build system: don't optimize when compiling iso639_language_list.cpp

Even `-O1` causes compilation time & memory usage to skyrocket,
possibly exponentially, with the number of entries to `emplace_back()`
into the vector.

This isn't so bad with the current number of entries (489). In that
case compilation with `-O3` only takes 7.2s.

However, extending the list to cover ISO 639-3 means that the list
will include 7160 entries. With that many entries things are much,
much more severe:

• with `-O1` alone compilation takes 11m 23s already.
• with `-O3` memory usage exceeded 20 GB after six minutes when I had
  to abort due to other running applications getting killed.

Runtime cost is negligible. I ran a micro benchmark. With all 7160
entries and no optimizations (`-O0`) the initialization takes ~1.4
milliseconds for the one-time initialization on startup; with
optimizations (`-O1`) it still took ~570 microseconds.

Part of the implementation of #3007.

Rakefile

@@ -347,6 +347,13 @@ cxx_compiler = lambda do |*args|
     flags.gsub!(%r{-Wpedantic}, '')
   end
 
+  if %r{src/common/iso639_language_list.cpp}.match(source)
+    # Even -O1 causes compilation time & memory usage to skyrocket,
+    # possibly exponentially, with the number of entries to
+    # emplace_back() into the vector.
+    flags.gsub!(%r{ -O\d+ }, ' -O0 ')
+  end
+
   args = [
     "cxx", source,
     "#{c(:CXX)} #{flags}#{pchu}#{pchx} #{$system_includes} -c -MMD -MF #{dep} -o #{t.name} -x #{lang} #{source}",

rake.d/iso639.rb

@@ -39,9 +39,9 @@ def create_iso639_language_list_file
 // NOTE: this file is auto-generated by the "dev:iso639_list" rake target.
 // -----------------------------------------------------------------------
 
-#include "common/common_pch.h"
+#include "common/iso639_types.h"
 
-#include "common/iso639.h"
+using namespace std::string_literals;
 
 namespace mtx::iso639 {
 

rake.d/pch.rb

@@ -403,7 +403,7 @@ PCH status: <%= c?(:USE_PRECOMPILED_HEADERS) ? "enabled" : "disabled" %>
 
   def self.info_for_user(user, ofile)
     f = Info.new
-    if c?(:USE_PRECOMPILED_HEADERS)
+    if c?(:USE_PRECOMPILED_HEADERS) && !%r{src/common/iso639_language_list.cpp}.match(user)
       user = Pathname.new(user).cleanpath.to_s
       f.language = "c++-header" if user.end_with?(".h")
       header = @db_scan.fetch(user, nil)

src/common/iso639.h

@@ -15,24 +15,10 @@
 
 #include "common/common_pch.h"
 
+#include "common/iso639_types.h"
+
 namespace mtx::iso639 {
 
-struct language_t {
-  std::string const english_name, alpha_3_code, alpha_2_code, terminology_abbrev;
-  bool is_part_of_iso639_2{};
-
-  language_t(std::string &&p_english_name, std::string &&p_alpha_3_code, std::string &&p_alpha_2_code, std::string &&p_terminology_abbrev, bool p_is_part_of_iso639_2)
-    : english_name{std::move(p_english_name)}
-    , alpha_3_code{std::move(p_alpha_3_code)}
-    , alpha_2_code{std::move(p_alpha_2_code)}
-    , terminology_abbrev{std::move(p_terminology_abbrev)}
-    , is_part_of_iso639_2{p_is_part_of_iso639_2}
-  {
-  }
-};
-
-extern std::vector<language_t> g_languages;
-
 void init();
 std::optional<language_t> look_up(std::string const &s, bool allow_short_english_names = false);
 void list_languages();

src/common/iso639_language_list.cpp

@@ -15,9 +15,9 @@
 // NOTE: this file is auto-generated by the "dev:iso639_list" rake target.
 // -----------------------------------------------------------------------
 
-#include "common/common_pch.h"
+#include "common/iso639_types.h"
 
-#include "common/iso639.h"
+using namespace std::string_literals;
 
 namespace mtx::iso639 {
 

src/common/iso639_types.h

@@ -0,0 +1,37 @@
+/*
+   mkvmerge -- utility for splicing together matroska files
+   from component media subtypes
+
+   Distributed under the GPL v2
+   see the file COPYING for details
+   or visit https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+
+   ISO 639 types
+
+   Written by Moritz Bunkus <moritz@bunkus.org>.
+*/
+
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace mtx::iso639 {
+
+struct language_t {
+  std::string const english_name, alpha_3_code, alpha_2_code, terminology_abbrev;
+  bool is_part_of_iso639_2{};
+
+  language_t(std::string &&p_english_name, std::string &&p_alpha_3_code, std::string &&p_alpha_2_code, std::string &&p_terminology_abbrev, bool p_is_part_of_iso639_2)
+    : english_name{std::move(p_english_name)}
+    , alpha_3_code{std::move(p_alpha_3_code)}
+    , alpha_2_code{std::move(p_alpha_2_code)}
+    , terminology_abbrev{std::move(p_terminology_abbrev)}
+    , is_part_of_iso639_2{p_is_part_of_iso639_2}
+  {
+  }
+};
+
+extern std::vector<language_t> g_languages;
+
+} // namespace mtx::iso639