[cfe-dev] RFC: Attribute to suppress coverage mapping for functions

[Vedant Kumar via cfe-dev]

Hi,

I'd like to add a new attribute which can be used to suppress code coverage
reporting on the function level. This is meant to work with clang's
frontend-based coverage implementation [1]. Here are some potential users of
the new attribute:

  * llvm_unreachable_internal.

  * The various dump() functions for llvm Values, AST nodes, etc.

  * Methods in a base class which just call llvm_unreachable.

These functions are usually not covered. It's not practical to write "death
tests" for all of them, and it's also unhelpful to report missing coverage for
them.

I'd like to be able to write this in C:

  void foo() __attribute__((nocoverage)) { llvm_unreachable("boom!"); }

And this in C++11:

  void foo() [[clang::nocoverage]] { llvm_unreachable("boom!"); }

Here are some alternatives and why I think they're not as good:

  * Define a preprocessor macro when -fcoverage-mapping is enabled.

    Conditionally compiling code based on whether code coverage is enabled
    sounds scary. We shouldn't make it easy (or possible?) to change the
    meaning of a program by enabling coverage.

  * Pass a function blacklist to llvm-cov.

    The blacklist would have to live separately from the source code, and may
    get out of sync. We also would go through the trouble of emitting coverage
    mappings for functions even though they aren't needed.

  * Add a pair of pragmas to arbitrarily stop/resume coverage mapping.

    We'd need some extra diagnostics to catch abuse of the pragmas. It also
    requires more typing in the common case (disabling coverage at the function
    level).

  * Look at the function CFG. If all paths through the function can be shown to
    reach __builtin_unreachable(), don't create coverage mappings for the
    function.

    I'm worried this might be complicated and inflexible. It wouldn't let us
    mark dump() functions as uncovered.

Wdyt?

vedant

[1] http://clang.llvm.org/docs/SourceBasedCodeCoverage.html