static_initializers.md: Document -Wglobal-constructors

Also moves the config for this from //base -> //build/config/compiler
for symmetry with //build/config/compiler:wexit_time_destructors.

Bug: None
Change-Id: If117e5963df44fdc2f03e01716e8af16e2c91e58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453460
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814731}

base/BUILD.gn

@@ -92,18 +92,6 @@ if (is_fuchsia) {
   import("//third_party/fuchsia-sdk/sdk/build/fidl_library.gni")
 }
 
-config("base_flags") {
-  if (is_clang) {
-    cflags = [
-      # Ideally all product code (but no test code) in chrome would have these
-      # flags. But this isn't trivial so start with //base as a minimum
-      # requirement.
-      # https://groups.google.com/a/chromium.org/d/topic/chromium-dev/B9Q5KTD7iCo/discussion
-      "-Wglobal-constructors",
-    ]
-  }
-}
-
 config("base_implementation") {
   defines = [ "BASE_IMPLEMENTATION" ]
   configs = [ "//build/config/compiler:wexit_time_destructors" ]
@@ -1249,10 +1237,10 @@ component("base") {
   frameworks = []
 
   configs += [
-    ":base_flags",
     ":base_implementation",
     "//build/config:precompiled_headers",
     "//build/config/compiler:noshadowing",
+    "//build/config/compiler:wglobal_constructors",
   ]
 
   deps = [

build/config/compiler/BUILD.gn

@@ -1830,9 +1830,18 @@ config("no_exceptions") {
 
 # Warnings ---------------------------------------------------------------------
 
+# Generate a warning for code that might emit a static initializer.
+# See: //docs/static_initializers.md
+# See: https://groups.google.com/a/chromium.org/d/topic/chromium-dev/B9Q5KTD7iCo/discussion
+config("wglobal_constructors") {
+  if (is_clang) {
+    cflags = [ "-Wglobal-constructors" ]
+  }
+}
+
 # This will generate warnings when using Clang if code generates exit-time
 # destructors, which will slow down closing the program.
-# TODO(thakis): Make this a blacklist instead, http://crbug.com/101600
+# TODO(thakis): Make this a blocklist instead, http://crbug.com/101600
 config("wexit_time_destructors") {
   if (is_clang) {
     cflags = [ "-Wexit-time-destructors" ]

docs/static_initializers.md

@@ -23,6 +23,17 @@ Common fixes include:
 
 ## Listing Static Initializers
 
+### Method 1 - Ask compiler to report them
+1. Edit [//build/config/BUILDCONFIG.gn](https://cs.chromium.org/chromium/src/build/config/BUILDCONFIG.gn)
+and add `"//build/config/compiler:wglobal_constructors"` to `default_compiler_configs`
+2. Set GN arg `treat_warnings_as_errors=false`
+3. Compile and look at warnings
+
+This will produce far more warnings than there are static initializers because
+it (seems to) trigger on global variables initialized with constexpr
+constructors.
+
+### Method 2 - Use objdump to report them
 For Linux:
 
     tools/linux/dump-static-initializers.py out/Release/chrome