Add C++20 audit (#2688)

b/309847996
b/329683364

Refactor c++17 flags from all platforms used while building cobalt into
a central place "default_cpp_standard" in
starboard/build/config/BUILD.gn
Remove default config while compiling c++20 audit, add
"cpp20_supported_config" in it's place.

Co-author: andrewsavage1

---------

Co-authored-by: Andrew Savage <[email protected]>

starboard/android/shared/platform_configuration/BUILD.gn

@@ -70,7 +70,6 @@ config("platform_configuration") {
     defines += [ "THREAD_SANITIZER" ]
   }
 
-  cflags_cc = [ "-std=c++17" ]
   cflags += [
     # libwebp uses the cpufeatures library to detect ARM NEON support
     "-I${android_ndk_path}/sources/android/cpufeatures",

starboard/build/config/BUILD.gn

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import("//build/config/compiler/compiler.gni")
+import("//starboard/build/config/os_definitions.gni")
 
 config("base") {
   defines = [ "USE_COBALT_CUSTOMIZATIONS" ]
@@ -84,7 +85,11 @@ config("host") {
       cflags = [
         "/wd4267",  # Possible loss of precision from size_t to a smaller type.
         "/wd4715",  # Not all control paths return value.
+        "/std:c++17",
       ]
+    } else if (is_apple) {
+      cflags_cc = [ "-std=gnu++17" ]
+      cflags_objcc = [ "-std=gnu++17" ]
     }
   }
 }
@@ -233,10 +238,49 @@ config("default_compiler_flags") {
   }
 }
 
-# TODO: b/329683364 - Move other flags setting the c++ standard from each
-# platform's platform_configuration to this config.
+# This config is intended to encapsulate all platform-specific logic for selecting the C++ standard
+# version in shared code while building with the default toolchain.
+# Starboard platform-specific build code should not attempt to overload this.
 config("default_cpp_standard") {
-  if (sb_is_modular && current_toolchain == cobalt_toolchain) {
-    cflags_cc = [ "-std=c++17" ]
+  if (current_toolchain == default_toolchain) {
+    if (sb_is_modular || is_android) {
+      cflags_cc = [ "-std=c++17" ]
+    }
+
+    # These flags apply to non-modular windows platforms which includes win32, xb1.
+    if (is_host_win && !sb_is_modular) {
+      cflags = [ "/std:c++17" ]
+    }
+
+    if (is_linux && !sb_is_modular) {
+      cflags_cc = [ "-std=gnu++17" ]
+    }
+
+    if (is_apple) {
+      cflags_cc = [ "-std=gnu++17" ]
+      cflags_objcc = [ "-std=gnu++17" ]
+    }
+  }
+}
+
+config("cpp20_supported_config") {
+  if (current_toolchain == default_toolchain) {
+    if (sb_is_modular || is_android) {
+      cflags_cc = [ "-std=c++20" ]
+    }
+
+    # These flags apply to non-modular windows platforms which includes win32, xb1.
+    if (is_host_win && !sb_is_modular) {
+      cflags = [ "/std:c++20" ]
+    }
+
+    if (is_linux && !sb_is_modular) {
+      cflags_cc = [ "-std=gnu++20" ]
+    }
+
+    if (is_apple) {
+      cflags_cc = [ "-std=gnu++20" ]
+      cflags_objcc = [ "-std=gnu++20" ]
+    }
   }
 }

starboard/build/config/base_configuration.gni

@@ -158,6 +158,9 @@ declare_args() {
   # Enables an NPLB audit of C++17 support.
   sb_enable_cpp17_audit = true
 
+  # Enables an NPLB audit of C++20 support.
+  sb_enable_cpp20_audit = true
+
   # Enable when using clang 16.
   is_clang_16 = false
 

starboard/build/config/mac/BUILD.gn

@@ -43,8 +43,6 @@ config("host") {
 
 config("common") {
   arflags = [ "-no_warning_for_no_symbols" ]
-  cflags_cc = [ "-std=gnu++17" ]
-  cflags_objcc = [ "-std=gnu++17" ]
   cflags = [ "-fno-common" ]
   asmflags = [ "-fno-common" ]
   ldflags = [ "-fno-common" ]

starboard/build/config/win/BUILD.gn

@@ -45,10 +45,7 @@ config("common") {
     "$wdk_include_path/winrt",
     "$msvc_path/include",
   ]
-  cflags += [
-    "/EHsc",
-    "/std:c++17",
-  ]
+  cflags += [ "/EHsc" ]
 
   # msvs_debug/_devel/etc
   ldflags += [ "/INCREMENTAL:NO" ]

starboard/linux/shared/platform_configuration/BUILD.gn

@@ -98,7 +98,6 @@ config("compiler_flags") {
     "MESA_EGL_NO_X11_HEADERS",
   ]
   cflags_c += [ "-std=c11" ]
-  cflags_cc = [ "-std=gnu++17" ]
 
   if (use_asan) {
     cflags += [

starboard/nplb/BUILD.gn

@@ -291,6 +291,10 @@ target(gtest_target_type, "nplb") {
     deps += [ "//starboard/nplb/compiler_compliance:cpp17_supported" ]
   }
 
+  if (sb_enable_cpp20_audit) {
+    deps += [ "//starboard/nplb/compiler_compliance:cpp20_supported" ]
+  }
+
   data_deps = [
     "//cobalt/network:copy_ssl_certificates",
     "//starboard/nplb/testdata/file_tests:nplb_file_tests_data",

starboard/nplb/compiler_compliance/BUILD.gn

@@ -40,3 +40,20 @@ if (sb_enable_cpp17_audit) {
     cflags_cc = [ "-std=c++17" ]
   }
 }
+
+if (sb_enable_cpp20_audit) {
+  static_library("cpp20_supported") {
+    sources = [ "cpp20_support.cc" ]
+    deps = [
+      ":cpp20_supported_config_shim",
+      "//starboard:starboard_group",
+    ]
+    configs -= [ "//starboard/build/config:default_cpp_standard" ]
+  }
+
+  # We do this to ensure the -std=c++20 flag is added after any other -std flag
+  # so it overrides any other one.
+  group("cpp20_supported_config_shim") {
+    public_configs = [ "//starboard/build/config:cpp20_supported_config" ]
+  }
+}

starboard/nplb/compiler_compliance/cpp20_support.cc

@@ -0,0 +1,65 @@
+// Copyright 2024 The Cobalt Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the aLicense is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <iostream>
+#include <memory>
+#include <numeric>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+namespace starboard {
+namespace nplb {
+namespace compiler_compliance {
+namespace {
+
+// These examples are taken after referring to
+// 1) C++20 allowlist from chromium m114 milestone branch :
+// https://chromium.googlesource.com/chromium/src/+/refs/branch-heads/5735/styleguide/c++/c++-features.md
+// 2) cpp reference : https://en.cppreference.com/w/cpp
+
+// Test std::string ends_with support
+void test_string_ends_with() {
+  bool result = std::string("foobar").ends_with("bar");
+}
+
+// Test std::erase_if support
+void test_erase_if() {
+  std::vector<char> cnt(10);
+  std::iota(cnt.begin(), cnt.end(), '0');
+  std::erase(cnt, '3');
+  std::erase_if(cnt, [](char x) { return (x - '0') % 2 == 0; });
+}
+
+// Test std::midpoint support
+void test_midpoint() {
+  std::uint32_t a = std::numeric_limits<std::uint32_t>::max();
+  std::uint32_t b = std::numeric_limits<std::uint32_t>::max() - 2;
+  std::midpoint(a, b);
+}
+
+// Test designated initializers
+void test_designated_initializer() {
+  struct S {
+    int x = 1;
+    int y = 2;
+  };
+  S s{.y = 3};  // OK, s.x == 1, s.y == 3
+}
+
+}  // namespace
+}  // namespace compiler_compliance
+}  // namespace nplb
+}  // namespace starboard

starboard/stub/platform_configuration/BUILD.gn

@@ -16,7 +16,6 @@ config("platform_configuration") {
   configs = [ "//starboard/build/config/sabi" ]
   cflags = []
 
-  cflags_cc = [ "-std=gnu++17" ]
   ldflags = [ "-static-libstdc++" ]
 
   libs = [ "pthread" ]