git cl format and gcc disagree on "multi-line comment"s

[ghost]

vm-kernel-gcc-linux is currently failing with:

 FAILED: obj/runtime/bin/ffi_test/libffi_test_functions.ffi_test_functions_vmspecific.o 
 g++ -MMD -MF obj/runtime/bin/ffi_test/libffi_test_functions.ffi_test_functions_vmspecific.o.d -DDART_SHARED_LIB -D_FORTIFY_SOURCE=2 -DDEBUG -I../../runtime -I../.. -Igen -fPIC -m32 -msse2 -mfpmath=sse -fno-exceptions -Wall -Wextra -Werror -Wendif-labels -Wno-missing-field-initializers -Wno-unused-parameter -Wno-ignored-qualifiers -fdebug-prefix-map=/b/s/w/ir/cache/builder/sdk=../.. -no-canonical-prefixes -O2 -fdata-sections -ffunction-sections -g3 -ggdb3 -fvisibility-inlines-hidden -fno-omit-frame-pointer -std=c++17 -fno-rtti -fno-exceptions -c ../../runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc -o obj/runtime/bin/ffi_test/libffi_test_functions.ffi_test_functions_vmspecific.o
 ../../runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc:1304:9: error: multi-line comment [-Werror=comment]
  #endif  // defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID) ||     \
          ^
 cc1plus: all warnings being treated as errors

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8816300984185262753/+/u/build_dart_ia32/stdout

But modifying the code to:

#endif  // defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID) ||
        // defined(DART_HOST_OS_MACOS)

Will cause git cl format (and therefore our presubmit) to complain.

I'm guessing we'll need to modify git cl format to not enforce multiline comments like this.

[whesse]

git cl format is part of depot_tools, so it would need to be modified there. It pulls down the latest version of clang-format binaries and runs them to format the C++ code according to clang standards.

I think rather than trying to exclude this file from clang formatting, it would be better for our build rules to disable the gcc warning for multi-line comments when building with gcc.

If clang standards and gcc warnings are conflicting, I think we will want to go with clang standards.
We should be able to disable this warning by adding it here, or 10 lines later:

https://github.com/dart-lang/sdk/blob/main/build/config/compiler/BUILD.gn#L572

[whesse]

Also, modifying the code to:

#endif  // defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID) ||
// defined(DART_HOST_OS_MACOS)

or to

#endif /* defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID) ||
          defined(DART_HOST_OS_MACOS) */

will avoid the problem.
If the second comment line is indented at all, then clang-format will first indent it the same as the first line, then, the next time it is run, add the backslash.

Other alternatives are to put all comments on the succeeding lines, none on the line with #endif.

I have filed an issue with the team that maintains clang-format for Chromium, and they may upstream it to clang-format at
https://github.com/llvm/llvm-project/labels/clang-format

I filed the issue as low-priority, since there seems to be workarounds.