Static Initializers

Some background on the original decision to ban static initializers:

http://neugierig.org/software/chromium/notes/2011/08/static-initializers.html

Note: Another name for static initializers is “global constructors”.

How Static Initializers are Checked

• For Linux and Mac:
  • The expected count is stored in //testing/scripts/check_static_initializers.py
• For Android:
  • The expected count is stored in the build target //chrome/android:monochrome_static_initializers

Removing Static Initializers

Common fixes include:

• Add constexpr.
• Move global variable to be a static variable within a function that returns it, often wrapped in base::NoDestructor.

Listing Static Initializers

Step 1 - Use objdump to report them

For Linux:

tools/linux/dump-static-initializers.py out/Release/chrome

For Android (from easiest to hardest):

# Build with: is_official_build=true is_chrome_branded=true
# This will dump the list of SI's only when they don't match the expected
# number in static_initializers.gni (this is what the bots use).
ninja chrome/android:monochrome_static_initializers
# or:
tools/binary_size/diagnose_bloat.py HEAD  # See README.md for flags.
# or (the other two use this under the hood):
tools/linux/dump-static-initializers.py --toolchain-prefix third_party/android_ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/arm-linux-androideabi- out/Release/lib.unstripped/libmonochrome.so
# arm32 ^^ vv arm64
tools/linux/dump-static-initializers.py --toolchain-prefix third_party/android_ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android- out/Release/lib.unstripped/libmonochrome.so
# Note: For arm64, having use_thin_lto=true seems to dump a couple extra
#     initializers that don't actually exist.

The last one may actually be the easiest if you've already properly built libmonochrome.so with is_official_build=true.

Step 2 - Ask compiler to report them

If the source of the new initializers is not obvious from Step 1, you can ask the compiler to pinpoint the exact source line.

1. Edit //build/config/BUILDCONFIG.gn and add "//build/config/compiler:wglobal_constructors" to default_compiler_configs
2. Remove the config from the configs in //base:base
3. Set GN arg treat_warnings_as_errors=false
4. Compile and look for warnings from the files identified by step 1 (may want to pipe ninja output to a file).

The compiler warning triggers for every static initializer that exists before optimization. We care only about those that survive optimization. More details in crbug/1136086.

• For more information about diagnose_bloat.py, refer to its README.md
• List of existing static initializers documented in static_initializers.gni

Step 3 - Manual verification

If the source of the new initializers is not revealed with dump-static-initializers.py (e.g. for static initializers introduced in compiler-rt), there's a manual option.

1. Locate the address range of the .init_array section with llvm-readelf --hex-dump=.init_array ./lib.unstripped/libmonochrome_64.so. It will yield an address range like 0x0917fd40 to 0x0918fd78.
2. Each .init_array slot may be zero if the contents are relocatable. To translate, use a command like llvm-readelf --relocations ./lib/unstripped | grep 0x0917fd40 to obtain a result mapping each .init_array slot to a function address.

    000000000918fd40  0000000000000403 R_AARCH64_RELATIVE                51732f0

3. Finally, convert the address into a function name with llvm-addr2line --functions -e ./lib.unstripped/libmonochrome_64.so 51732f0

    __cxx_global_var_init
    ./../../buildtools/third_party/libc++/trunk/src/iostream.cpp:80