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>