Commits on Mar 16, 2021

1. Adjust -Ctarget-cpu=native handling in cg_llvm …

   When cg_llvm encounters the `-Ctarget-cpu=native` it computes an
   explciit set of features that applies to the target in order to
   correctly compile code for the host CPU (because e.g. `skylake` alone is
   not sufficient to tell if some of the instructions are available or
   not).
   
   However there were a couple of issues with how we did this. Firstly, the
   order in which features were overriden wasn't quite right – conceptually
   you'd expect `-Ctarget-cpu=native` option to override the features that
   are implicitly set by the target definition. However due to how other
   `-Ctarget-cpu` values are handled we must adopt the following order
   of priority:
   
   * Features from -Ctarget-cpu=*; are overriden by
   * Features implied by --target; are overriden by
   * Features from -Ctarget-feature; are overriden by
   * function specific features.
   
   Another problem was in that the function level `target-features`
   attribute would overwrite the entire set of the globally enabled
   features, rather than just the features the
   `#[target_feature(enable/disable)]` specified. With something like
   `-Ctarget-cpu=native` we'd end up in a situation wherein a function
   without `#[target_feature(enable)]` annotation would have a broader
   set of features compared to a function with one such attribute. This
   turned out to be a cause of heavy run-time regressions in some code
   using these function-level attributes in conjunction with
   `-Ctarget-cpu=native`, for example.
   
   With this PR rustc is more careful about specifying the entire set of
   features for functions that use `#[target_feature(enable/disable)]` or
   `#[instruction_set]` attributes.
   
   Sadly testing the original reproducer for this behaviour is quite
   impossible – we cannot rely on `-Ctarget-cpu=native` to be anything in
   particular on developer or CI machines.

   

   nagisa committed Mar 16, 2021
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