[vm/ffi] Fix --enable-ffi=false

[dcharkes]

Our test testing whether the flag works has a bug, it triggers a compile error but not because of the flag.

Fixing the compile error makes the test succeed, even with --enable-ffi=false.

/cc @sjindel-google

[sjindel-google]

This is probably because the check for imports of "dart:ffi" which enforces the flag is performed in the Kernel FE, not the KBC FE.

[dcharkes]

It indeed looks like that the frontend does not pass that the FFI library is used.

  if (((flags & kUsesDartFfiFlag) != 0) && !Api::IsFfiEnabled()) {
    H.ReportError(
        "import of dart:ffi is not supported in the current Dart runtime");
  }

runtime/vm/compiler/frontend/bytecode_reader.cc

flags is 0.

It should be set here:

    for (var dependency in library.dependencies) {
      final targetLibrary = dependency.targetLibrary;
      assert(targetLibrary != null);
      if (targetLibrary == coreTypes.mirrorsLibrary) {
        flags |= LibraryDeclaration.usesDartMirrorsFlag;
      } else if (targetLibrary == dartFfiLibrary) {
        flags |= LibraryDeclaration.usesDartFfiFlag;
      }
    }

pkg/vm/lib/bytecode/gen_bytecode.dart

@alexmarkov what Dart script do I invoke to run gen_bytecode.dart to verify if it sets the usesDartFfiFlag?

[alexmarkov]

Currently bytecode generator is used in kernel service of standalone VM by default. You can add debug prints into gen_bytecode.dart, rebuild Dart SDK and see what's printed when you run the test using dart command.

Also, you can build a dill file with bytecode, run it and dump it:

pkg/vm/tool/gen_kernel --no-aot --no-link-platform --platform=out/ReleaseX64/vm_platform_strong.dill -o out.dill foo.dart --packages=.packages -Ddart.developer.causal_async_stacks=true --gen-bytecode  --drop-ast --bytecode-options=source-positions,local-var-info,debugger-stops,annotations
out/ReleaseX64/dart out.dill
pkg/vm/tool/dump_kernel out.dill out.dill.dump

Could you check if dart:ffi is indexed in CoreTypes.index ? Maybe

sdk/pkg/vm/lib/bytecode/gen_bytecode.dart

Lines 996 to 997 in a46bb7c

	Library get dartFfiLibrary =>
	_dartFfiLibrary ??= libraryIndex.tryGetLibrary('dart:ffi');

returns null. libraryIndex is taken from CoreTypes instance:

sdk/pkg/vm/lib/bytecode/gen_bytecode.dart

Line 890 in a46bb7c

LibraryIndex get libraryIndex => coreTypes.index;

[alexmarkov]

If dart:ffi is used without being included into CoreTypes.index, then we can fix detection of dart:fii import in bytecode generator in the following way: https://dart-review.googlesource.com/c/sdk/+/119180. @dcharkes could you try this?

[dcharkes]

@alexmarkov thanks! Yes, dart:ffi was not indexed in CoreTypes.index.

Offtopic: For the next time I need to work with gen_bytecode.dart

You can add debug prints into gen_bytecode.dart, rebuild Dart SDK and see what's printed when you run the test using dart command.

Building create_sdk takes like 2 minutes. Isn't there a way to run just run dart x/y/z/gen_bytecode.dart my/dart/source_program.dart without any compilation?

(I use dart pkg/vm/bin/gen_kernel.dart my/dart/source_program.dart to iterate fast and run the debugger without recompiling, when I'm working on the pkg/vm ffi transformations for example.)

[alexmarkov]

Isn't there a way to run just run dart x/y/z/gen_bytecode.dart my/dart/source_program.dart without any compilation?

@dcharkes as I mentioned above, you can also generate a dill file with bytecode using gen_kernel tool if you prefer (note that pkg/vm/tool/gen_kernel runs pkg/vm/bin/gen_kernel.dart from source). Just add --gen-bytecode flag to generate bytecode, optionally with --drop-ast --bytecode-options=source-positions,local-var-info,debugger-stops,annotations depending on your needs.