__EXCEPTIONS and mac

[Scott Violet]

Hello,

One of the third_party libraries we pull in is eigen. This library conditionally enables exceptions. One of the ifdefs it looks for to determine if it should enable exceptions is __EXCEPTIONS. It seems that this define is set on mac and none of the other platforms. I verified --fno-exceptions is on the command line of the file that triggers the exceptions. Does this seem right?

https://gitlab.com/libeigen/eigen/-/blob/master/Eigen/src/Core/util/Macros.h?ref_type=heads#L1238 is the line in eigen looking for the conditional.

https://chromium-review.googlesource.com/c/chromium/src/+/5771167/6/third_party/mediapipe/src/mediapipe/framework/formats/matrix.cc#20 is my hack to verify which code path we're hitting. It only fails on mac: https://ci.chromium.org/ui/p/chromium/builders/try/mac-rel/1885383/overview . And this code is compiled on windows/linux as well.

Thanks,

   -Scott

[Nico Weber]

I think __EXCEPTIONS is getting set on macOS when compiling Objective-C(++) files (.m, .mm), but not when building C++ files:

```

 % ~/src/chrome/src/third_party/llvm-build/Release+Asserts/bin/clang -c -x objective-c++ /dev/null -E -dM -fno-exceptions | grep EXCEPT
#define OBJC_ZEROCOST_EXCEPTIONS 1
#define __EXCEPTIONS 1

 % ~/src/chrome/src/third_party/llvm-build/Release+Asserts/bin/clang -c -x c++ /dev/null -E -dM -fno-exceptions | grep EXCEPT

# nothing

```

(`-x` tells clang the file type of the input file. I'm using `/dev/null` as input, so clang can't get the file type from the input file's extension like it usually does. Instead of `-x c++ /dev/null`, `touch empty.cc` and passing that would work the same.)

This is because -fno-exceptions disables C++ exceptions but not Objective-C exceptions.

If you disable Objective-C exceptions as well, it stops being defined:

```

% ~/src/chrome/src/third_party/llvm-build/Release+Asserts/bin/clang -c -x objective-c++ /dev/null -E -dM -fno-exceptions -fno-objc-exceptions | grep EXCEPT

```

In general, system libraries can throw (objc) exceptions, and user code sometimes has to catch them, so we don't disable ObjC exceptions globally.

Possible options:

* Move the code this is about from a .mm to a .cc file

* Ask upstream to use `__cpp_exceptions` instead of `__EXCEPTIONS`, which cleanly maps to just the C++ -fexceptions toggle and is unaffected by ObjC exception state

* Check if that code can be built with -fno-objc-exceptions

I think this is all working as intended from the toolchain side. (I agree it's confusing, and it took me a few minutes to figure it out.)

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[Mark Mentovai]

But this is failing while building matrix.cc, which is definitely not an .mm file.

(Curses are directed at autoninja/siso/reclient for not showing the compiler invocation when something fails.)

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[Nico Weber]

The build is passing -ObjC++ for that file, which turns it into an ObjC++ file.

(See raw_io.output_text; siso sadly hides the compile line quite a bit 😞)

[Mark Mentovai]

Oh, I was looking for -x objective-c++. Yes, we should stop building C++ as Objective-C++.

mark@arm-and-hammer zsh% git grep -F -- -ObjC++ '**/*.gn' '**/*.gni'
third_party/mediapipe/BUILD.gn:    cflags = [ "-ObjC++" ]

That’s here, and it was presumably done because resource_util_apple.cc is actually Objective-C++ source.

That file should be renamed to have a proper .mm extension, rather than polluting all of the other files that share its target with Objective-C semantics.

[Scott Violet]

Yes, I came across this when messaging with Nico. I'm going to see about putting the file that needs -ObjC++ into isn't target.

  -Scott

[Dirk Pranke]

On Fri, Aug 9, 2024 at 8:04 AM Nico Weber <tha...@chromium.org> wrote:

The build is passing -ObjC++ for that file, which turns it into an ObjC++ file.

(See raw_io.output_text; siso sadly hides the compile line quite a bit 😞)

Have we filed a bug w/ the Siso folks to get them to fix this?

-- Dirk

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[Nico Weber]

I filed b/333693728 a while ago.