This patch adds support in gcc+gcov for modified condition/decision
coverage (MC/DC) with the -fcondition-coverage flag. MC/DC is a type of
test/code coverage and it is particularly important for safety-critical
applicaitons in industries like aviation and automotive. Notably, MC/DC
is required or recommended by:

    * DO-178C for the most critical software (Level A) in avionics.
    * IEC 61508 for SIL 4.
    * ISO 26262-6 for ASIL D.

From the SQLite webpage:

    Two methods of measuring test coverage were described above:
    "statement" and "branch" coverage. There are many other test
    coverage metrics besides these two. Another popular metric is
    "Modified Condition/Decision Coverage" or MC/DC. Wikipedia defines
    MC/DC as follows:

        * Each decision tries every possible outcome.
        * Each condition in a decision takes on every possible outcome.
        * Each entry and exit point is invoked.
        * Each condition in a decision is shown to independently affect
          the outcome of the decision.

    In the C programming language where && and || are "short-circuit"
    operators, MC/DC and branch coverage are very nearly the same thing.
    The primary difference is in boolean vector tests. One can test for
    any of several bits in bit-vector and still obtain 100% branch test
    coverage even though the second element of MC/DC - the requirement
    that each condition in a decision take on every possible outcome -
    might not be satisfied.

    https://sqlite.org/testing.html#mcdc

MC/DC comes in different flavors, the most important being unique cause
MC/DC and masking MC/DC. This patch implements masking MC/DC, which is
works well with short circuiting semantics, and according to John
Chilenski's "An Investigation of Three Forms of the Modified Condition
Decision Coverage (MCDC) Criterion" (2001) is as good as unique cause at
catching bugs.

Whalen, Heimdahl, and De Silva "Efficient Test Coverage Measurement for
MC/DC" describes an algorithm for finding the masking table from an AST
walk, but my algorithm figures this out by analyzing the control flow
graph.  The CFG is considered a reduced ordered binary decision diagram
and an input vector a path through the BDD, which is recorded.  Specific
edges will mask ("null out") the contribution from earlier path
segments, which can be determined by finding short circuit endpoints.
Masking is most easily understood as circuiting of terms in the
reverse-ordered Boolean function, and the masked conditions do not
affect the decision like short-circuited conditions do not affect the
decision.

A tag/discriminator mapping from gcond->uid is created during
gimplification and made available through the function struct. The
values are unimportant as long as basic conditions constructed from a
single Boolean expression are given the same identifier. This happens in
the breaking down of ANDIF/ORIF trees, so the coverage generally works
well for frontends that create such trees.

Like Whalen et al this implementation records coverage in fixed-size
bitsets which gcov knows how to interpret. Recording conditions only
requires a few bitwise operations per condition and is very fast, but
comes with a limit on the number of terms in a single boolean
expression; the number of bits in a gcov_unsigned_type (which is usually
typedef'd to uint64_t). For most practical purposes this is acceptable,
and by default a warning will be issued if gcc cannot instrument the
expression.  This is a practical limitation in the implementation, and
not a limitation of the algorithm, so support for more conditions can be
supported by introducing arbitrary-sized bitsets.

In action it looks pretty similar to the branch coverage. The -g short
opt carries no significance, but was chosen because it was an available
option with the upper-case free too.

gcov --conditions:

        3:   17:void fn (int a, int b, int c, int d) {
        3:   18:    if ((a && (b || c)) && d)
conditions covered 3/8
condition  0 not covered (true false)
condition  1 not covered (true)
condition  2 not covered (true)
condition  3 not covered (true)
        1:   19:        x = 1;
        -:   20:    else
        2:   21:        x = 2;
        3:   22:}

gcov --conditions --json-format:

"conditions": [
    {
        "not_covered_false": [
            0
        ],
        "count": 8,
        "covered": 3,
        "not_covered_true": [
            0,
            1,
            2,
            3
        ]
    }
],

Expressions with constants may be heavily rewritten before it reaches
the gimplification, so constructs like int x = a ? 0 : 1 becomes
_x = (_a == 0). From source you would expect coverage, but it gets
neither branch nor condition coverage. The same applies to expressions
like int x = 1 || a which are simply replaced by a constant.

The test suite contains a lot of small programs and functions. Some of
these were designed by hand to test for specific behaviours and graph
shapes, and some are previously-failed test cases in other programs
adapted into the test suite.

gcc/ChangeLog:

	* builtins.cc (expand_builtin_fork_or_exec): Check
	condition_coverage_flag.
	* collect2.cc (main): Add -fno-condition-coverage to OBSTACK.
	* common.opt: Add new options -fcondition-coverage and
	-Wcoverage-too-many-conditions.
	* doc/gcov.texi: Add --conditions documentation.
	* doc/invoke.texi: Add -fcondition-coverage documentation.
	* function.cc (free_after_compilation): Free cond_uids.
	* function.h (struct function): Add cond_uids.
	* gcc.cc: Link gcov on -fcondition-coverage.
	* gcov-counter.def (GCOV_COUNTER_CONDS): New.
	* gcov-dump.cc (tag_conditions): New.
	* gcov-io.h (GCOV_TAG_CONDS): New.
	(GCOV_TAG_CONDS_LENGTH): New.
	(GCOV_TAG_CONDS_NUM): New.
	* gcov.cc (class condition_info): New.
	(condition_info::condition_info): New.
	(condition_info::popcount): New.
	(struct coverage_info): New.
	(add_condition_counts): New.
	(output_conditions): New.
	(print_usage): Add -g, --conditions.
	(process_args): Likewise.
	(output_intermediate_json_line): Output conditions.
	(read_graph_file): Read condition counters.
	(read_count_file): Likewise.
	(file_summary): Print conditions.
	(accumulate_line_info): Accumulate conditions.
	(output_line_details): Print conditions.
	* gimplify.cc (next_cond_uid): New.
	(reset_cond_uid): New.
	(shortcut_cond_r): Set condition discriminator.
	(tag_shortcut_cond): New.
	(gimple_associate_condition_with_expr): New.
	(shortcut_cond_expr): Set condition discriminator.
	(gimplify_cond_expr): Likewise.
	(gimplify_function_tree): Call reset_cond_uid.
	* ipa-inline.cc (can_early_inline_edge_p): Check
	condition_coverage_flag.
	* ipa-split.cc (pass_split_functions::gate): Likewise.
	* passes.cc (finish_optimization_passes): Likewise.
	* profile.cc (struct condcov): New declaration.
	(cov_length): Likewise.
	(cov_blocks): Likewise.
	(cov_masks): Likewise.
	(cov_maps): Likewise.
	(cov_free): Likewise.
	(instrument_decisions): New.
	(read_thunk_profile): Control output to file.
	(branch_prob): Call find_conditions, instrument_decisions.
	(init_branch_prob): Add total_num_conds.
	(end_branch_prob): Likewise.
	* tree-core.h (struct tree_exp): Add condition_uid.
	* tree-profile.cc (struct conds_ctx): New.
	(CONDITIONS_MAX_TERMS): New.
	(EDGE_CONDITION): New.
	(topological_cmp): New.
	(index_of): New.
	(single_p): New.
	(single_edge): New.
	(contract_edge_up): New.
	(struct outcomes): New.
	(conditional_succs): New.
	(condition_index): New.
	(condition_uid): New.
	(masking_vectors): New.
	(emit_assign): New.
	(emit_bitwise_op): New.
	(make_top_index_visit): New.
	(make_top_index): New.
	(paths_between): New.
	(struct condcov): New.
	(cov_length): New.
	(cov_blocks): New.
	(cov_masks): New.
	(cov_maps): New.
	(cov_free): New.
	(find_conditions): New.
	(struct counters): New.
	(find_counters): New.
	(resolve_counter): New.
	(resolve_counters): New.
	(instrument_decisions): New.
	(tree_profiling): Check condition_coverage_flag.
	(pass_ipa_tree_profile::gate): Likewise.
	* tree.h (SET_EXPR_UID): New.
	(EXPR_COND_UID): New.

libgcc/ChangeLog:

	* libgcov-merge.c (__gcov_merge_ior): New.

gcc/testsuite/ChangeLog:

	* lib/gcov.exp: Add condition coverage test function.
	* g++.dg/gcov/gcov-18.C: New test.
	* gcc.misc-tests/gcov-19.c: New test.
	* gcc.misc-tests/gcov-20.c: New test.
	* gcc.misc-tests/gcov-21.c: New test.
	* gcc.misc-tests/gcov-22.c: New test.
	* gcc.misc-tests/gcov-23.c: New test.