cmd/compile, cmd/link: use libFuzzer 8-bit instead of extra counters

By using libFuzzer’s 8-bit counters instead of extra counters, the
coverage instrumentation in libFuzzer mode is improved in three ways:
  1- 8-bit counters are supported on all platforms, including macOS and
     Windows, with all relevant versions of libFuzzer, whereas extra
     counters are a Linux-only feature that only recently received
     support on Windows.
  2- Newly covered blocks are now properly reported as new coverage by
     libFuzzer, not only as new features.
  3- The NeverZero strategy is used to ensure that coverage counters
     never become 0 again after having been positive once. This resolves
     issues encountered when fuzzing loops with iteration counts that
     are multiples of 256 (e.g., larger powers of two).

Change-Id: I9021210d7fbffd07c891ad08750402ee91cb3df5
GitHub-Last-Rev: 9057e4b21d146ce9ffb3993982bfb84b96705989
GitHub-Pull-Request: golang/go#51318
Reviewed-on: https://go-review.googlesource.com/c/go/+/387334
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

0 files changed, 0 insertions, 0 deletions