugging shell\n\n");
+  printf("Options:\n");
   for (size_t i = 0; i < num_flags; ++i) {
     Flag* f = &flags[i];
     char* value = ToString(f);
-    printf("  --%s (%s)  type: %s  default: %s\n",
+    printf("  --%s (%s)\n        type: %s  default: %s\n",
            f->name(), f->comment(), Type2String(f->type()), value);
     DeleteArray(value);
   }
 }
 
 } }  // namespace v8::internal