Graph trimming and remote caching

[Konstantin]

This conversation from four years ago suggests that graph trimming with unused_inputs_list attribute to ctx.actions.run() hurts remote caching as the cache key is constructed from all declared inputs instead of only used ones. I wonder if it is still the case and what can possibly be done to make it work.

C++ compilation rules use unused_inputs_list to remove unused headers from the list of compilation action inputs and it seriously harms remote caching efficiency. Looking for a workaround. 

Any thoughts?

Konstantin

[Yannic]

Hey Konstantin,

At least for C++, using declared inputs to compute the cache key is mandatory for correctness. One can use __has_include() to change the behavior of programs depending on the pure existence of a declared input. Please see this example: https://github.com/bazelbuild/bazel/pull/13592#issuecomment-976694722

Thanks,

Yannic